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diff --git a/.gitattributes b/.gitattributes new file mode 100644 index 0000000..d7b82bc --- /dev/null +++ b/.gitattributes @@ -0,0 +1,4 @@ +*.txt text eol=lf +*.htm text eol=lf +*.html text eol=lf +*.md text eol=lf diff --git a/LICENSE.txt b/LICENSE.txt new file mode 100644 index 0000000..6312041 --- /dev/null +++ b/LICENSE.txt @@ -0,0 +1,11 @@ +This eBook, including all associated images, markup, improvements, +metadata, and any other content or labor, has been confirmed to be +in the PUBLIC DOMAIN IN THE UNITED STATES. + +Procedures for determining public domain status are described in +the "Copyright How-To" at https://www.gutenberg.org. + +No investigation has been made concerning possible copyrights in +jurisdictions other than the United States. Anyone seeking to utilize +this eBook outside of the United States should confirm copyright +status under the laws that apply to them. diff --git a/README.md b/README.md new file mode 100644 index 0000000..b02a961 --- /dev/null +++ b/README.md @@ -0,0 +1,2 @@ +Project Gutenberg (https://www.gutenberg.org) public repository for +eBook #55738 (https://www.gutenberg.org/ebooks/55738) diff --git a/old/55738-0.txt b/old/55738-0.txt deleted file mode 100644 index bde25d4..0000000 --- a/old/55738-0.txt +++ /dev/null @@ -1,2279 +0,0 @@ -The Project Gutenberg EBook of The Genetic Effects of Radiation, by -Isaac Asimov and Theodosius Dobzhansky - -This eBook is for the use of anyone anywhere in the United States and most -other parts of the world at no cost and with almost no restrictions -whatsoever. You may copy it, give it away or re-use it under the terms of -the Project Gutenberg License included with this eBook or online at -www.gutenberg.org. If you are not located in the United States, you'll have -to check the laws of the country where you are located before using this ebook. - -Title: The Genetic Effects of Radiation - -Author: Isaac Asimov - Theodosius Dobzhansky - -Release Date: October 13, 2017 [EBook #55738] - -Language: English - -Character set encoding: UTF-8 - -*** START OF THIS PROJECT GUTENBERG EBOOK THE GENETIC EFFECTS OF RADIATION *** - - - - -Produced by Stephen Hutcheson and the Online Distributed -Proofreading Team at http://www.pgdp.net - - - - - - - - - - The Genetic Effects of Radiation - - - By ISAAC ASIMOV and THEODOSIUS DOBZHANSKY - - - - - Contents - - - THE MACHINERY OF INHERITANCE 1 - Introduction 1 - Cells and Chromosomes 2 - Enzymes and Genes 5 - Parents and Offspring 8 - MUTATIONS 10 - Sudden Change 10 - Spontaneous Mutations 13 - Genetic Load 16 - Mutation Rates 19 - RADIATION 22 - Ionizing Radiation 22 - Background Radiation 27 - Man-made Radiation 30 - DOSE AND CONSEQUENCE 32 - Radiation Sickness 32 - Radiation and Mutation 33 - Dosage Rates 37 - Effects on Mammals 40 - Conclusion 43 - SUGGESTED REFERENCES 47 - - -THE COVER - -[Illustration: The cover design embodies a radiation symbol, a stylized -karyotype of human chromosomes, and a genealogical table.] - -THE AUTHORS - -[Illustration: ISAAC ASIMOV received his academic degrees from Columbia -University and is Associate Professor of Biochemistry at the Boston -University School of Medicine. He is a prolific author who has written -over 65 books in the past 15 years, including about 20 science fiction -works, and books for children. His many excellent science books for the -public cover subjects in mathematics, physics, astronomy, chemistry, and -biology, such as _The Genetic Code_, _Inside the Atom_, _Building Blocks -of the Universe_, _The Living River_, _The New Intelligent Man’s Guide -to Science_, and _Asimov’s Biographical Encyclopedia of Science and -Technology_. In 1965 Dr. Asimov received the James T. Grady Award of the -American Chemical Society for his major contribution in reporting -science progress to the public.] - -[Illustration: THEODOSIUS DOBZHANSKY was graduated from Kiev University -and is now a professor at the Rockefeller University. He has done -research in genetics and biological evolution on every continent except -Antarctica. Among his distinguished published works are _Radiation, -Genes, and Man_, _Heredity and the Nature of Man_, _Mankind Evolving_, -and _Evolution, Genetics, and Man_. Mr. Dobzhansky received the Daniel -G. Elliot Prize and Medal and the Kimber Genetics Award from the -National Academy of Sciences in 1958, and the National Medal of Science -awarded by the President of the United States, in 1965.] - - - - - The Genetic Effects of Radiation - - - - - THE MACHINERY OF INHERITANCE - - -Introduction - -There is nothing new under the sun, says the Bible. Nor is the sun -itself new, we might add. As long as life has existed on earth, it has -been exposed to radiation from the sun, so that life and radiation are -old acquaintances and have learned to live together. - -We are accustomed to looking upon sunlight as something good, useful, -and desirable, and certainly we could not live long without it. The -energy of sunlight warms the earth, produces the winds that tend to -equalize earth’s temperatures, evaporates the oceans and produces rain -and fresh water. Most important of all, it supplies what is needed for -green plants to convert carbon dioxide and water into food and oxygen, -making it possible for all animal life (including ourselves) to live. - -Yet sunlight has its dangers, too. Lizards avoid the direct rays of the -noonday sun on the desert, and we ourselves take precautions against -sunburn and sunstroke. - -The same division into good and bad is to be found in connection with -other forms of radiation—forms of which mankind has only recently become -aware. Such radiations, produced by radioactivity in the soil and -reaching us from outer space, have also been with us from the beginning -of time. They are more energetic than sunlight, however, and can do more -damage, and because our senses do not detect them, we have not learned -to take precautions against them. - -To be sure, energetic radiation is present in nature in only very small -amounts and is not, therefore, much of a danger. Man, however, has the -capacity of imitating nature. Long ago in dim prehistory, for instance, -he learned to manufacture a kind of sunlight by setting wood and other -fuels on fire. This involved a new kind of good and bad. A whole new -technology became possible, on the one hand, and, on the other, the -chance of death by burning was also possible. The good in this case far -outweighs the evil. - -In our own twentieth century, mankind learned to produce energetic -radiation in concentrations far surpassing those we usually encounter in -nature. Again, a new technology is resulting and again there is the -possibility of death. - -The balance in this second instance is less certainly in favor of the -good over the evil. To shift the balance clearly in favor of the good, -it is necessary for mankind to learn as much as possible about the new -dangers in order that we might minimize them and most effectively guard -against them. - -To see the nature of the danger, let us begin by considering living -tissue itself—the living tissue that must withstand the radiation and -that can be damaged by it. - - -Cells and Chromosomes - -The average human adult consists of about 50 trillion _cells_—50 -trillion microscopic, more or less self-contained, blobs of life. He -begins life, however, as a single cell, the _fertilized ovum_. - -After the fertilized ovum is formed, it divides and becomes two cells. -Each daughter cell divides to produce a total of four cells, and each of -those divides and so on. - -There is a high degree of order and direction to those divisions. When a -human fertilized ovum completes its divisions an adult human being is -the inevitable result. The fertilized ovum of a giraffe will produce a -giraffe, that of a fruit fly will produce a fruit fly, and so on. There -are no mistakes, so it is quite clear that the fertilized ovum must -carry “instructions” that guide its development in the appropriate -direction. - -These “instructions” are contained in the cell’s _chromosomes_, tiny -structures that appear most clearly (like stubby bits of tangled -spaghetti) when the cell is in the actual process of division. Each -species has some characteristic number of chromosomes in its cells, and -these chromosomes can be considered in pairs. Human cells, for instance, -contain 23 pairs of chromosomes—46 in all. - -When a cell is undergoing division (_mitosis_), the number of -chromosomes is temporarily doubled, as each chromosome brings about the -formation of a replica of itself. (This process is called -_replication_.) As the cell divides, the chromosomes are evenly shared -by the new cells in such a way that if a particular chromosome goes into -one daughter cell, its replica goes into the other. In the end, each -cell has a complete set of pairs of chromosomes; and the set in each -cell is identical with the set in the original cell before division. - -[Illustration: Mitosis] - - Interphase - Prophase - Metaphase - Anaphase - Telophase - Interphase - -[Illustration: _To study chromosomes, scientists begin with a cell that -is in the process of dividing, when chromosomes are in their most -visible form. Then they treat the cell with a chemical, a derivative of -colchicine, to arrest the cell division at the metaphase stage (see -mitosis diagram on preceding page). This brings a result like the -photomicrograph above; the chromosomes are visible but still too tangled -to be counted or measured. Then the cell is treated with a -low-concentration salt solution, which swells the chromosomes and -disperses them so they become distinct structures, as below._] - - [Illustration: Cell after treatment with salt solution] - -[Illustration: _The separate chromosomes in a dividing cell are -photographed and then can be identified by their overall length, the -position of the centromere, or point where the two strands join, and -other characteristics. The photomicrograph can then be cut apart and the -chromosomes grouped in a karyotype, which is an arrangement according to -a standard classification to show chromosome complement and -abnormalities. The karotype below is of a normal male, since it shows X -and Y sex chromosomes and 22 pairs of other, autosomal, chromosomes. By -contrast, the cells in the upper pictures are abnormal, with only 45 -chromosomes each._] - -In this way, the fundamental “instructions” that determine the -characteristics of a cell are passed on to each new cell. Ideally, all -the trillions of cells in a particular human being have identical sets -of “instructions”.[1] - - -Enzymes and Genes - -Each cell is a tiny chemical factory in which several thousand different -kinds of chemical changes are constantly taking place among the numerous -sorts of molecules that move about in its fluid or that are pinned to -its solid structures. These chemical changes are guided and controlled -by the existence of as many thousands of different _enzymes_ within the -cell. - -Enzymes possess large molecules built up of some 20 different, but -chemically related, units called _amino acids_. A particular enzyme -molecule may contain a single amino acid of one type, five of another, -several dozen of still another and so on. All the units are strung -together in some specific pattern in one long chain, or in a small -number of closely connected chains. - -Every different pattern of amino acids forms a molecule with its own set -of properties, and there are an enormous number of patterns possible. In -an enzyme molecule made up of 500 amino acids, the number of possible -patterns can be expressed by a 1 followed by 1100 zeroes (10¹¹⁰⁰). - -Every cell has the capacity of choosing among this unimaginable number -of possible patterns and selecting those characteristic of itself. It -therefore ends with a complement of specific enzymes that guide its own -chemical changes and, consequently, its properties and its behavior. The -“instructions” that enable a fertilized ovum to develop in the proper -manner are essentially “instructions” for choosing a particular set of -enzyme patterns out of all those possible. - -The differences in the enzyme-guided behavior of the cells making up -different species show themselves in differences in body structure. We -cannot completely follow the long and intricate chain of -cause-and-effect that leads from one set of enzymes to the long neck of -a giraffe and from another set of enzymes to the large brain of a man, -but we are sure that the chain is there. Even within a species, -different individuals will have slight distinctions among their sets of -enzymes and this accounts for the fact that no two human beings are -exactly alike (leaving identical twins out of consideration). - -Each chromosome can be considered as being composed of small sections -called _genes_, usually pictured as being strung along the length of the -chromosome. Each gene is considered to be responsible for the formation -of a chain of amino acids in a fixed pattern. The formation is guided by -the details of the gene’s own structure (which are the “instructions” -earlier referred to). This gene structure, which can be translated into -an enzyme’s structure, is now called the _genetic code_. - -[Illustration: _Stained section of one cell from salivary gland of_ -Drosophila, _or fruit flies, reveals dark bands that may be genes -controlling specific traits_.] - -If a particular enzyme (or group of enzymes) is, for any reason, formed -imperfectly or not at all, this may show up as some visible abnormality -of the body—an inability to see color, for instance, or the possession -of two joints in each finger rather than three. It is much easier to -observe physical differences than some delicate change in the enzyme -pattern of the cells. Genes are therefore usually referred to by the -body change they bring about, and one can, for instance, speak of a -“gene for color blindness”. - -A gene may exist in two or more varieties, each producing a slightly -different enzyme, a situation that is reflected, in turn, in slight -changes in body characteristics. Thus, there are genes governing eye -color, one of which is sufficiently important to be considered a “gene -for blue eyes” and another a “gene for brown eyes”. One or the other, -but not both, will be found in a specific place on a specific -chromosome. - -The two chromosomes of a particular pair govern identical sets of -characteristics. Both, for instance, will have a place for genes -governing eye color. If we consider only the most important of the -varieties involved, those on each chromosome of the pair may be -identical; both may be for blue eyes or both may be for brown eyes. In -that case, the individual is _homozygous_ for that characteristic and -may be referred to as a _homozygote_. The chromosomes of the pair may -carry different varieties: A gene for blue eyes on one chromosome and -one for brown eyes on the other. The individual is then _heterozygous_ -for that characteristic and may be referred to as a _heterozygote_. -Naturally, particular individuals may be homozygous for some types of -characteristics and heterozygous for others. - -When an individual is heterozygous for a particular characteristic, it -frequently happens that he shows the effect associated with only one of -the gene varieties. If he possesses both a gene for brown eyes and one -for blue eyes, his eyes are just as brown as though he had carried two -genes for brown eyes. The gene for brown eyes is _dominant_ in this case -while the gene for blue eyes is _recessive_. - - -Parents and Offspring - -How does the fertilized ovum obtain its particular set of chromosomes in -the first place? - -Each adult possesses gonads in which _sex cells_ are formed. In the -male, sperm cells are formed in the testes; in the female, egg cells are -formed in the ovaries. - -In the formation of the sperm cells and egg cells there is a key -step—_meiosis_—a cell division in which the chromosomes group into pairs -and are then apportioned between the daughter cells, one of each pair to -each cell. Such a division, unaccompanied by replication, means that in -place of the usual 23 pairs of chromosomes in each other cell, each sex -cell has 23 individual chromosomes, a “half-set”, so to speak. - -In the process of fertilization, a sperm cell from the father enters and -merges with an egg cell from the mother. The fertilized ovum that -results now has a full set of 23 pairs of chromosomes, but of each pair, -one comes from the father and one from the mother. - -In this way, each newborn child is a true individual, with its -characteristics based on a random reshuffling of chromosomes. In forming -the sex cells, the chromosome pairs can separate in either fashion (_a_ -into cell 1 and _b_ into cell 2, or vice versa). If each of 23 pairs -does this randomly, nearly 10 million different combinations of -chromosomes are possible in the sex cells of a single individual. - -Furthermore, one can’t predict which chromosome combination in the sperm -cell will end up in combination with which in the egg cell, so that by -this reasoning, a single married couple could produce children with any -of 100 trillion (100,000,000,000,000) possible chromosome combinations. - -It is this that begins to explain the endless variety among living -beings, even within a particular species. - -It only begins to explain it, because there are other sources of -difference, too. A chromosome is capable of exchanging pieces with its -pair, producing chromosomes with a brand new pattern of gene varieties. -Before such a _crossover_, one chromosome may have carried a gene for -blue eyes and one for wavy hair, while the other chromosome may have -carried a gene for brown eyes and one for straight hair. After the -crossover, one would carry genes for blue eyes and straight hair, the -other for brown eyes and wavy hair. - - [Illustration: Meiosis] - - Interphase - Prophase - Metaphase - Anaphase - Interphase - Metaphase - Interphase - - - - - MUTATIONS - - -Sudden Change - -Shifts in chromosome combinations, with or without crossovers, can -produce unique organisms with characteristics not quite like any -organism that appeared in the past nor likely to appear in the -reasonable future. They may even produce novelties in individual -characteristics since genes can affect one another, and a gene -surrounded by unusual neighbors can produce unexpected effects. - -Matters can go further still, however, in the direction of novelty. It -is possible for chromosomes to undergo more serious changes, either -structural or chemical, so that entirely new characteristics are -produced that might not otherwise exist. Such changes are called -_mutations_. - -We must be careful how we use this term. A child may possess some -characteristics not present in either parent through the mere shuffling -of chromosomes and not through mutation. - -Suppose, for instance, that a man is heterozygous to eye color, carrying -one gene for brown eyes and one for blue eyes. His eyes would, of -course, be brown since the gene for brown eyes is dominant over that for -blue. Half the sperm cells he produces would carry a single gene for -brown eyes in its half set of chromosomes. The other half would carry a -single gene for blue eyes. If his wife were similarly heterozygous (and -therefore also had brown eyes), half her egg cells would carry the gene -for brown eyes and half the gene for blue. - -It might follow in this marriage, then, that a sperm carrying the gene -for blue eyes might fertilize an egg carrying the gene for blue eyes. -The child would then be homozygous, with two genes for blue eyes, and he -would definitely be blue-eyed. In this way, two brown-eyed parents might -have a blue-eyed child and this would _not_ be a mutation. If the -parents’ ancestry were traced further back, blue-eyed individuals would -undoubtedly be found on both sides of the family tree. - -If, however, there were no record of, say, anything but normal color -vision in a child’s ancestry, and he were born color-blind, that could -be assumed to be the result of a mutation. Such a mutation could then be -passed on by the normal modes of inheritance and a certain proportion of -the child’s eventual descendants would be color-blind. - -A mutation may be associated with changes in chromosome structure -sufficiently drastic to be visible under the microscope. Such -_chromosome mutations_ can arise in several ways. Chromosomes may -undergo replication without the cell itself dividing. In that way, cells -can develop with two, three, or four times the normal complement of -chromosomes, and organisms made up of cells displaying such _polyploidy_ -can be markedly different from the norm. This situation is found chiefly -among plants and among some groups of invertebrates. It does not usually -occur in mammals, and when it does it leads to quick death. - -Less extreme changes take place, too, as when a particular chromosome -breaks and fails to reunite, or when several break and then reunite -incorrectly. Under such conditions, the mechanism by which chromosomes -are distributed among the daughter cells is not likely to work -correctly. Sex cells may then be produced with a piece of chromosome (or -a whole one) missing, or with an extra piece (or whole chromosome) -present. - -In 1959, such a situation was found to exist in the case of persons -suffering from a long-known disease called Down’s syndrome.[2] Each -person so afflicted has 47 chromosomes in place of the normal 46. It -turned out that the 21st pair of chromosomes (using a convention whereby -the chromosome pairs are numbered in order of decreasing size) consists -of three individuals rather than two. The existence of this chromosome -abnormality clearly demonstrated what had previously been strongly -suspected—that Down’s syndrome originates as a mutation and is inborn -(see the figure on the next page). - -[Illustration: _Karyotype of a female patient with Down’s syndrome -(Mongolism). During meiosis both chromosomes No. 21 of the mother, -instead of just one, went to the ovum. Fertilization added the father’s -chromosome, which made three Nos. 21 instead of the normal pair. -(Compare with the normal karyotype on page 4.)_] - -Most mutations, however, are not associated with any noticeable change -in chromosome structure. There are, instead, more subtle changes in the -chemical structure of the genes that make up the chromosome. Then we -have _gene mutations_. - -The process by which a gene produces its own replica is complicated and, -while it rarely goes wrong, it does misfire on occasion. Then, too, even -when a gene molecule is replicated perfectly, it may undergo change -afterward through the action upon it of some chemical or other -environmental influence. In either case, a new variety of a particular -gene is produced and, if present in a sex cell, it may be passed on to -descendants through an indefinite number of generations. - -Of course, chromosome or gene mutations may take place in ordinary cells -rather than in sex cells. Such changes in ordinary cells are _somatic -mutations_. When mutated body cells divide, new cells with changed -characteristics are produced. These changes may be trivial, or they may -be serious. It is often suggested, for instance, that cancer may result -from a somatic mutation in which certain cells lose the capacity to -regulate their growth properly. Since somatic mutations do not involve -the sex cells, they are confined to the individual and are not passed on -to the offspring. - - -Spontaneous Mutations - -Mutations that take place in the ordinary course of nature, without -man’s interference, are _spontaneous mutations_. Most of these arise out -of the very nature of the complicated mechanism of gene replication. -Copies of genes are formed out of a large number of small units that -must be lined up in just the right pattern to form one particular gene -and no other. - -Ideally, matters are so arranged within the cell that the necessary -changes giving rise to the desired pattern are just those that have a -maximum probability. Other changes are less likely to happen but are not -absolutely excluded. Sometimes through the accidental jostling of -molecules a wrong turn may be taken, and the result is a spontaneous -mutation. - -We might consider a mutation to be either “good” or “bad” in the sense -that any change that helps a creature live more easily and comfortably -is good and that the reverse is bad. - -It seems reasonable that random changes in the gene pattern are almost -sure to be bad. Consider that any creature, including man, is the -product of millions of years of evolution. In every generation those -individuals with a gene pattern that fit them better for their -environment won out over those with less effective patterns—won out in -the race for food, for mates, and for safety. The “more fit” had more -offspring and crowded out the “less fit”. - -By now, then, the set of genes with which we are normally equipped is -the end product of long ages of such _natural selection_. A random -change cannot be expected to improve it any more than random changes -would improve any very complex, intricate, and delicate structure. - -[Illustration: _Evolution of the horse (skull, hindfoot, and forefoot -shown). Note the changes over a 60-million-year period from the Eocene -era to the present._] - - Pleistocene and Recent - Pliocene - Miocene - Oligocene - Eocene - -Yet over the eons, creatures have indeed changed, largely through the -effects of mutation. If mutations are almost always for the worse, how -can one explain that evolution seems to progress toward the better and -that out of a primitive form as simple as an amoeba, for instance, there -eventually emerged man? - -In the first place, environment is not fixed. Climate changes, -conditions change, the food supply may change, the nature of living -enemies may change. A gene pattern that is very useful under one set of -conditions may be less useful under another. - -Suppose, for instance, that man had lived in tropical areas for -thousands of years and had developed a heavily pigmented skin as a -protection against sunburn. Any child who, through a mutation, found -himself incapable of forming much pigment, would be at a severe -disadvantage in the outdoor activities engaged in by his tribe. He would -not do well and such a mutated gene would never establish itself for -long. - -If a number of these men migrated to northern Europe, however, children -with dark skin would absorb insufficient sunlight during the long winter -when the sun was low in the sky, and visible for brief periods only. -Dark-skinned children would, under such conditions, tend to suffer from -rickets. - -Mutant children with pale skin would absorb more of what weak sunlight -there was and would suffer less. There would be little danger of sunburn -so there would be no penalty counteracting this new advantage of pale -skins. It would be the dark-skinned people who would tend to die out. In -the end, you would have dark skins in Africa and pale skins in -Scandinavia, and both would be “fit”. - -In the same way, any child born into a primitive hunting society who -found himself with a mutated gene that brought about nearsightedness -would be at a distinct disadvantage. In a modern technological society, -however, nearsighted individuals, doing more poorly at outdoor games, -are often driven into quieter activities that involve reading, thinking, -and studying. This may lead to a career as a scientist, scholar, or -professional man, categories that are valuable in such a society and are -encouraged. Nearsightedness would therefore spread more generally -through civilized societies than through primitive ones. - -Then, too, a gene may be advantageous when it occurs in low numbers and -disadvantageous when it occurs in high numbers. Suppose there were a -gene among humans that so affected the personality as to make it -difficult for a human being to endure crowded conditions. Such -individuals would make good explorers, farmers, and herdsmen, but poor -city dwellers. Even in our modern urbanized society, such a gene in -moderate concentration would be good, since we still need our -outdoorsmen. In high concentration, it would be bad, for then the -existence of areas of high population density (on which our society now -seems to depend) might become impossible. - -In any species, then, each gene exists in a number of varieties upon -which an absolute “good” or “bad” cannot be unequivocally stamped. These -varieties make up the _gene pool_, and it is this gene pool that makes -evolution possible. - -A species with an invariable set of genes could not change to suit -altered conditions. Even a slight shift in the nature of the environment -might suffice to wipe it out. - -The possession of a gene pool lends flexibility, however. As conditions -change, one combination of varieties might gain over another and this, -in turn, might produce changes in body characteristics that would then -further alter the relative “goodness” or “badness” of certain gene -patterns. - -Thus, over the past million years, for example, the human brain has, -through mutations and appropriate shifts in emphasis within the gene -pool, increased notably in size. - - -Genetic Load - -Some gene mutations produce characteristics so undesirable that it is -difficult to imagine any reasonable change in environmental conditions -that would make them beneficial. There are mutations that lead to the -nondevelopment of hands and feet, to the production of blood that will -not clot, to serious malformations of essential organs, and so on. Such -mutations are unqualifiedly bad. - -The badness may be so severe that a fertilized ovum may be incapable of -development; or, if it develops, the fetus miscarries or the child is -stillborn; or, if the child is born alive, it dies before it matures so -that it can never have children of its own. Any mutation that brings -about death before the gene producing it can be passed on to another -generation is a _lethal mutation_. - -A gene governing a lethal characteristic may be dominant. It will then -kill even though the corresponding gene on the other chromosome of the -pair is normal. Under such conditions, the lethal gene is removed in the -same generation in which it is formed. - -The lethal gene may, on the other hand, be recessive. Its effect is then -not evident if the gene it is paired with is normal. The normal gene -carries on for both. - -When this is the case, the lethal gene will remain in existence and -will, every once in a while, make itself evident. If two people, each -serving as a _carrier_ for such a gene, have children, a sperm cell -carrying a lethal may fertilize an egg cell carrying the same type of -lethal, with sad results. - -Every species, including man, includes individuals who carry undesirable -genes. These undesirable genes may be passed along for generations, even -if dominant, before natural selection culls them out. The more seriously -undesirable they are, the more quickly they are removed, but even -outright lethal genes will be included among the chromosomes from -generation to generation provided they are recessive. These deleterious -genes make up the _genetic load_. - -The only way to avoid a genetic load is to have no mutations and -therefore no gene pool. The gene pool is necessary for the flexibility -that will allow a species to survive and evolve over the eons and the -genetic load is the price that must be paid for that. Generally, the -capacity for a species to reproduce itself is sufficiently high to make -up, quite easily, the numbers lost through the combination of -deleterious genes. - -The size of a genetic load depends on two factors: The rate at which a -deleterious gene is produced through mutation, and the rate at which it -is removed by natural selection. When the rate of removal equals the -rate of production, a condition of _genetic equilibrium_ is reached and -the level of occurrence of that gene then remains stable over the -generations. - -Even though deleterious genes are removed relatively rapidly, if -dominant, and lethal genes are removed in the same generation in which -they are formed, a new crop of deleterious genes will appear by mutation -with every succeeding generation. The equilibrium level for such -dominant deleterious genes is relatively low, however. - -Deleterious genes that are recessive are removed much more slowly. Those -persons with two such genes, who alone show the bad effects, are like -the visible portion of an iceberg and represent only a small part of the -whole. The heterozygotes, or carriers, who possess a single gene of this -sort, and who live out normal lives, keep that gene in being. If people -in a particular population marry randomly and if one out of a million is -born homozygous for a certain deleterious recessive gene (and dies of -it), one out of five hundred is heterozygous for that same gene, shows -no ill effects, and is capable of passing it on. - -It may be that the heterozygote is not quite normal but does show some -ill effects—not enough to incommode him seriously, perhaps, but enough -to lower his chances slightly for mating and bearing children. In that -case, the equilibrium level for that gene will be lower than it would -otherwise be. - -It may also be that the heterozygote experiences an actual advantage -over the normal individual under some conditions. There is a recessive -gene, for instance, that produces a serious disease called sickle-cell -anemia. People possessing two such genes usually die young. A -heterozygote possessing only one of these genes is not seriously -affected and has red blood cells that are, apparently, less appetizing -to malaria parasites. The heterozygote therefore experiences a positive -advantage if he lives in a region where the incidence of certain kinds -of malaria is high. The equilibrium level of the sickle-cell anemia gene -can, in other words, be higher in malarial regions than elsewhere. - -Here is one subject area in which additional research is urgently -needed. It may be that the usefulness of a single deleterious gene is -greater than we may suspect in many cases, and that there are greater -advantages to heterozygousness than we know. This may be the basis of -what is sometimes called “hybrid vigor”. In a world in which human -beings are more mobile than they have ever been in history and in which -intercultural marriages are increasingly common, information on this -point is particularly important. - - -Mutation Rates - -It is easier to observe the removal of genes through death or through -failure to reproduce than to observe their production through mutation. -It is particularly difficult to study their production in human beings, -since men have comparatively long lifetimes and few children, and since -their mating habits cannot well be controlled. - -For this reason, geneticists have experimented with species much simpler -than man—smaller organisms that are short-lived, produce many offspring, -and that can be penned up and allowed to mate only under fixed -conditions. Such creatures may have fewer chromosomes than man does and -the sites of mutation are more easily pinned down. - -An important assumption made in such experiments is that the machinery -of inheritance and mutation is essentially the same in all creatures and -that therefore knowledge gained from very simple species (even from -bacteria) is applicable to man. There is overwhelming evidence to -indicate that this is true in general, although there are specific -instances where it is not completely true and scientists must tread -softly while drawing conclusions. - -The animals most commonly used in studies of genetics and mutations are -certain species of fruit flies, called _Drosophila_. The American -geneticist, Hermann J. Muller, devised techniques whereby he could study -the occurrence of lethal mutations anywhere along one of the four pairs -of chromosomes possessed by _Drosophilia_. - -A lethal gene, he found, might well be produced somewhere along the -length of a particular chromosome once out of every two hundred times -that chromosome underwent replication. This means that out of every 200 -sex cells produced by _Drosophilia_, one would contain a lethal gene -somewhere along the length of that chromosome. - -[Illustration: _Geneticist Hermann J. Muller studying_ Drosophila _in -his laboratory. Dr. Muller won a Nobel Prize in 1946 for showing that -radiation can cause mutations. (See page 34.)_] - -That particular chromosome, however, contained at least 500 genes -capable of undergoing a lethal mutation. If each of those genes is -equally likely to undergo such a mutation, then the chance that any one -particular gene is lethal is one out of 200 × 500, or 1 out of 100,000. - -This is a typical mutation rate for a gene in higher organisms -generally, as far as geneticists can tell (though the rates are lower -among bacteria and viruses). Naturally, a chance for mutation takes -place every time a new individual is born. Fruit flies have many more -offspring per year than human beings, since their generations are -shorter and they produce more young at a time. For that reason, though -the mutation rate may be the same in fruit flies as in man, many more -actual mutations are produced per unit time in fruit flies than in men. - -This does not mean that the situation may be ignored in the case of man. -Suppose the rate for production of a particular deleterious gene in man -is 1 out of 100,000. It is estimated that a human being has at least -10,000 different genes, and therefore the chance that at least one of -the genes in a sex cell is deleterious is 10,000 out of 100,000 or 1 out -of 10. - -Furthermore, it is estimated that the number of gene mutations that are -weakly deleterious are four times as numerous as those that are strongly -deleterious or lethal. The chances that at least one gene in a sex cell -is at least weakly deleterious then would be 4 + 1 out of 10, or 1 out -of 2. - -Naturally, these deleterious genes are not necessarily spread out evenly -among human beings with one to a sex cell. Some sex cells will be -carrying more than one, thus increasing the number that may be expected -to carry none at all. Even so, it is supposed that very nearly half the -sex cells produced by humanity carry at least one deleterious gene. - -Even though only half the sex cells are free of deleterious genes, it is -still possible to produce a satisfactory new generation of men. Yet one -can see that the genetic load is quite heavy and that anything that -would tend to increase it would certainly be undesirable, and perhaps -even dangerous. - -We tend to increase the genetic load by reducing the rate at which -deleterious genes are removed, that is, by taking care of the sick and -retarded, and by trying to prevent discomfort and death at all levels. - -There is, however, no humane alternative to this. What’s more, it is, by -and large, only those with slightly deleterious genes who are preserved -genetically. It is those persons with nearsightedness, with diabetes, -and so on, who, with the aid of glasses, insulin, or other props, can go -on to live normal lives and have children in the usual numbers. Those -with strongly deleterious genes either die despite all that can be done -for them even today or, at the least, do not have a chance to have many -children. - -The danger of an increase in the genetic load rests more heavily, then, -at the other end—at measures that (usually inadvertently or -unintentionally) increase the rate of production of mutant genes. It is -to this matter we will now turn. - - - - - RADIATION - - -Ionizing Radiation - -Our modern technological civilization exposes mankind to two general -types of genetic dangers unknown earlier: Synthetic chemicals (or -unprecedentedly high concentrations of natural ones) absent in earlier -eras, and intensities of energetic radiation equally unknown or -unprecedented. - -Chemicals can interfere with the process of replication by offering -alternate pathways with which the cellular machinery is not prepared to -cope. In general, however, it is only those cells in direct contact with -the chemicals that are so affected, such as the skin, the intestinal -linings, the lungs, and the liver (which is active in altering and -getting rid of foreign chemicals). These may undergo somatic mutations, -and an increased incidence of cancer in those tissues is among the -drastic results of exposure to certain chemicals. - -Such chemicals are not, however, likely to come in contact with the -gonads where the sex cells are produced. While individual persons may be -threatened by the manner in which the environment is being permeated -with novel chemicals, the next generation is not affected in advance. - -Radiation is another matter. In its broadest sense, radiation is any -phenomenon spreading out from some source in all directions. Physically, -such radiation may consist of waves or of particles.[3] Of the wave -forms the two best-known are sound and electromagnetic radiations. - -Sound carries very low concentrations of energy. This energy is absorbed -by living tissue and converted into heat. Heat in itself can increase -the mutation rate but the effect is a small one. The body has effective -machinery for keeping its temperature constant and the gonads are not -likely to suffer unduly from exposure to heat. - -Electromagnetic radiation comes in a wide range of energies, with -visible light (the best-known example of such radiation because we can -detect it directly and with great sensitivity) about in the middle of -the range. Electromagnetic radiations less energetic than light (such as -infrared waves and microwaves) are converted into heat when absorbed by -living tissue. The heat thus formed is sufficient to cause atoms and -molecules to vibrate more rapidly, but this added vibration is not -usually sufficient to pull molecules apart and therefore does not bring -about chemical changes. - -Light will bring about some chemical changes. It is energetic enough to -cause a mixture of hydrogen and chlorine to explode. It will break up -silver compounds and produce tiny black grains of metallic silver (the -chemical basis of photography). Living tissue, however, is largely -unaffected—the retina of the eye being one obvious exception. - -Ultraviolet light, which is more energetic than visible light, -correspondingly can bring about chemical changes more easily. It will -redden the skin, stimulate the production of pigment, and break up -certain steroid molecules to form vitamin D. It will even interfere with -replication to some extent. At least there is evidence that persistent -exposure to sunlight brings about a heightened tendency to skin cancer. -Ultraviolet light is not very penetrating, however, and its effects are -confined to the skin. - -Electromagnetic radiations more energetic than ultraviolet light, such -as X rays and gamma rays, carry sufficient concentrations of energy to -bring about changes not only in molecules but in the very structure of -the atoms making up those molecules. - -Atoms consist of particles (electrons), each carrying a negative -electric charge and circling a tiny centrally located nucleus, which -carries a positive electric charge. - -Ordinarily, the negative charges of the electrons just balance the -positive charge on the nucleus so that atoms and molecules tend to be -electrically neutral. An X ray or gamma ray, crashing into an atom, -will, however, jar electrons loose. What is left of the atom will carry -a positive electric charge with the charge size proportional to the -number of electrons lost. - -An atom fragment carrying an electric charge is called an _ion_. X rays -and gamma rays are therefore examples of _ionizing radiation_. - -Radiations may consist of flying particles, too, and if these carry -sufficient energy they are also ionizing in character. Examples are -_cosmic rays_, _alpha rays_, and _beta rays_. Cosmic rays are streams of -positively charged nuclei, predominantly those of the element hydrogen. -Alpha rays are streams of positively charged helium nuclei. Beta rays -are streams of negatively charged electrons. The individual particles -contained in these rays may be referred to as _cosmic particles_, _alpha -particles_, and _beta particles_, respectively. - -[Illustration: _Cosmic ray and trapped Van Allen Belt energetic -particles produced the dark tracks in this photo of a nuclear emulsion -that had been carried aloft on an Air Force satellite. The energetic -particles cause ionization of the silver bromide molecules in the -emulsion._] - -[Illustration: _Alpha particles emitted by the source at right leave -tracks in a cloud chamber. Some tracks are bent near the end as a result -of collisions with atomic nuclei. Such collisions are more likely at the -end of a track when the alpha particle has been slowed down._] - -[Illustration: _Beta particles originating at left leave these tracks in -a cloud chamber. Note that the tracks are much farther apart than those -of alpha particles. As the particle slows down, its path becomes more -erratic and the ions are formed closer together. At the very end of an -electron track the proximity of the ions approximates that in an -alpha-particle track._] - -Ionizing radiation is capable of imparting so much energy to molecules -as to cause them to vibrate themselves apart, producing not only ions -but also high-energy uncharged molecular fragments called _free -radicals_. - -The direct effect of ionizing radiation on chromosomes can be serious. -Enough chemical bonds may be disrupted so that a chromosome struck by a -high-energy wave or particle may break into fragments. Even if the -chromosome manages to remain intact, an individual gene along its length -may be badly damaged and a mutation may be produced. - -[Illustration: _Effects of ionizing radiation on chromosomes: Left, a -normal plant cell showing chromosomes divided into two groups; right, -the same type of cell after X-ray exposure, showing broken fragments and -bridges between groups, typical abnormalities induced by radiation._] - -If only direct hits mattered, radiation effects would be less dangerous -than they are, since such direct hits are comparatively few. However, -near-misses may also be deadly. A streaking bit of radiation may strike -a water molecule near a gene and may break up the molecule to form a -free radical. The free radical will be sufficiently energetic to bring -about a chemical reaction with almost any molecule it strikes. If it -happens to strike the neighboring gene before it has disposed of that -energy, it will produce the mutation as surely as the original radiation -might have. - -Furthermore, ionizing radiations (particularly of the electromagnetic -variety) tend to be penetrating, so that the interior of the body is as -exposed as is the surface. The gonads cannot hide from X rays, gamma -rays, or cosmic particles. - -All these radiations can bring about somatic mutations—all can cause -cancer, for instance. - -What is worse, all of them increase the rate of genetic mutations so -that their presence threatens generations unborn as well as the -individuals actually exposed. - - -Background Radiation - -Ionizing radiation in low intensities is part of our natural -environment. Such natural radiation is referred to as _background -radiation_. Part of it arises from certain constituents of the soil. -Atoms of the heavy metals, uranium and thorium, are constantly, though -very slowly, breaking down and in the process giving off alpha rays, -beta rays, and gamma rays. These elements, while not among the most -common, are very widely spread; minerals containing small quantities of -uranium and thorium are to be found nearly everywhere. - -In addition, all the earth is bombarded with cosmic rays from outer -space and with streams of high-energy particles from the sun. - -Various units can be used to measure the intensity of this background -radiation. The _roentgen_, abbreviated _r_, and named in honor of the -discoverer of X rays, Wilhelm Roentgen, is a unit based on the number of -ions produced by radiation. Rather more convenient is another unit that -has come more recently into prominence. This is the _rad_ (an -abbreviation for “radiation absorbed dose”) that is a measure of the -amount of energy delivered to the body upon the absorption of a -particular dose of ionizing radiation. One rad is very nearly equal to -one roentgen. - -Since background radiation is undoubtedly one of the factors in -producing spontaneous mutations, it is of interest to try to determine -how much radiation a man or woman will have absorbed from the time he is -first conceived to the time he conceives his own children. The average -length of time between generations is taken to be about 30 years, so we -can best express absorption of background radiation in units of _rads -per 30 years_. - -[Illustration: _Natural radioactivity in the atmosphere is shown by this -nuclear-emulsion photograph of alpha-particle tracks (enlarged 2000 -diameters) emitted by a grain of radioactive dust._] - -The intensity of background radiation varies from place to place on the -earth for several reasons. Cosmic rays are deflected somewhat toward the -magnetic poles by the earth’s magnetic field. They are also absorbed by -the atmosphere to some extent. For this reason, people living in -equatorial regions are less exposed to cosmic rays than those in polar -regions; and those in the plains, with a greater thickness of atmosphere -above them, are less exposed than those on high plateaus. - -Then, too, radioactive minerals may be spread widely, but they are not -spread evenly. Where they are concentrated to a greater extent than -usual, background radiation is abnormally high. - -Thus, an inhabitant of Harrisburg, Pennsylvania, may absorb 2.64 rads -per 30 years, while one of Denver, Colorado, a mile high at the foot of -the Rockies, may absorb 5.04 rads per 30 years. Greater extremes are -encountered at such places as Kerala, India, where nearby soil, rich in -thorium minerals, so increases the intensity of background radiation -that as much as 84 rads may be absorbed in 30 years. - -In addition to high-energy radiation from the outside, there are sources -within the body itself. Some of the potassium and carbon atoms of our -body are inevitably radioactive. As much as 0.5 rad per 30 years arises -from this source. - -Rads and roentgens are not completely satisfactory units in estimating -the biological effects of radiation. Some types of radiation—those made -up of comparatively large particles, for instance—are more effective in -producing ions and bring about molecular changes with greater ease than -do electromagnetic radiations delivering equal energy to the body. Thus -if 1 rad of alpha particles is absorbed by the body, 10 to 20 times as -much biological effect is produced as there would be in the absorption -of 1 rad of X rays, gamma rays, or beta particles. - -Sometimes, then, one speaks of the _relative biological effectiveness_ -(RBE) of radiation, or the _roentgen equivalent, man_ (rem). A rad of X -rays, gamma rays, or beta particles has a rem of 1, while a rad of alpha -particles has a rem of 10 to 20. - -If we allow for the effect of the larger particles (which are not very -common under ordinary conditions) we can estimate that the gonads of the -average human being receive a total dose of natural radiation of about 3 -rems per 30 years. This is just about an irreducible minimum. - - -Man-made Radiation - -Man began to add to the background radiation in the 1890s. In 1895, X -rays were discovered and since then have become increasingly useful in -medical diagnosis and therapy and in industry. In 1896, radioactivity -was discovered and radioactive substances were concentrated in -laboratories in order that they might be studied. In 1934, it was found -that radioactive forms of nonradioactive elements (_radioisotopes_) -could be formed and their use came to be widespread in universities, -hospitals, and industries.[4] - -Then, in 1945, the nuclear bomb was developed. With the uranium or -plutonium fission that produces a nuclear explosion, there is an -accompaniment of intense gamma radiation. In addition, a variety of -radioisotopes are left behind in the form of the residue (_fission -fragments_) of the fissioning atoms. These fission fragments are -distributed widely in the atmosphere. Some rise high into the -stratosphere and descend (as _fallout_) over the succeeding months and -years.[5] - -It is hard to try to estimate how much additional radiation is being -absorbed by human beings out of these man-made sources. Fallout is not -uniformly spread over the earth but is higher in those latitudes where -nuclear bombs have been most frequently tested. Then, too, people in -industries and research who are involved with the use of radioisotopes, -and people in medical centers who constantly deal with X rays, are -likely to get more exposure than others. - -These adjuncts of modern science and medicine are more common and -widespread in technologically advanced countries than elsewhere, and -nuclear bombs have most often been exploded in just those latitudes -where the advanced countries are to be found. - -Attempts have been made to work out estimates of this exposure. One -estimate, involving a number of technologically advanced countries -(including the United States) showed that an average of somewhere -between 0.02 and 0.18 rem per year was absorbed, as a result of -radiations (usually X rays) used in medical diagnosis and therapy. -Occupational exposure added, on the average, not more than 0.003 rem, -though the individuals constantly exposed in the course of their work -would naturally absorb considerably more than this overall average. - -[Illustration: _Man-made radioactivity in the atmosphere produced this -nuclear-emulsion photograph. This radiation source is a fission product -produced in a nuclear explosion. The enlargement is 1200 diameters. -Compare this with the natural radioactivity depicted on page 28._] - -On the whole, the highest absorption was found, as was to be expected, -in the United States. - -If these findings are expanded to cover a 30-year period, assuming the -absorption will remain the same from year to year, it turns out that the -average absorption of man-made radiation in the nations studied varies -from 0.6 rem to 5.5 rems per 30 years per individual. - -Considering the higher figure to be applicable to the United States, it -would seem that man-made radiation from all sources is now being -absorbed at nearly twice the rate that natural radiation is. To put it -another way, Americans are just about tripling their radiation dosage by -reason of the human activities that are now adding man-made radiation to -the natural supply. By far the major part of this additional dosage is -the result of the use of X rays in searching for decayed teeth, broken -bones, lung lesions, swallowed objects, and so on. - - - - - DOSE AND CONSEQUENCE - - -Radiation Sickness - -The danger to the individual as a result of overexposure to high-energy -radiation was understood fairly soon but not before some tragic -experiences were recorded. - -One of the early workers with radioactive materials, Pierre Curie, -deliberately exposed a patch of his skin to the action of radioactive -radiations and obtained a serious and slow-healing burn. His wife, Marie -Curie, and their daughter, Irène Joliot-Curie, who spent their lives -working with radioactive materials, both died of leukemia, very possibly -as the result of cumulative exposure to radiation. Other research -workers in the field died of cancer before the full necessity of extreme -caution was understood. - -The damage done to human beings by radiation could first be studied on a -large scale among the survivors of the nuclear bombings of Hiroshima and -Nagasaki in 1945. Here marked symptoms of _radiation sickness_ were -observed. This sickness often leads to death, though a slow recovery is -sometimes possible. - -In general, high-energy radiation damages the complex molecules within a -cell, interfering with its chemical machinery to the point, in extreme -cases, of killing it. (Thus, cancers, which cannot safely be reached -with the surgeon’s knife, are sometimes exposed to high-energy radiation -in the hope that the cancer cells will be effectively killed in that -manner.) - -The delicate structure of the genes and chromosomes is particularly -vulnerable to the impact of high-energy radiation. Chromosomes can be -broken by such radiation and this is the main cause of actual cell -death. A cell that is not killed outright by radiation may nevertheless -be so damaged as to be unable to undergo replication and mitosis. - -If a cell is of a type that will not, in the course of nature, undergo -division, the destruction of the mitosis machinery is not in itself -fatal to the organism. A creature like _Drosophila_, which, in its adult -stage, has very few cell divisions going on among the ordinary cells of -its body, can survive radiation doses a hundred times as great as would -suffice to kill a man. - -In a human being, however—even in an adult who is no longer experiencing -overall growth—there are many tissues whose cells must undergo division -throughout life. Hair and fingernails grow constantly, as a result of -cell division at their roots. The outer layers of skin are steadily lost -through abrasion and are replaced through constant cell division in the -deeper layers. The same is true of the lining of the mouth, throat, -stomach, and intestines. Too, blood cells are continually breaking up -and must be replaced in vast numbers. - -If radiation kills the mechanism of division in only some of these -cells, it is possible that those that remain reasonably intact can -divide and eventually replace or do the work of those that can no longer -divide. In that case, the symptoms of radiation sickness are relatively -mild in the first place and eventually disappear. - -Past a certain critical point, when too many cells are made incapable of -division, this is no longer possible. The symptoms, which show up in the -growing tissues particularly (as in the loss of hair, the misshaping or -loss of fingernails, the reddening and hemorrhaging of skin, the -ulceration of the mouth, and the lowering of the blood cell count), grow -steadily more severe and death follows. - - -Radiation and Mutation - -Where radiation is insufficient to render a cell incapable of division, -it may still induce mutations, and it is in this fashion that skin -cancer, leukemia, and other disorders may be brought about.[6] - -[Illustration: _Studies at the California Institute of Technology -furnish information on the nature of radiation effects on genes. The -experiments produced fruit flies with three or four wings and double or -partially doubled thoraxes by causing gene mutation through -X-irradiation and chromosome rearrangements. A is a normal male_ -Drosophila; _B is a four-winged male with a double thorax; and C and D -are three-winged flies with partial double thoraxes._] - - [Illustration: Four-winged male with a double thorax] - - [Illustration: Three-winged fly with partial double thoraxes] - - [Illustration: Three-winged fly with partial double thoraxes] - -Mutations can be brought about in the sex cells, too, of course, and -when this happens it is succeeding generations that are affected and not -merely the exposed individual. Indeed, where the sex cells are -concerned, the relatively mild effect of mutation is more serious than -the drastic one of nondivision. A fertilized ovum that cannot divide -eventually dies and does no harm; one that can divide but is altered, -may give rise to an individual with one of the usual kinds of major or -minor physical defects. - -The effect of high-energy radiation on the genetic mechanism was first -demonstrated experimentally in 1927 by Muller. Using _Drosophila_ he -showed that after large doses of X rays, flies experienced many more -lethal mutations per chromosome than did similar flies not exposed to -radiation. The drastic differences he observed proved the connection -between radiation and mutation at once. - -Later experiments, by Muller and by others, showed that the number of -mutations was directly proportional to the quantity of radiation -absorbed. Doubling the quantity of radiation absorbed doubled the number -of mutations, tripling the one tripled the other, and so on. This means -that if the number of mutations is plotted against the amount of -radiation absorbed, a straight line can be drawn. - -It is generally believed that the straight line continues all the way -down without deviation to very low radiation absorptions. This means -there is no “threshold” for the mutational effect of radiation. No -matter how small a dosage of radiation the gonads receive, this will be -reflected in a proportionately increased likelihood of mutated sex cells -with effects that will show up in succeeding generations. - -In this respect, the genetic effect of radiation is quite different from -the somatic effect. A small dose of radiation may affect growing tissues -and prevent a small proportion of the cells of those tissues from -dividing. The remaining, unaffected cells take up the slack, however, -and if the proportion of affected cells is small enough, symptoms are -not visible and never become visible. There is thus a threshold effect: -The radiation absorbed must be more than a certain amount before any -somatic symptoms are manifest. - -Matters are quite different where the genetic effect is concerned. If a -sex cell is damaged and if that sex cell is one of the pair that goes -into the production of a fertilized ovum, a damaged organism results. -There is no margin for correction. There is no unaffected cell that can -take over the work of the damaged sex cell once fertilization has taken -place. - -Suppose only one sex cell out of a million is damaged. If so, a damaged -sex cell will, on the average, take part in one out of every million -fertilizations. And when it is used, it will not matter that there are -999,999 perfectly good sex cells that might have been used—it was the -damaged cell that _was_ used. That is why there is no threshold in the -genetic effect of radiation and why there is no “safe” amount of -radiations insofar as genetic effects are concerned. However small the -quantity of radiation absorbed, mankind must be prepared to pay the -price in a corresponding increase of the genetic load. - -[Illustration: Percent lethal chromosomes vs. Amount of x radiation, r] - -If the straight line obtained by plotting mutation rate against -radiation dose is followed down to a radiation dose of zero, it is found -that the line strikes the vertical axis slightly above the origin. The -mutation rate is more than zero even when the radiation dose is zero. -The reason for this is that it is the dose of man-made radiation that is -being considered. Even when man-made radiation is completely absent -there still remains the natural background radiation. - -It is possible in this manner to determine that background radiation -accounts for considerably less than 1% of the spontaneous mutations that -take place. The other mutations must arise out of chemical -misadventures, out of the random heat-jiggling of molecules, and so on. -These, it can be presumed, will remain constant when the radiation dose -is increased. - -This is a hopeful aspect of the situation for it means that, if the -background radiation is doubled or tripled for mankind as a whole, only -that small portion of the spontaneous mutation rate that is due to the -background radiation will be doubled or tripled. - -Let us suppose, for instance, that fully 1% of the spontaneous mutations -occurring in mankind is due to background radiation. In that case, the -tripling of the background radiation produced in the United States by -man-made causes (see Table) would triple that 1%. In place of 99 -non-radiational mutations plus 1 radiational, we would have 99 plus 3. -The total number of mutations would increase from 100 to 102—an increase -of 2%, not an increase of 200% that one would expect if all spontaneous -mutations were caused by background radiation. - - RADIATION EXPOSURES IN THE UNITED STATES[7] - Millirems[8] - - Natural Sources - A. External to the body - 1. From cosmic radiation 50.0 - 2. From the earth 47.0 - 3. From building materials 3.0 - B. Inside the body - 1. Inhalation of air 5.0 - 2. Elements found naturally in human tissues 21.0 - Total, Natural sources 126.0 - Man-made Sources - A. Medical Procedures - 1. Diagnostic X rays 50.0 - 2. Radiotherapy X ray, radioisotopes 10.0 - 3. Internal diagnosis, therapy 1.0 - Subtotal 61.0 - B. Atomic energy industry, laboratories 0.2 - C. Luminous watch dials, television tubes, 2.0 - radioactive industrial wastes, etc. - D. Radioactive fallout 4.0 - Subtotal 6.2 - Total, man-made sources 67.2 - Overall total 193.2 - - -Dosage Rates - -Another difference between the genetic and somatic effects of radiation -rests in the response to changes in the rate at which radiation is -absorbed. It makes a considerable difference to the body whether a large -dose of radiation is absorbed over the space of a few minutes or a few -years. - -When a large dose is absorbed over a short interval of time, so many of -the growing tissues lose the capacity for cell division that death may -follow. If the same dose is delivered over years, only a small bit of -radiation is absorbed on any given day and only small proportions of -growing cells lose the capacity for division at any one time. The -unaffected cells will continually make up for this and will replace the -affected ones. The body is, so to speak, continually repairing the -radiation damage and no serious symptoms will develop. - -Then, too, if a moderate dose is delivered, the body may show visible -symptoms of radiation sickness but can recover. It will then be capable -of withstanding another moderate dose, and so on. - -The situation is quite different with respect to the genetic effects, at -least as far as experiments with _Drosophila_ and bacteria seem to show. -Even the smallest doses will produce a few mutations in the chromosomes -of those cells in the gonads that eventually develop into sex cells. The -affected gonad cells will continue to produce sex cells with those -mutations for the rest of the life of the organism. Every tiny bit of -radiation adds to the number of mutated sex cells being constantly -produced. There is no recovery, because the sex cells, after formation, -do not work in cooperation, and affected cells are not replaced by those -that are unaffected. - -This means (judging by the experiments on lower creatures) that what -counts, where genetic damage is in question, is not the rate at which -radiation is absorbed but the total sum of radiation. Every exposure an -organism experiences, however small, adds its bit of damage. - -Accepting this hard view, it would seem important to make every effort -to minimize radiation exposure for the population generally. - -Since most of the man-made increase in background radiation is the -result of the use of X rays in medical diagnosis and therapy, many -geneticists are looking at this with suspicion and concern. No one -suggests that their use be abandoned, for certainly such techniques are -important in the saving of life and the mitigation of suffering. Still, -X rays ought not to be used lightly, or routinely as a matter of course. - -It might seem that X rays applied to the jaw or the chest would not -affect the gonads, and this might be so if all the X rays could indeed -be confined to the portion of the body at which they are aimed. -Unfortunately, X rays do not uniformly travel a straight line in passing -through matter. They are scattered to a certain extent; if a stream of X -rays passes through the body anywhere, or even through objects near the -body, some X rays will be scattered through the gonads. - -It is for this reason that some geneticists suggest that the history of -exposure to X rays be kept carefully for each person. A decision on a -new exposure would then be determined not only by the current situation -but by the individual’s past history. - -Such considerations were also an important part of the driving force -behind the movement to end atmospheric testing of nuclear bombs. While -the total addition to the background radiation resulting from such tests -is small, the prospect of continued accumulation is unpleasant. - -What’s more, whereas X rays used in diagnosis and therapy have a humane -purpose and chiefly affect the patient who hopes to be helped in the -process, nuclear fallout affects all of humanity without distinction and -seems, to many people, to have as its end only the promise of a totally -destructive nuclear war. - -It is not to be expected that the large majority of humanity that makes -up the populations outside the United States, Great Britain, France, -China, and the Soviet Union can be expected to accept stoically the risk -of even limited quantities of genetic damage, out of any feeling of -loyalty to nations not their own. Even within the populations of the -three major nuclear powers there are strong feelings that the possible -benefits of nuclear testing do not balance the certain dangers. - -Public opinion throughout the world is a key factor, then, in enforcing -the Nuclear Test Ban Treaty, signed by the governments of the United -States, Great Britain, and the Soviet Union on October 10, 1963. - - -Effects on Mammals - -Although genetic findings on such comparatively simple creatures as -fruit flies and bacteria seem to apply generally to all forms of life, -it seems unsafe to rely on these findings completely in anything as -important as possible genetic damage to man through radiation. During -the 1950s and 1960s, therefore, there have been important studies on -mice, particularly by W. L. Russell at Oak Ridge National Laboratory, -Oak Ridge, Tennessee. - -While not as short-lived or as fecund as fruit flies, mice can -nevertheless produce enough young over a reasonable period of time to -yield statistically useful results. Experimenters have worked with -hundreds of thousands of offspring born of mice that have been -irradiated with gamma rays and X rays in different amounts and at -different intensities, as well as with additional hundreds of thousands -born to mice that were not irradiated. - -Since mice, like men, are mammals, results gained by such experiments -are particularly significant. Mice are far closer to man in the scheme -of life than is any other creature that has been studied genetically on -a large scale, and their reactions (one might cautiously assume) are -likely to be closer to those that would be found in man. - -Almost at once, when the studies began, it turned out that mice were -more susceptible to genetic damage than fruit flies were. The induced -mutation rate per gene seems to be about fifteen times that found in -_Drosophila_ for comparable X ray doses. The only safe course for -mankind then is to err, if it must, strongly on the side of -conservatism. Once we have decided what might be safe on the basis of -_Drosophila_ studies, we ought then to tighten precautions several -notches by remembering that we are very likely more vulnerable than -fruit flies are. - -Counteracting the depressing nature of this finding was that of a later, -quite unexpected discovery. It was well established that in fruit flies -and other simple organisms, it was the total dosage of absorbed -radiation that counted and that whether this was delivered quickly or -slowly did not matter. - -[Illustration: _Arrangement for long-term low-dose-rate irradiation of -mice used for mutation-rate studies at Oak Ridge National Laboratory. -The cages are arranged at equal distances from a cesium-137 gamma-ray -source in the lead pot on the floor. The horizontal rod rotates the -source._] - -This proved to be _not_ so in the case of mice. In male mice, a -radiation dose delivered at the rate of 0.009 rad per minute produced -only from one-quarter to one-third as many mutations as did the same -total dose delivered at 90 rads per minute. - -In the male, cells in the gonads are constantly dividing to produce sex -cells. The latter are produced by the billions. It might be, then, that -at low radiation dose rates, a few of the gonad cells are damaged but -that the undamaged ones produce a flood of sperm cells, “drowning out” -the few produced by the damaged gonad cells. The same radiation dose -delivered in a short time might, however, damage so many of the gonad -cells as to make the damaged sex cells much more difficult to “flood -out”. - -A second possible explanation is that there is present within the cells -themselves some process that tends to repair damage to the genes and to -counteract mutations. It might be a slow-working, laborious process that -could keep up with the damage inflicted at low dosage rates but not at -high ones. High dosage rates might even damage the repair mechanism -itself. That, too, would account for the fewer mutations at low dosage -rates than at high ones. - -To check which of the two possible explanations was nearer the truth, -Russell performed similar tests on female mice. In the female mouse (or -the female human being, for that matter) the egg cells have completed -almost all their divisions before the female is born. There are only so -many cells in the female gonads that can give rise to egg cells, and -each one gives rise to only a single egg cell. There is no possibility -of damaged egg cells being drowned out by floods of undamaged ones -because there are no floods. - -Yet it was found that in the female mouse the mutation rate also dropped -when the radiation dose rate was decreased. In fact, it dropped even -more drastically than was the case in the male mouse. - -Apparently, then, there must be actual repair within the cell. There -must be some chemical mechanism inside the cell capable of counteracting -radiation damage to some extent. In the female mouse, the mutation rate -drops very low as the radiation dose rate drops, so that it would seem -that almost all mutations might be repaired, given enough time. In the -male, the mutation rate drops only so far and no farther, so that some -mutations (about one-third is the best estimate so far) cannot be -repaired. - -If this is also true in the human being (and it is at least reasonably -likely that it is), then the greater vulnerability of our genes as -compared with those of fruit flies is at least partially made up for by -our greater ability to repair the damage. - -This opens a door for the future, too. The workings of the gene-repair -mechanism ought (it is to be hoped) eventually to be puzzled out. When -it is, methods may be discovered for reinforcing that mechanism, -speeding it, and increasing its effectiveness. We may then find -ourselves no longer completely helpless in the face of genetic damage, -or even of radiation sickness. - -On the other hand, it is only fair to point out that the foregoing -appraisal may be an over-optimistic view. Russell’s experiments involved -just 7 genes and it is possible that these are not representative of the -thousands that exist altogether. While the work done so far is most -suggestive and interesting, much research remains to be carried out. - -If, then, we cannot help hoping that natural devices for counteracting -radiation damage may be developed in the future, we must, for the -present, remain rigidly cautious. - - -Conclusion - -It is unrealistic to suppose that all sources of man-made radiation -should be abolished. The good they do now, the greater good they will do -in the future, cannot be abandoned. It is, however, reasonable to expect -that the present Nuclear Test Ban Treaty will continue and that nations, -such as France and China, which have nuclear capabilities but are not -signatories of the Treaty will eventually sign. It is also reasonable to -expect that X ray diagnosis and therapy will be carried on with the -greatest circumspection, and that the use of radiation in industry and -research will be carried on with great care and with the use of ample -shielding. - -[Illustration: _A film badge (left) and a personal radiation monitor -(right) record the amount of radiation absorbed by the wearer. These -safety devices, worn by persons working in radiation environments, are -designed to keep a constant check on each individual’s absorbed dose and -to prevent overexposure._] - -As long as man-made radiation exists, there will be some absorption of -it by human beings. The advantages of its use in our modern society are -such that we must be prepared to pay some price. This is not a matter of -callousness. We have come to depend a great deal for comfort and even -for extended life, upon the achievements of our technology, and any -serious crippling of that technology will cost us lives. An attempt must -be made to balance the values of radiation against its dangers; we must -balance lives against lives. This involves hard judgments. - -Those working under conditions of greatest radiation risk—in atomic -research, in industrial plants using isotopes, and so on—can be allowed -to set relatively high limits for total radiation dosages and dose rates -that they may absorb (with time) with reasonable safety, but such rates -will never do for the population generally. A relative few can -voluntarily endure risks, both somatic and genetic, that we cannot -sanely expect of mankind as a whole.[9] - -From fruit fly experiments it would seem that a total exposure of 30 to -100 rads of radiation will double the spontaneous mutation rate. So much -radiation and such a doubling of the rate would be considered -intolerable for humanity. - -Some geneticists have recommended that the average total exposure of -human beings in the first 30 years of life be set at 10 rads. Note that -this figure is set as a _maximum_. Every reasonable method, it is -expected, will be used to allow mankind to fall as far short of this -figure as possible. Note also that the 10-rad figure is an _average_ -maximum. The exposure of some individuals to a greater total dose would -be viewed as tolerable for society if it were balanced by the exposure -of other individuals to a lesser total dose. - -A total exposure of 10 rads might increase the overall mutation rate, it -is roughly estimated, by 10%. This is serious enough, but is bearable if -we can convince ourselves that the alternative of abandoning radiation -technology altogether will cause still greater suffering. - -A 10% increase in mutation rate, whatever it might mean in personal -suffering and public expense, is not likely to threaten the human race -with extinction, or even with serious degeneration. - -The human race as a whole may be thought of as somewhat analogous to a -population of dividing cells in a growing tissue. Those affected by -genetic damage drop out and the slack is taken up by those not affected. - -If the number of those affected is increased, there would come a crucial -point, or threshold, where the slack could no longer be taken up. The -genetic load might increase to the point where the species as a whole -would degenerate and fade toward extinction—a sort of “racial radiation -sickness”. - -We are not near this threshold now, however, and can, therefore, as a -species, absorb a moderate increase in mutation rate without danger of -extinction. - -On the other hand, it is _not_ correct to argue, as some do, that an -increase in mutation rate might be actually beneficial. The argument -runs that a higher mutation rate might broaden the gene pool and make it -more flexible, thus speeding up the course of evolution and hastening -the advent of “supermen”—brainier, stronger, healthier than we ourselves -are. - -The truth seems to be that the gene pool, as it exists now, supplies us -with all the variability we need for the effective working of the -evolutionary mechanism. That mechanism is functioning with such -efficiency that broadening the gene pool cannot very well add to it, and -if the hope of increased evolutionary efficiency were the only reason to -tolerate man-made radiation, it would be insufficient. - -The situation is rather analogous to that of a man who owns a good house -that is heavily mortgaged. If he were offered a second house with a -similar mortgage, he would have to refuse. To be sure, he would have -twice the number of houses, but he would not need a second house since -he has all the comfort he can reasonably use in his first house—and he -would not be able to afford a second mortgage. - -What humanity must do, if additional radiation damage is absolutely -necessary, is to take on as little of that added damage as possible, and -not pretend that any direct benefits will be involved. Any pretense of -that sort may well lure us into assuming still greater damage—damage we -may not be able to afford under any circumstances and for any reason. - -Actually, as the situation appears right now, it is not likely that the -use of radiation in modern medicine, research, and industry will -overstep the maximum bounds set by scientists who have weighed the -problem carefully. Only nuclear warfare is likely to do so, and -apparently those governments with large capacities in this direction are -thoroughly aware of the danger and (so far, at least) have guided their -foreign policies accordingly. - - - - - SUGGESTED REFERENCES - - -Books - -_Radiation, Genes, and Man_, Bruce Wallace and Theodosius Dobzhansky, - Holt, Rinehart and Winston, Inc., New York 10017, 1963, 205 pp., $5.00 - (hardback); $1.28 (paperback). - -_Genetics in the Atomic Age_ (second edition), Charlotte Auerbach, - Oxford University Press, Inc., Fair Lawn, New Jersey 07410, 1965, 111 - pp., $2.50. - -_Atomic Radiation and Life_ (revised edition), Peter Alexander, Penguin - Books, Inc., Baltimore, Maryland 21211, 1966, 288 pp., $1.65. - -_The Genetic Code_, Isaac Asimov, Grossman Publishers, Inc., The Orion - Press, New York 10003, 1963, 187 pp., $3.95 (hardback); $0.60 - (paperback) from the New American Library of World Literature, Inc., - New York 10022. - -_Radiation: What It Is and How It Affects You._ Ralph E. Lapp and Jack - Schubert, The Viking Press, New York 10022, 1957, 314 pp., $4.50 - (hardback); $1.45 (paperback). - -_Report of the United Nations Scientific Committee on the Effects of - Atomic Radiation_, General Assembly, 19th Session, Supplement No. 14 - (A/5814), United Nations, International Documents Service, Columbia - University Press, New York 10027, 1964, 120 pp., $1.50. - -_The Effects of Nuclear Weapons_, Samuel Glasstone (Ed.), U. S. Atomic - Energy Commission, 1962, 730 pp., $3.00. Available from the - Superintendent of Documents, U. S. Government Printing Office, - Washington, D. C. 20402. - -_Effect of Radiation on Human Heredity_, World Health Organization, - International Documents Service, Columbia University Press, New York - 10027, 1957, 168 pp., $4.00. - -_The Nature of Radioactive Fallout and Its Effects on Man_, Hearings - before the Special Subcommittee on Radiation of the Joint Committee on - Atomic Energy, Congress of the United States, 85th Congress, 1st - Session, U. S. Government Printing Office, 1957, Volume I, 1008 pp., - $3.75; Volume II, 1057 pp., $3.50. Available from the Office of the - Joint Committee on Atomic Energy, Congress of the United States, - Senate Post Office, Washington, D. C. 20510. - -_Genetics, Radiobiology, and Radiology_, Proceedings of the Midwestern - Conference, Wendell G. Scott and Evans Titus, Charles C. Thomas - Publisher, Springfield, Illinois 62703, 1959, 166 pp., $5.50. - - -Articles - -Genetic Hazards of Nuclear Radiations, Bentley Glass, _Science_, 126: - 241 (August 9, 1957). - -Genetic Loads in Natural Populations, Theodosius Dobzhansky, _Science_, - 126: 191 (August 2, 1957). - -Radiation Dose Rate and Mutation Frequency, W. L. Russell and others, - _Science_, 128: 1546 (December 19, 1958). - -Ionizing Radiation and the Living Cell, Alexander Hollaender and George - E. Stapleton, _Scientific American_, 201: 95 (September 1959). - -Radiation and Human Mutation, H. J. Muller, _Scientific American_, 193: - 58 (November 1955). - -Ionizing Radiation and Evolution, James F. Crow, _Scientific American_, - 201: 138 (September 1959). - - -Motion Pictures - -_Radiation and the Population_, 29 minutes, sound, black and white, - 1962. Produced by the Argonne National Laboratory. This film explains - how radiation causes mutations and how these mutations are passed on - to succeeding generations. Mutation research is illustrated with - results of experimentation on generations of mice. A discussion of - work with fruit flies and induced mutations is also included. This - film is available for loan without charge from the AEC Headquarters - Film Library, Division of Public Information, U. S. Atomic Energy - Commission, Washington, D. C. 20545 and from other AEC film libraries. - -The following films were produced by the American Institute of - Biological Sciences and may be rented from the Text-Film Division, - McGraw-Hill Book Company, 330 West 42nd Street, New York 10036. - -_Mutation_, 28 minutes, sound, color, 1962. This film discusses - chromosomal and genetic mutations as applied to man. Muller’s work in - inducing mutations by X rays is described. - -These three films are 30 minutes long, have sound, are in black and - white, and were released in 1960. They are part of a 48-film series - that is correlated with the textbook, _Principles of Genetics_, (fifth - edition), Edmund W. Sinnott, L. C. Dunn, and Theodosius Dobzhansky, - McGraw-Hill Book Company, 1958, 459 pp., $8.50. - -_Mutagen-Induced Gene Mutation._ The narrator of this film is Hermann J. - Muller, who won a Nobel Prize in 1946 for his work in the field of - genetics. The measurement of X-ray dose in roentgens and the dose - required to double the spontaneous mutation rate in _Drosophila_ and - mice are discussed. The magnitude and meaning of permissible doses of - high-energy radiation are discussed. Other mutagenic agents - (ultraviolet light and chemical substances) are discussed, concluding - with comments on the importance of gene mutation in the present and - future. - -_Selection, Genetic Death and Genetic Radiation Damage._ The narrator of - this film is Theodosius Dobzhansky, the coauthor of this booklet. - Genetic death is discussed in detail, as are examples of how genetic - loads are changed subsequent to radiation exposure. While it is - generally agreed that the great majority of mutants are harmful when - homozygous, more evidence is needed about the beneficial and - detrimental effects of mutants when heterozygous. In the case of - sickle cell anemia, heterozygotes are adaptively superior to normal - homozygotes. This makes for balanced polymorphism, by which a gene is - retained in the population despite its lethality when homozygous - because of the advantage it confers when heterozygous. - -_Gene Structure and Gene Action._ The lecturer of this film is G. W. - Beadle of Cornell University. The Watson-Crick structure of DNA is - discussed in terms of mutation. Several tests of the chain separation - hypothesis for DNA replication are described (experiments with heavy - DNA, radioactive chromosomes, and the replication of DNA in vitro). - This working hypothesis is presented: The coded information in DNA is - transferred to RNA, which serves as a template for polypeptide - synthesis. - - PHOTO CREDITS - - Dr. Asimov’s photograph by David R. Phillips, courtesy _Chemical and - Engineering News_ - - Page - - 4 James German, M.D. - 6 Bausch & Lomb, Inc. - 12 James German, M.D. - 20 Indiana University - 24 Robert C. Filz, Air Force Cambridge Research Laboratories - 25 J. K. Boggild, Niels Bohr Institute, Copenhagen University - 26 Brookhaven National University - 28, 31 Herman Yagoda, Air Force Cambridge Research Laboratories - 41 Oak Ridge National Laboratory - - - - - Footnotes - - -[1]For more detail about cell division, see _Radioisotopes and Life - Processes_, another booklet in this series. - -[2]This is more commonly known as “Mongolism” or “Mongolian idiocy” - though it has nothing to do with the Mongolian people. - -[3]Actually, all waves have some of the characteristics of particles and - all particles have some of the characteristics of waves. Usually, - however, the radiation is predominantly one or the other and little - confusion arises under ordinary circumstances in speaking of waves - and particles as though they were separate phenomena. - -[4]For more about this subject, see _Radioisotopes in Industry_ and - _Radioisotopes in Medicine_, companion booklets in this series. - -[5]For more about this subject, see _Fallout from Nuclear Tests_, - another booklet in this series. - -[6]For details on _somatic_ effects of radiation, see _Your Body and - Radiation_, a companion booklet in this series. - -[7]Estimated average exposures to the gonads, based on 1963 report of - Federal Radiation Council. - -[8]One thousandth of a rem. - -[9]Nevertheless, it should be pointed out that the precautions taken in - the atomic energy industry are such that absorption of radiation is - not as severe a problem as one might suspect. Fully 95% of those - engaged in this work receive less than 1 rem a year. Only 1% receive - more than 5 rems. - - -UNITED STATES ATOMIC ENERGY COMMISSION - - _Dr. Glenn T. Seaborg, Chairman_ - _James T. Ramey_ - _Dr. Gerald F. Tape_ - _Dr. Samuel M. Nabrit_ - _Wilfrid E. Johnson_ - -_ONE OF A SERIES ON -UNDERSTANDING THE ATOM_ - -Nuclear energy is playing a vital role in the life of every man, woman, -and child in the United States today. In the years ahead it will affect -increasingly all the peoples of the earth. It is essential that all -Americans gain an understanding of this vital force if they are to -discharge thoughtfully their responsibilities as citizens and if they -are to realize fully the myriad benefits that nuclear energy offers -them. - -The United States Atomic Energy Commission provides this booklet to help -you achieve such understanding. - - [Illustration: Edward J. Brunenkant] - - Edward J. Brunenkant - Director - Division of Technical Information - - -This booklet is one of the “Understanding the Atom” Series. Comments are -invited on this booklet and others in the series; please send them to -the Division of Technical Information, U. S. Atomic Energy Commission, -Washington, D. C. 20545. - -Published as part of the AEC’s educational assistance program, the -series includes these titles: - - NUCLEAR POWER AND MERCHANT SHIPPING - PLUTONIUM - OUR ATOMIC WORLD - NUCLEAR ENERGY FOR DESALTING - CONTROLLED NUCLEAR FUSION - WHOLE BODY COUNTERS - PLOWSHARE - POPULAR BOOKS ON NUCLEAR SCIENCE - SNAP, NUCLEAR SPACE REACTORS - NUCLEAR REACTORS - ATOMS, NATURE, AND MAN - MICROSTRUCTURE OF MATTER - SYNTHETIC TRANSURANIUM ELEMENTS - COMPUTERS - RESEARCH REACTORS - GENETIC EFFECTS OF RADIATION - POWER FROM RADIOISOTOPES - NONDESTRUCTIVE TESTING - RARE EARTHS - FOOD PRESERVATION BY IRRADIATION - FALLOUT FROM NUCLEAR TESTS - RADIOACTIVE WASTES - RADIOISOTOPES IN INDUSTRY - ATOMS AT THE SCIENCE FAIR - RADIOISOTOPES AND LIFE PROCESSES - ATOMIC FUEL - ATOMIC POWER SAFETY - DIRECT CONVERSION OF ENERGY - CAREERS IN ATOMIC ENERGY - RADIOISOTOPES IN MEDICINE - ACCELERATORS - NUCLEAR TERMS, A BRIEF GLOSSARY - NEUTRON ACTIVATION ANALYSIS - ATOMS IN AGRICULTURE - POWER REACTORS IN SMALL PACKAGES - -Single copies of any booklet may be obtained free by writing to: - - USAEC, P. O. BOX 62, OAK RIDGE, TENNESSEE 37830 - -Requests for more than three titles generally can not be honored. - -Complete sets of the series are available to school and public -librarians, and to teachers who can make them available for reference or -for use by groups. Requests should be made on school or library -letterheads and indicate the proposed use. - -Students and teachers who need publications on specific topics related -to nuclear science, or references to other reading material, may also -write to the Oak Ridge address. Requests should state the topic of -interest exactly, and the use intended. - -_IMPORTANT_: All requests should include the “Zip Code” in the address -to which the material is to be mailed. - - - Printed in the United States of America - - -USAEC Division of Technical Information Extension, Oak Ridge, Tennessee - September 1966 - - - - - Transcriber’s Notes - - ---Retained publication information from the printed edition: this eBook - is public-domain in the country of publication. - ---Where possible, UTF superscript and subscript numbers are used; some - e-reader fonts may not support these characters. - ---In the text version only, underlined or italicized text is delimited - by _underscores_. - ---In the text version only, superscript text is preceded by caret and - delimited by ^{brackets}. - ---In the text version only, subscripted text is preceded by underscore - and delimited by _{brackets}. - ---In the text version only, added a brief label to each illustration; - and for graphs, provided tabular summaries of the data where possible. - - - - - - - -End of the Project Gutenberg EBook of The Genetic Effects of Radiation, by -Isaac Asimov and Theodosius Dobzhansky - -*** END OF THIS PROJECT GUTENBERG EBOOK THE GENETIC EFFECTS OF RADIATION *** - -***** This file should be named 55738-0.txt or 55738-0.zip ***** -This and all associated files of various formats will be found in: - http://www.gutenberg.org/5/5/7/3/55738/ - -Produced by Stephen Hutcheson and the Online Distributed -Proofreading Team at http://www.pgdp.net - -Updated editions will replace the previous one--the old editions will -be renamed. - -Creating the works from print editions not protected by U.S. copyright -law means that no one owns a United States copyright in these works, -so the Foundation (and you!) can copy and distribute it in the United -States without permission and without paying copyright -royalties. 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} -.author { text-align:right; margin-top:0em; margin-bottom:0em; display:block; } - -dl.biblio dt { margin-top:.6em; margin-left:2em; text-indent:-2em; text-align:justify; clear:both; } -dl.biblio dt div { display:block; float:left; margin-left:-6em; width:6em; clear:both; } -dl.biblio dt.center { margin-left:0em; text-align:center; } -dl.biblio dd { margin-top:.3em; margin-left:3em; text-align:justify; font-size:90%; } -.clear { clear:both; } -p.book { margin-left:2em; text-indent:-2em; } -p.review { margin-left:2em; text-indent:-2em; } - -span.col {width:6em; }</style> -</head> -<body> - - -<pre> - -The Project Gutenberg EBook of The Genetic Effects of Radiation, by -Isaac Asimov and Theodosius Dobzhansky - -This eBook is for the use of anyone anywhere in the United States and most -other parts of the world at no cost and with almost no restrictions -whatsoever. You may copy it, give it away or re-use it under the terms of -the Project Gutenberg License included with this eBook or online at -www.gutenberg.org. If you are not located in the United States, you'll have -to check the laws of the country where you are located before using this ebook. - -Title: The Genetic Effects of Radiation - -Author: Isaac Asimov - Theodosius Dobzhansky - -Release Date: October 13, 2017 [EBook #55738] - -Language: English - -Character set encoding: UTF-8 - -*** START OF THIS PROJECT GUTENBERG EBOOK THE GENETIC EFFECTS OF RADIATION *** - - - - -Produced by Stephen Hutcheson and the Online Distributed -Proofreading Team at http://www.pgdp.net - - - - - - -</pre> - -<div id="cover" class="img"> -<img id="coverpage" src="images/cover.jpg" alt="The Genetic Effects of Radiation" width="500" height="768" /> -</div> -<h1>The Genetic Effects of Radiation</h1> -<p class="jr1">By ISAAC ASIMOV and THEODOSIUS DOBZHANSKY</p> -<h2>Contents</h2> -<dl class="toc"> -<dt><a href="#c1">THE MACHINERY OF INHERITANCE</a> 1</dt> -<dd><a href="#c2">Introduction</a> 1</dd> -<dd><a href="#c3">Cells and Chromosomes</a> 2</dd> -<dd><a href="#c4">Enzymes and Genes</a> 5</dd> -<dd><a href="#c5">Parents and Offspring</a> 8</dd> -<dt><a href="#c6">MUTATIONS</a> 10</dt> -<dd><a href="#c7">Sudden Change</a> 10</dd> -<dd><a href="#c8">Spontaneous Mutations</a> 13</dd> -<dd><a href="#c9">Genetic Load</a> 16</dd> -<dd><a href="#c10">Mutation Rates</a> 19</dd> -<dt><a href="#c11">RADIATION</a> 22</dt> -<dd><a href="#c12">Ionizing Radiation</a> 22</dd> -<dd><a href="#c13">Background Radiation</a> 27</dd> -<dd><a href="#c14">Man-made Radiation</a> 30</dd> -<dt><a href="#c15">DOSE AND CONSEQUENCE</a> 32</dt> -<dd><a href="#c16">Radiation Sickness</a> 32</dd> -<dd><a href="#c17">Radiation and Mutation</a> 33</dd> -<dd><a href="#c18">Dosage Rates</a> 37</dd> -<dd><a href="#c19">Effects on Mammals</a> 40</dd> -<dd><a href="#c20">Conclusion</a> 43</dd> -<dt><a href="#c21">SUGGESTED REFERENCES</a> 47</dt> -</dl> -<div class="pb" id="Page_002">002</div> -<h4>THE COVER</h4> -<div class="img" id="pic_1"> -<img src="images/p02.jpg" alt="" width="324" height="499" /> -<p class="caption small">The cover design embodies a radiation -symbol, a stylized karyotype of human -chromosomes, and a genealogical table.</p> -</div> -<h4>THE AUTHORS</h4> -<div class="img" id="pic_2"> -<img src="images/p02a.jpg" alt="" width="388" height="499" /> -<p class="caption small"><span class="ss">ISAAC ASIMOV</span> received his academic degrees -from Columbia University and is Associate -Professor of Biochemistry at the Boston -University School of Medicine. He is a prolific -author who has written over 65 books in the -past 15 years, including about 20 science -fiction works, and books for children. His -many excellent science books for the public -cover subjects in mathematics, physics, astronomy, -chemistry, and biology, such as <i>The Genetic Code</i>, <i>Inside -the Atom</i>, <i>Building Blocks of the Universe</i>, <i>The Living River</i>, <i>The -New Intelligent Man’s Guide to Science</i>, and <i>Asimov’s Biographical -Encyclopedia of Science and Technology</i>. In 1965 Dr. Asimov received -the James T. Grady Award of the American Chemical -Society for his major contribution in reporting science progress -to the public.</p> -</div> -<div class="img" id="pic_3"> -<img src="images/p02b.jpg" alt="" width="391" height="500" /> -<p class="caption small"><span class="ss">THEODOSIUS DOBZHANSKY</span> was graduated -from Kiev University and is now a professor -at the Rockefeller University. He has done -research in genetics and biological evolution -on every continent except Antarctica. Among -his distinguished published works are <i>Radiation, -Genes, and Man</i>, <i>Heredity and the Nature -of Man</i>, <i>Mankind Evolving</i>, and <i>Evolution, Genetics, -and Man</i>. Mr. Dobzhansky received the Daniel G. Elliot -Prize and Medal and the Kimber Genetics Award from the National -Academy of Sciences in 1958, and the National Medal of Science -awarded by the President of the United States, in 1965.</p> -</div> -<div class="pb" id="Page_1">1</div> -<h1 title="">The Genetic Effects of Radiation</h1> -<h2 id="c1">THE MACHINERY OF INHERITANCE</h2> -<h3 id="c2">Introduction</h3> -<p>There is nothing new under the sun, says the Bible. Nor -is the sun itself new, we might add. As long as life has -existed on earth, it has been exposed to radiation from the -sun, so that life and radiation are old acquaintances and -have learned to live together.</p> -<p>We are accustomed to looking upon sunlight as something -good, useful, and desirable, and certainly we could not -live long without it. The energy of sunlight warms the -earth, produces the winds that tend to equalize earth’s -temperatures, evaporates the oceans and produces rain -and fresh water. Most important of all, it supplies what is -needed for green plants to convert carbon dioxide and -water into food and oxygen, making it possible for all -animal life (including ourselves) to live.</p> -<p>Yet sunlight has its dangers, too. Lizards avoid the -direct rays of the noonday sun on the desert, and we ourselves -take precautions against sunburn and sunstroke.</p> -<p>The same division into good and bad is to be found in -connection with other forms of radiation—forms of which -mankind has only recently become aware. Such radiations, -produced by radioactivity in the soil and reaching us from -outer space, have also been with us from the beginning of -<span class="pb" id="Page_2">2</span> -time. They are more energetic than sunlight, however, and -can do more damage, and because our senses do not detect -them, we have not learned to take precautions against -them.</p> -<p>To be sure, energetic radiation is present in nature in -only very small amounts and is not, therefore, much of a -danger. Man, however, has the capacity of imitating nature. -Long ago in dim prehistory, for instance, he learned to -manufacture a kind of sunlight by setting wood and other -fuels on fire. This involved a new kind of good and bad. -A whole new technology became possible, on the one hand, -and, on the other, the chance of death by burning was also -possible. The good in this case far outweighs the evil.</p> -<p>In our own twentieth century, mankind learned to produce -energetic radiation in concentrations far surpassing those -we usually encounter in nature. Again, a new technology is -resulting and again there is the possibility of death.</p> -<p>The balance in this second instance is less certainly in -favor of the good over the evil. To shift the balance clearly -in favor of the good, it is necessary for mankind to learn -as much as possible about the new dangers in order that -we might minimize them and most effectively guard against -them.</p> -<p>To see the nature of the danger, let us begin by considering -living tissue itself—the living tissue that must withstand -the radiation and that can be damaged by it.</p> -<h3 id="c3">Cells and Chromosomes</h3> -<p>The average human adult consists of about 50 trillion -<i>cells</i>—50 trillion microscopic, more or less self-contained, -blobs of life. He begins life, however, as a single -cell, the <i>fertilized ovum</i>.</p> -<p>After the fertilized ovum is formed, it divides and -becomes two cells. Each daughter cell divides to produce -a total of four cells, and each of those divides and so on.</p> -<p>There is a high degree of order and direction to those -divisions. When a human fertilized ovum completes its -divisions an adult human being is the inevitable result. -The fertilized ovum of a giraffe will produce a giraffe, -that of a fruit fly will produce a fruit fly, and so on. There -<span class="pb" id="Page_3">3</span> -are no mistakes, so it is quite clear that the fertilized -ovum must carry “instructions” that guide its development -in the appropriate direction.</p> -<p>These “instructions” are contained in the cell’s <i>chromosomes</i>, -tiny structures that appear most clearly (like -stubby bits of tangled spaghetti) when the cell is in the -actual process of division. Each species has some characteristic -number of chromosomes in its cells, and these -chromosomes can be considered in pairs. Human cells, -for instance, contain 23 pairs of chromosomes—46 in all.</p> -<p>When a cell is undergoing division (<i>mitosis</i>), the number -of chromosomes is temporarily doubled, as each chromosome -brings about the formation of a replica of itself. -(This process is called <i>replication</i>.) As the cell divides, -the chromosomes are evenly shared by the new cells in -such a way that if a particular chromosome goes into one -daughter cell, its replica goes into the other. In the end, -each cell has a complete set of pairs of chromosomes; -and the set in each cell is identical with the set in the -original cell before division.</p> -<div class="img" id="pic_4"> -<img src="images/p03.jpg" alt="" width="600" height="645" /> -<p class="caption small">Mitosis</p> -</div> -<dl class="pcap"><dt>Interphase</dt> -<dt>Prophase</dt> -<dt>Metaphase</dt> -<dt>Anaphase</dt> -<dt>Telophase</dt> -<dt>Interphase</dt></dl> -<div class="pb" id="Page_4">4</div> -<div class="img" id="pic_5"> -<img src="images/p04.jpg" alt="" width="500" height="494" /> -<p class="caption small"><i>To study chromosomes, scientists begin with a cell that is in the -process of dividing, when chromosomes are in their most visible -form. Then they treat the cell with a chemical, a derivative of -colchicine, to arrest the cell division at the metaphase stage (see -<a href="#pic_4">mitosis diagram</a> on preceding page). This brings a result like the -photomicrograph above; the chromosomes are visible but -still too tangled to be counted or measured. Then the cell is -treated with a low-concentration salt solution, which swells the -chromosomes and disperses them so they become distinct structures, -as below.</i></p> -</div> -<div class="img" id="pic_6"> -<img src="images/p04a.jpg" alt="Cell after treatment with salt solution" width="504" height="500" /> -</div> -<div class="img" id="pic_7"> -<img src="images/p04b.jpg" alt="" width="600" height="403" /> -<p class="caption small"><i>The separate chromosomes in a dividing cell are photographed and -then can be identified by their overall length, the position of the -centromere, or point where the two strands join, and other characteristics. -The photomicrograph can then be cut apart and the -chromosomes grouped in a karyotype, which is an arrangement -according to a standard classification to show chromosome complement -and abnormalities. The karotype below is of a normal -male, since it shows X and Y sex chromosomes and 22 pairs of -other, autosomal, chromosomes. By contrast, the cells in the -upper pictures are abnormal, with only 45 chromosomes each.</i></p> -</div> -<div class="pb" id="Page_5">5</div> -<p>In this way, the fundamental “instructions” that determine -the characteristics of a cell are passed on to each -new cell. Ideally, all the trillions of cells in a particular -human being have identical sets of “instructions”.<a class="fn" id="fr_1" href="#fn_1">[1]</a></p> -<h3 id="c4">Enzymes and Genes</h3> -<p>Each cell is a tiny chemical factory in which several -thousand different kinds of chemical changes are constantly -taking place among the numerous sorts of molecules that -move about in its fluid or that are pinned to its solid structures. -These chemical changes are guided and controlled -by the existence of as many thousands of different <i>enzymes</i> -within the cell.</p> -<p>Enzymes possess large molecules built up of some 20 -different, but chemically related, units called <i>amino acids</i>. -A particular enzyme molecule may contain a single amino -acid of one type, five of another, several dozen of still -another and so on. All the units are strung together in -some specific pattern in one long chain, or in a small -number of closely connected chains.</p> -<p>Every different pattern of amino acids forms a molecule -with its own set of properties, and there are an enormous -number of patterns possible. In an enzyme molecule made -up of 500 amino acids, the number of possible patterns -can be expressed by a 1 followed by 1100 zeroes (10¹¹⁰⁰).</p> -<p>Every cell has the capacity of choosing among this -unimaginable number of possible patterns and selecting -those characteristic of itself. It therefore ends with a -complement of specific enzymes that guide its own chemical -changes and, consequently, its properties and its -behavior. The “instructions” that enable a fertilized ovum -to develop in the proper manner are essentially “instructions” -for choosing a particular set of enzyme patterns -out of all those possible.</p> -<div class="pb" id="Page_6">6</div> -<p>The differences in the enzyme-guided behavior of the -cells making up different species show themselves in differences -in body structure. We cannot completely follow -the long and intricate chain of cause-and-effect that leads -from one set of enzymes to the long neck of a giraffe and -from another set of enzymes to the large brain of a man, -but we are sure that the chain is there. Even within a -species, different individuals will have slight distinctions -among their sets of enzymes and this accounts for the fact -that no two human beings are exactly alike (leaving identical -twins out of consideration).</p> -<p>Each chromosome can be considered as being composed -of small sections called <i>genes</i>, usually pictured as being -strung along the length of the chromosome. Each gene is -considered to be responsible for the formation of a chain -of amino acids in a fixed pattern. The formation is guided -by the details of the gene’s own structure (which are the -“instructions” earlier referred to). This gene structure, -which can be translated into an enzyme’s structure, is -now called the <i>genetic code</i>.</p> -<div class="img" id="pic_8"> -<img src="images/p05.jpg" alt="" width="800" height="568" /> -<p class="caption small"><i>Stained section of one cell from salivary gland of</i> Drosophila, <i>or -fruit flies, reveals dark bands that may be genes controlling specific -traits</i>.</p> -</div> -<div class="pb" id="Page_7">7</div> -<p>If a particular enzyme (or group of enzymes) is, for any -reason, formed imperfectly or not at all, this may show up -as some visible abnormality of the body—an inability to -see color, for instance, or the possession of two joints in -each finger rather than three. It is much easier to observe -physical differences than some delicate change in the -enzyme pattern of the cells. Genes are therefore usually -referred to by the body change they bring about, and one -can, for instance, speak of a “gene for color blindness”.</p> -<p>A gene may exist in two or more varieties, each producing -a slightly different enzyme, a situation that is reflected, -in turn, in slight changes in body characteristics. -Thus, there are genes governing eye color, one of which is -sufficiently important to be considered a “gene for blue -eyes” and another a “gene for brown eyes”. One or the -other, but not both, will be found in a specific place on a -specific chromosome.</p> -<p>The two chromosomes of a particular pair govern -identical sets of characteristics. Both, for instance, will -have a place for genes governing eye color. If we consider -only the most important of the varieties involved, -those on each chromosome of the pair may be identical; -both may be for blue eyes or both may be for brown eyes. -In that case, the individual is <i>homozygous</i> for that characteristic -and may be referred to as a <i>homozygote</i>. The -chromosomes of the pair may carry different varieties: -A gene for blue eyes on one chromosome and one for -brown eyes on the other. The individual is then <i>heterozygous</i> -for that characteristic and may be referred to as a -<i>heterozygote</i>. Naturally, particular individuals may be -homozygous for some types of characteristics and heterozygous -for others.</p> -<p>When an individual is heterozygous for a particular -characteristic, it frequently happens that he shows the -effect associated with only one of the gene varieties. If -he possesses both a gene for brown eyes and one for blue -eyes, his eyes are just as brown as though he had carried -two genes for brown eyes. The gene for brown eyes is -<i>dominant</i> in this case while the gene for blue eyes is -<i>recessive</i>.</p> -<div class="pb" id="Page_8">8</div> -<h3 id="c5">Parents and Offspring</h3> -<p>How does the fertilized ovum obtain its particular set of -chromosomes in the first place?</p> -<p>Each adult possesses gonads in which <i>sex cells</i> are -formed. In the male, sperm cells are formed in the testes; -in the female, egg cells are formed in the ovaries.</p> -<p>In the formation of the sperm cells and egg cells there -is a key step—<i>meiosis</i>—a cell division in which the -chromosomes group into pairs and are then apportioned -between the daughter cells, one of each pair to each cell. -Such a division, unaccompanied by replication, means that -in place of the usual 23 pairs of chromosomes in each other -cell, each sex cell has 23 individual chromosomes, a -“half-set”, so to speak.</p> -<p>In the process of fertilization, a sperm cell from the -father enters and merges with an egg cell from the mother. -The fertilized ovum that results now has a full set of 23 -pairs of chromosomes, but of each pair, one comes from -the father and one from the mother.</p> -<p>In this way, each newborn child is a true individual, with -its characteristics based on a random reshuffling of -chromosomes. In forming the sex cells, the chromosome -pairs can separate in either fashion (<i>a</i> into cell 1 and <i>b</i> -into cell 2, or vice versa). If each of 23 pairs does this -randomly, nearly 10 million different combinations of -chromosomes are possible in the sex cells of a single -individual.</p> -<p>Furthermore, one can’t predict which chromosome combination -in the sperm cell will end up in combination with -which in the egg cell, so that by this reasoning, a single -married couple could produce children with any of 100 trillion -(100,000,000,000,000) possible chromosome combinations.</p> -<p>It is this that begins to explain the endless variety among -living beings, even within a particular species.</p> -<p>It only begins to explain it, because there are other -sources of difference, too. A chromosome is capable of -exchanging pieces with its pair, producing chromosomes -with a brand new pattern of gene varieties. Before such a -<i>crossover</i>, one chromosome may have carried a gene for -blue eyes and one for wavy hair, while the other chromosome -may have carried a gene for brown eyes and one for -straight hair. After the crossover, one would carry genes -for blue eyes and straight hair, the other for brown eyes -and wavy hair.</p> -<div class="pb" id="Page_9">9</div> -<div class="img" id="pic_9"> -<img src="images/p06.jpg" alt="Meiosis" width="404" height="800" /> -</div> -<dl class="pcap"><dt>Interphase</dt> -<dt>Prophase</dt> -<dt>Metaphase</dt> -<dt>Anaphase</dt> -<dt>Interphase</dt> -<dt>Metaphase</dt> -<dt>Interphase</dt></dl> -<div class="pb" id="Page_10">10</div> -<h2 id="c6">MUTATIONS</h2> -<h3 id="c7">Sudden Change</h3> -<p>Shifts in chromosome combinations, with or without -crossovers, can produce unique organisms with characteristics -not quite like any organism that appeared in the past -nor likely to appear in the reasonable future. They may -even produce novelties in individual characteristics since -genes can affect one another, and a gene surrounded by -unusual neighbors can produce unexpected effects.</p> -<p>Matters can go further still, however, in the direction of -novelty. It is possible for chromosomes to undergo more -serious changes, either structural or chemical, so that -entirely new characteristics are produced that might not -otherwise exist. Such changes are called <i>mutations</i>.</p> -<p>We must be careful how we use this term. A child may -possess some characteristics not present in either parent -through the mere shuffling of chromosomes and not through -mutation.</p> -<p>Suppose, for instance, that a man is heterozygous to eye -color, carrying one gene for brown eyes and one for blue -eyes. His eyes would, of course, be brown since the gene -for brown eyes is dominant over that for blue. Half the -sperm cells he produces would carry a single gene for -brown eyes in its half set of chromosomes. The other half -would carry a single gene for blue eyes. If his wife were -similarly heterozygous (and therefore also had brown eyes), -half her egg cells would carry the gene for brown eyes and -half the gene for blue.</p> -<p>It might follow in this marriage, then, that a sperm -carrying the gene for blue eyes might fertilize an egg -carrying the gene for blue eyes. The child would then be -homozygous, with two genes for blue eyes, and he would -definitely be blue-eyed. In this way, two brown-eyed parents -might have a blue-eyed child and this would <i>not</i> be a -<span class="pb" id="Page_11">11</span> -mutation. If the parents’ ancestry were traced further back, -blue-eyed individuals would undoubtedly be found on both -sides of the family tree.</p> -<p>If, however, there were no record of, say, anything but -normal color vision in a child’s ancestry, and he were born -color-blind, that could be assumed to be the result of a -mutation. Such a mutation could then be passed on by the -normal modes of inheritance and a certain proportion of -the child’s eventual descendants would be color-blind.</p> -<p>A mutation may be associated with changes in chromosome -structure sufficiently drastic to be visible under the -microscope. Such <i>chromosome mutations</i> can arise in -several ways. Chromosomes may undergo replication without -the cell itself dividing. In that way, cells can develop -with two, three, or four times the normal complement of -chromosomes, and organisms made up of cells displaying -such <i>polyploidy</i> can be markedly different from the norm. -This situation is found chiefly among plants and among -some groups of invertebrates. It does not usually occur in -mammals, and when it does it leads to quick death.</p> -<p>Less extreme changes take place, too, as when a particular -chromosome breaks and fails to reunite, or when several -break and then reunite incorrectly. Under such conditions, -the mechanism by which chromosomes are distributed -among the daughter cells is not likely to work correctly. -Sex cells may then be produced with a piece of chromosome -(or a whole one) missing, or with an extra piece (or whole -chromosome) present.</p> -<p>In 1959, such a situation was found to exist in the case of -persons suffering from a long-known disease called Down’s -syndrome.<a class="fn" id="fr_2" href="#fn_2">[2]</a> Each person so afflicted has 47 chromosomes -in place of the normal 46. It turned out that the 21st pair of -chromosomes (using a convention whereby the chromosome -pairs are numbered in order of decreasing size) consists -of three individuals rather than two. The existence of this -chromosome abnormality clearly demonstrated what had -previously been strongly suspected—that Down’s syndrome -originates as a mutation and is inborn (see the <a href="#pic_10">figure</a> on the -next page).</p> -<div class="pb" id="Page_12">12</div> -<div class="img" id="pic_10"> -<img src="images/p07.jpg" alt="" width="600" height="390" /> -<p class="caption small"><i>Karyotype of a female patient with Down’s syndrome (Mongolism). -During meiosis both chromosomes No. 21 of the mother, instead of -just one, went to the ovum. Fertilization added the father’s chromosome, -which made three Nos. 21 instead of the normal pair. -(Compare with the normal karyotype on <a href="#Page_4">page 4</a>.)</i></p> -</div> -<p>Most mutations, however, are not associated with any -noticeable change in chromosome structure. There are, -instead, more subtle changes in the chemical structure of -the genes that make up the chromosome. Then we have -<i>gene mutations</i>.</p> -<p>The process by which a gene produces its own replica is -complicated and, while it rarely goes wrong, it does misfire -on occasion. Then, too, even when a gene molecule is -replicated perfectly, it may undergo change afterward -through the action upon it of some chemical or other environmental -influence. In either case, a new variety of a -particular gene is produced and, if present in a sex cell, it -may be passed on to descendants through an indefinite -number of generations.</p> -<p>Of course, chromosome or gene mutations may take -place in ordinary cells rather than in sex cells. Such -changes in ordinary cells are <i>somatic mutations</i>. When -mutated body cells divide, new cells with changed characteristics -are produced. These changes may be trivial, -or they may be serious. It is often suggested, for instance, -<span class="pb" id="Page_13">13</span> -that cancer may result from a somatic mutation in which -certain cells lose the capacity to regulate their growth -properly. Since somatic mutations do not involve the sex -cells, they are confined to the individual and are not -passed on to the offspring.</p> -<h3 id="c8">Spontaneous Mutations</h3> -<p>Mutations that take place in the ordinary course of nature, -without man’s interference, are <i>spontaneous mutations</i>. -Most of these arise out of the very nature of the -complicated mechanism of gene replication. Copies of genes -are formed out of a large number of small units that must -be lined up in just the right pattern to form one particular -gene and no other.</p> -<p>Ideally, matters are so arranged within the cell that the -necessary changes giving rise to the desired pattern are -just those that have a maximum probability. Other changes -are less likely to happen but are not absolutely excluded. -Sometimes through the accidental jostling of molecules a -wrong turn may be taken, and the result is a spontaneous -mutation.</p> -<p>We might consider a mutation to be either “good” or -“bad” in the sense that any change that helps a creature -live more easily and comfortably is good and that the -reverse is bad.</p> -<p>It seems reasonable that random changes in the gene -pattern are almost sure to be bad. Consider that any creature, -including man, is the product of millions of years of -evolution. In every generation those individuals with a gene -pattern that fit them better for their environment won out -over those with less effective patterns—won out in the -race for food, for mates, and for safety. The “more fit” -had more offspring and crowded out the “less fit”.</p> -<p>By now, then, the set of genes with which we are normally -equipped is the end product of long ages of such -<i>natural selection</i>. A random change cannot be expected to -improve it any more than random changes would improve -any very complex, intricate, and delicate structure.</p> -<div class="pb" id="Page_14">14</div> -<div class="img" id="pic_11"> -<img src="images/p08.jpg" alt="" width="491" height="801" /> -<p class="caption small"><i>Evolution of the horse (skull, hindfoot, and forefoot shown). Note -the changes over a 60-million-year period from the Eocene era to -the present.</i></p> -</div> -<dl class="pcap"><dt>Pleistocene and Recent</dt> -<dt>Pliocene</dt> -<dt>Miocene</dt> -<dt>Oligocene</dt> -<dt>Eocene</dt></dl> -<div class="pb" id="Page_15">15</div> -<p>Yet over the eons, creatures have indeed changed, -largely through the effects of mutation. If mutations are -almost always for the worse, how can one explain that -evolution seems to progress toward the better and that -out of a primitive form as simple as an amoeba, for instance, -there eventually emerged man?</p> -<p>In the first place, environment is not fixed. Climate -changes, conditions change, the food supply may change, -the nature of living enemies may change. A gene pattern -that is very useful under one set of conditions may be less -useful under another.</p> -<p>Suppose, for instance, that man had lived in tropical -areas for thousands of years and had developed a heavily -pigmented skin as a protection against sunburn. Any child -who, through a mutation, found himself incapable of forming -much pigment, would be at a severe disadvantage in -the outdoor activities engaged in by his tribe. He would -not do well and such a mutated gene would never establish -itself for long.</p> -<p>If a number of these men migrated to northern Europe, -however, children with dark skin would absorb insufficient -sunlight during the long winter when the sun was low in the -sky, and visible for brief periods only. Dark-skinned -children would, under such conditions, tend to suffer from -rickets.</p> -<p>Mutant children with pale skin would absorb more of -what weak sunlight there was and would suffer less. There -would be little danger of sunburn so there would be no -penalty counteracting this new advantage of pale skins. It -would be the dark-skinned people who would tend to die -out. In the end, you would have dark skins in Africa and -pale skins in Scandinavia, and both would be “fit”.</p> -<p>In the same way, any child born into a primitive hunting -society who found himself with a mutated gene that brought -about nearsightedness would be at a distinct disadvantage. -In a modern technological society, however, nearsighted -individuals, doing more poorly at outdoor games, are often -driven into quieter activities that involve reading, thinking, -and studying. This may lead to a career as a scientist, -scholar, or professional man, categories that are valuable -in such a society and are encouraged. Nearsightedness -would therefore spread more generally through civilized -societies than through primitive ones.</p> -<div class="pb" id="Page_16">16</div> -<p>Then, too, a gene may be advantageous when it occurs in -low numbers and disadvantageous when it occurs in high -numbers. Suppose there were a gene among humans that -so affected the personality as to make it difficult for a -human being to endure crowded conditions. Such individuals -would make good explorers, farmers, and herdsmen, but -poor city dwellers. Even in our modern urbanized society, -such a gene in moderate concentration would be good, since -we still need our outdoorsmen. In high concentration, it -would be bad, for then the existence of areas of high population -density (on which our society now seems to depend) -might become impossible.</p> -<p>In any species, then, each gene exists in a number of -varieties upon which an absolute “good” or “bad” cannot -be unequivocally stamped. These varieties make up the -<i>gene pool</i>, and it is this gene pool that makes evolution -possible.</p> -<p>A species with an invariable set of genes could not -change to suit altered conditions. Even a slight shift in -the nature of the environment might suffice to wipe it out.</p> -<p>The possession of a gene pool lends flexibility, however. -As conditions change, one combination of varieties might -gain over another and this, in turn, might produce changes -in body characteristics that would then further alter the -relative “goodness” or “badness” of certain gene patterns.</p> -<p>Thus, over the past million years, for example, the -human brain has, through mutations and appropriate shifts -in emphasis within the gene pool, increased notably in size.</p> -<h3 id="c9">Genetic Load</h3> -<p>Some gene mutations produce characteristics so undesirable -that it is difficult to imagine any reasonable change -in environmental conditions that would make them beneficial. -There are mutations that lead to the nondevelopment -of hands and feet, to the production of blood that will not -clot, to serious malformations of essential organs, and so -on. Such mutations are unqualifiedly bad.</p> -<p>The badness may be so severe that a fertilized ovum -may be incapable of development; or, if it develops, the -fetus miscarries or the child is stillborn; or, if the child is -<span class="pb" id="Page_17">17</span> -born alive, it dies before it matures so that it can never -have children of its own. Any mutation that brings about -death before the gene producing it can be passed on to -another generation is a <i>lethal mutation</i>.</p> -<p>A gene governing a lethal characteristic may be dominant. -It will then kill even though the corresponding gene -on the other chromosome of the pair is normal. Under such -conditions, the lethal gene is removed in the same generation -in which it is formed.</p> -<p>The lethal gene may, on the other hand, be recessive. Its -effect is then not evident if the gene it is paired with is -normal. The normal gene carries on for both.</p> -<p>When this is the case, the lethal gene will remain in -existence and will, every once in a while, make itself evident. -If two people, each serving as a <i>carrier</i> for such a -gene, have children, a sperm cell carrying a lethal may -fertilize an egg cell carrying the same type of lethal, with -sad results.</p> -<p>Every species, including man, includes individuals who -carry undesirable genes. These undesirable genes may be -passed along for generations, even if dominant, before -natural selection culls them out. The more seriously undesirable -they are, the more quickly they are removed, but -even outright lethal genes will be included among the -chromosomes from generation to generation provided they -are recessive. These deleterious genes make up the -<i>genetic load</i>.</p> -<p>The only way to avoid a genetic load is to have no mutations -and therefore no gene pool. The gene pool is necessary -for the flexibility that will allow a species to survive -and evolve over the eons and the genetic load is the price -that must be paid for that. Generally, the capacity for a -species to reproduce itself is sufficiently high to make up, -quite easily, the numbers lost through the combination of -deleterious genes.</p> -<p>The size of a genetic load depends on two factors: The -rate at which a deleterious gene is produced through mutation, -and the rate at which it is removed by natural selection. -When the rate of removal equals the rate of production, -a condition of <i>genetic equilibrium</i> is reached and the -<span class="pb" id="Page_18">18</span> -level of occurrence of that gene then remains stable over -the generations.</p> -<p>Even though deleterious genes are removed relatively -rapidly, if dominant, and lethal genes are removed in the -same generation in which they are formed, a new crop of -deleterious genes will appear by mutation with every succeeding -generation. The equilibrium level for such dominant -deleterious genes is relatively low, however.</p> -<p>Deleterious genes that are recessive are removed much -more slowly. Those persons with two such genes, who alone -show the bad effects, are like the visible portion of an iceberg -and represent only a small part of the whole. The -heterozygotes, or carriers, who possess a single gene of -this sort, and who live out normal lives, keep that gene in -being. If people in a particular population marry randomly -and if one out of a million is born homozygous for a certain -deleterious recessive gene (and dies of it), one out of -five hundred is heterozygous for that same gene, shows -no ill effects, and is capable of passing it on.</p> -<p>It may be that the heterozygote is not quite normal but -does show some ill effects—not enough to incommode him -seriously, perhaps, but enough to lower his chances slightly -for mating and bearing children. In that case, the equilibrium -level for that gene will be lower than it would otherwise -be.</p> -<p>It may also be that the heterozygote experiences an actual -advantage over the normal individual under some conditions. -There is a recessive gene, for instance, that produces -a serious disease called sickle-cell anemia. People -possessing two such genes usually die young. A heterozygote -possessing only one of these genes is not seriously -affected and has red blood cells that are, apparently, less -appetizing to malaria parasites. The heterozygote therefore -experiences a positive advantage if he lives in a region -where the incidence of certain kinds of malaria is -high. The equilibrium level of the sickle-cell anemia gene -can, in other words, be higher in malarial regions than -elsewhere.</p> -<p>Here is one subject area in which additional research is -urgently needed. It may be that the usefulness of a single -deleterious gene is greater than we may suspect in many -<span class="pb" id="Page_19">19</span> -cases, and that there are greater advantages to heterozygousness -than we know. This may be the basis of what is -sometimes called “hybrid vigor”. In a world in which -human beings are more mobile than they have ever been -in history and in which intercultural marriages are increasingly -common, information on this point is particularly -important.</p> -<h3 id="c10">Mutation Rates</h3> -<p>It is easier to observe the removal of genes through -death or through failure to reproduce than to observe -their production through mutation. It is particularly difficult -to study their production in human beings, since men -have comparatively long lifetimes and few children, and -since their mating habits cannot well be controlled.</p> -<p>For this reason, geneticists have experimented with -species much simpler than man—smaller organisms that -are short-lived, produce many offspring, and that can be -penned up and allowed to mate only under fixed conditions. -Such creatures may have fewer chromosomes than man -does and the sites of mutation are more easily pinned -down.</p> -<p>An important assumption made in such experiments is -that the machinery of inheritance and mutation is essentially -the same in all creatures and that therefore knowledge -gained from very simple species (even from bacteria) -is applicable to man. There is overwhelming evidence to -indicate that this is true in general, although there are -specific instances where it is not completely true and -scientists must tread softly while drawing conclusions.</p> -<p>The animals most commonly used in studies of genetics -and mutations are certain species of fruit flies, called -<i>Drosophila</i>. The American geneticist, Hermann J. Muller, -devised techniques whereby he could study the occurrence -of lethal mutations anywhere along one of the four pairs -of chromosomes possessed by <i>Drosophilia</i>.</p> -<p>A lethal gene, he found, might well be produced somewhere -along the length of a particular chromosome once -out of every two hundred times that chromosome underwent -replication. This means that out of every 200 sex -<span class="pb" id="Page_20">20</span> -cells produced by <i>Drosophilia</i>, one would contain a lethal -gene somewhere along the length of that chromosome.</p> -<div class="img" id="pic_12"> -<img src="images/p09.jpg" alt="" width="800" height="598" /> -<p class="caption small"><i>Geneticist Hermann J. Muller studying</i> Drosophila <i>in his laboratory. -Dr. Muller won a Nobel Prize in 1946 for showing that radiation -can cause mutations. (See <a href="#Page_34">page 34</a>.)</i></p> -</div> -<p>That particular chromosome, however, contained at least -500 genes capable of undergoing a lethal mutation. If each -of those genes is equally likely to undergo such a mutation, -then the chance that any one particular gene is lethal is one -out of 200 × 500, or 1 out of 100,000.</p> -<p>This is a typical mutation rate for a gene in higher -organisms generally, as far as geneticists can tell (though -the rates are lower among bacteria and viruses). Naturally, -a chance for mutation takes place every time a new individual -is born. Fruit flies have many more offspring -per year than human beings, since their generations are -shorter and they produce more young at a time. For that -reason, though the mutation rate may be the same in fruit -flies as in man, many more actual mutations are produced -per unit time in fruit flies than in men.</p> -<p>This does not mean that the situation may be ignored in -the case of man. Suppose the rate for production of a particular -<span class="pb" id="Page_21">21</span> -deleterious gene in man is 1 out of 100,000. It is -estimated that a human being has at least 10,000 different -genes, and therefore the chance that at least one of the -genes in a sex cell is deleterious is 10,000 out of 100,000 -or 1 out of 10.</p> -<p>Furthermore, it is estimated that the number of gene -mutations that are weakly deleterious are four times as -numerous as those that are strongly deleterious or lethal. -The chances that at least one gene in a sex cell is at least -weakly deleterious then would be 4 + 1 out of 10, or 1 out of -2.</p> -<p>Naturally, these deleterious genes are not necessarily -spread out evenly among human beings with one to a sex -cell. Some sex cells will be carrying more than one, thus -increasing the number that may be expected to carry none -at all. Even so, it is supposed that very nearly half the sex -cells produced by humanity carry at least one deleterious -gene.</p> -<p>Even though only half the sex cells are free of deleterious -genes, it is still possible to produce a satisfactory new -generation of men. Yet one can see that the genetic load is -quite heavy and that anything that would tend to increase it -would certainly be undesirable, and perhaps even dangerous.</p> -<p>We tend to increase the genetic load by reducing the rate -at which deleterious genes are removed, that is, by taking -care of the sick and retarded, and by trying to prevent -discomfort and death at all levels.</p> -<p>There is, however, no humane alternative to this. What’s -more, it is, by and large, only those with slightly deleterious -genes who are preserved genetically. It is those persons -with nearsightedness, with diabetes, and so on, who, -with the aid of glasses, insulin, or other props, can go on -to live normal lives and have children in the usual numbers. -Those with strongly deleterious genes either die -despite all that can be done for them even today or, at the -least, do not have a chance to have many children.</p> -<p>The danger of an increase in the genetic load rests more -heavily, then, at the other end—at measures that (usually -inadvertently or unintentionally) increase the rate of production -of mutant genes. It is to this matter we will now -turn.</p> -<div class="pb" id="Page_22">22</div> -<h2 id="c11">RADIATION</h2> -<h3 id="c12">Ionizing Radiation</h3> -<p>Our modern technological civilization exposes mankind to -two general types of genetic dangers unknown earlier: -Synthetic chemicals (or unprecedentedly high concentrations -of natural ones) absent in earlier eras, and intensities -of energetic radiation equally unknown or unprecedented.</p> -<p>Chemicals can interfere with the process of replication -by offering alternate pathways with which the cellular -machinery is not prepared to cope. In general, however, it -is only those cells in direct contact with the chemicals that -are so affected, such as the skin, the intestinal linings, the -lungs, and the liver (which is active in altering and getting -rid of foreign chemicals). These may undergo somatic -mutations, and an increased incidence of cancer in those -tissues is among the drastic results of exposure to certain -chemicals.</p> -<p>Such chemicals are not, however, likely to come in contact -with the gonads where the sex cells are produced. -While individual persons may be threatened by the manner -in which the environment is being permeated with novel -chemicals, the next generation is not affected in advance.</p> -<p>Radiation is another matter. In its broadest sense, radiation -is any phenomenon spreading out from some source -in all directions. Physically, such radiation may consist -of waves or of particles.<a class="fn" id="fr_3" href="#fn_3">[3]</a> Of the wave forms the two best-known -are sound and electromagnetic radiations.</p> -<p>Sound carries very low concentrations of energy. This -energy is absorbed by living tissue and converted into heat. -Heat in itself can increase the mutation rate but the effect -is a small one. The body has effective machinery for keeping -its temperature constant and the gonads are not likely -to suffer unduly from exposure to heat.</p> -<div class="pb" id="Page_23">23</div> -<p>Electromagnetic radiation comes in a wide range of -energies, with visible light (the best-known example of -such radiation because we can detect it directly and with -great sensitivity) about in the middle of the range. Electromagnetic -radiations less energetic than light (such as -infrared waves and microwaves) are converted into heat -when absorbed by living tissue. The heat thus formed is -sufficient to cause atoms and molecules to vibrate more -rapidly, but this added vibration is not usually sufficient to -pull molecules apart and therefore does not bring about -chemical changes.</p> -<p>Light will bring about some chemical changes. It is -energetic enough to cause a mixture of hydrogen and chlorine -to explode. It will break up silver compounds and -produce tiny black grains of metallic silver (the chemical -basis of photography). Living tissue, however, is largely -unaffected—the retina of the eye being one obvious exception.</p> -<p>Ultraviolet light, which is more energetic than visible -light, correspondingly can bring about chemical changes -more easily. It will redden the skin, stimulate the production -of pigment, and break up certain steroid molecules to -form vitamin D. It will even interfere with replication to -some extent. At least there is evidence that persistent -exposure to sunlight brings about a heightened tendency -to skin cancer. Ultraviolet light is not very penetrating, -however, and its effects are confined to the skin.</p> -<p>Electromagnetic radiations more energetic than ultraviolet -light, such as X rays and gamma rays, carry sufficient -concentrations of energy to bring about changes not -only in molecules but in the very structure of the atoms -making up those molecules.</p> -<p>Atoms consist of particles (electrons), each carrying a -negative electric charge and circling a tiny centrally located -nucleus, which carries a positive electric charge.</p> -<p>Ordinarily, the negative charges of the electrons just -balance the positive charge on the nucleus so that atoms -and molecules tend to be electrically neutral. An X ray or -gamma ray, crashing into an atom, will, however, jar -electrons loose. What is left of the atom will carry a -<span class="pb" id="Page_24">24</span> -positive electric charge with the charge size proportional -to the number of electrons lost.</p> -<p>An atom fragment carrying an electric charge is called -an <i>ion</i>. X rays and gamma rays are therefore examples of -<i>ionizing radiation</i>.</p> -<p>Radiations may consist of flying particles, too, and if -these carry sufficient energy they are also ionizing in -character. Examples are <i>cosmic rays</i>, <i>alpha rays</i>, and <i>beta -rays</i>. Cosmic rays are streams of positively charged -nuclei, predominantly those of the element hydrogen. Alpha -rays are streams of positively charged helium nuclei. -Beta rays are streams of negatively charged electrons. -The individual particles contained in these rays may be -referred to as <i>cosmic particles</i>, <i>alpha particles</i>, and <i>beta -particles</i>, respectively.</p> -<div class="img" id="pic_13"> -<img src="images/p10.jpg" alt="" width="600" height="515" /> -<p class="caption small"><i>Cosmic ray and trapped Van Allen Belt energetic particles produced -the dark tracks in this photo of a nuclear emulsion that had -been carried aloft on an Air Force satellite. The energetic particles -cause ionization of the silver bromide molecules in the -emulsion.</i></p> -</div> -<div class="pb" id="Page_25">25</div> -<div class="img" id="pic_14"> -<img src="images/p10a.jpg" alt="" width="600" height="528" /> -<p class="caption small"><i>Alpha particles emitted by the source at right leave tracks in a -cloud chamber. Some tracks are bent near the end as a result of -collisions with atomic nuclei. Such collisions are more likely at -the end of a track when the alpha particle has been slowed down.</i></p> -</div> -<div class="img" id="pic_15"> -<img src="images/p10b.jpg" alt="" width="600" height="459" /> -<p class="caption small"><i>Beta particles originating at left leave these tracks in a cloud -chamber. Note that the tracks are much farther apart than those -of alpha particles. As the particle slows down, its path becomes -more erratic and the ions are formed closer together. At the very -end of an electron track the proximity of the ions approximates -that in an alpha-particle track.</i></p> -</div> -<div class="pb" id="Page_26">26</div> -<p>Ionizing radiation is capable of imparting so much energy -to molecules as to cause them to vibrate themselves -apart, producing not only ions but also high-energy uncharged -molecular fragments called <i>free radicals</i>.</p> -<p>The direct effect of ionizing radiation on chromosomes -can be serious. Enough chemical bonds may be disrupted -so that a chromosome struck by a high-energy wave or -particle may break into fragments. Even if the chromosome -manages to remain intact, an individual gene along -its length may be badly damaged and a mutation may be -produced.</p> -<div class="img" id="pic_16"> -<img src="images/p11.jpg" alt="" width="800" height="413" /> -<p class="caption small"><i>Effects of ionizing radiation on chromosomes: Left, a normal -plant cell showing chromosomes divided into two groups; right, -the same type of cell after X-ray exposure, showing broken fragments -and bridges between groups, typical abnormalities induced -by radiation.</i></p> -</div> -<p>If only direct hits mattered, radiation effects would be -less dangerous than they are, since such direct hits are -comparatively few. However, near-misses may also be -deadly. A streaking bit of radiation may strike a water -molecule near a gene and may break up the molecule to -form a free radical. The free radical will be sufficiently -energetic to bring about a chemical reaction with almost -any molecule it strikes. If it happens to strike the neighboring -gene before it has disposed of that energy, it will -produce the mutation as surely as the original radiation -might have.</p> -<div class="pb" id="Page_27">27</div> -<p>Furthermore, ionizing radiations (particularly of the -electromagnetic variety) tend to be penetrating, so that the -interior of the body is as exposed as is the surface. The -gonads cannot hide from X rays, gamma rays, or cosmic -particles.</p> -<p>All these radiations can bring about somatic mutations—all -can cause cancer, for instance.</p> -<p>What is worse, all of them increase the rate of genetic -mutations so that their presence threatens generations -unborn as well as the individuals actually exposed.</p> -<h3 id="c13">Background Radiation</h3> -<p>Ionizing radiation in low intensities is part of our -natural environment. Such natural radiation is referred to -as <i>background radiation</i>. Part of it arises from certain -constituents of the soil. Atoms of the heavy metals, uranium -and thorium, are constantly, though very slowly, -breaking down and in the process giving off alpha rays, -beta rays, and gamma rays. These elements, while not -among the most common, are very widely spread; minerals -containing small quantities of uranium and thorium -are to be found nearly everywhere.</p> -<p>In addition, all the earth is bombarded with cosmic rays -from outer space and with streams of high-energy particles -from the sun.</p> -<p>Various units can be used to measure the intensity of -this background radiation. The <i>roentgen</i>, abbreviated <i>r</i>, and -named in honor of the discoverer of X rays, Wilhelm -Roentgen, is a unit based on the number of ions produced -by radiation. Rather more convenient is another unit that -has come more recently into prominence. This is the -<i>rad</i> (an abbreviation for “radiation absorbed dose”) that -is a measure of the amount of energy delivered to the body -upon the absorption of a particular dose of ionizing radiation. -One rad is very nearly equal to one roentgen.</p> -<p>Since background radiation is undoubtedly one of the -factors in producing spontaneous mutations, it is of interest -to try to determine how much radiation a man or woman -will have absorbed from the time he is first conceived to -the time he conceives his own children. The average length -<span class="pb" id="Page_28">28</span> -of time between generations is taken to be about 30 years, -so we can best express absorption of background radiation -in units of <i>rads per 30 years</i>.</p> -<div class="img" id="pic_17"> -<img src="images/p12.jpg" alt="" width="500" height="625" /> -<p class="caption small"><i>Natural radioactivity in the atmosphere is shown by this nuclear-emulsion -photograph of alpha-particle tracks (enlarged 2000 -diameters) emitted by a grain of radioactive dust.</i></p> -</div> -<p>The intensity of background radiation varies from place -to place on the earth for several reasons. Cosmic rays are -deflected somewhat toward the magnetic poles by the -earth’s magnetic field. They are also absorbed by the -atmosphere to some extent. For this reason, people living -<span class="pb" id="Page_29">29</span> -in equatorial regions are less exposed to cosmic rays -than those in polar regions; and those in the plains, with a -greater thickness of atmosphere above them, are less exposed -than those on high plateaus.</p> -<p>Then, too, radioactive minerals may be spread widely, -but they are not spread evenly. Where they are concentrated -to a greater extent than usual, background radiation -is abnormally high.</p> -<p>Thus, an inhabitant of Harrisburg, Pennsylvania, may -absorb 2.64 rads per 30 years, while one of Denver, Colorado, -a mile high at the foot of the Rockies, may absorb -5.04 rads per 30 years. Greater extremes are encountered -at such places as Kerala, India, where nearby soil, rich in -thorium minerals, so increases the intensity of background -radiation that as much as 84 rads may be absorbed in 30 -years.</p> -<p>In addition to high-energy radiation from the outside, -there are sources within the body itself. Some of the -potassium and carbon atoms of our body are inevitably -radioactive. As much as 0.5 rad per 30 years arises from -this source.</p> -<p>Rads and roentgens are not completely satisfactory units -in estimating the biological effects of radiation. Some types -of radiation—those made up of comparatively large particles, -for instance—are more effective in producing ions -and bring about molecular changes with greater ease than -do electromagnetic radiations delivering equal energy to -the body. Thus if 1 rad of alpha particles is absorbed by -the body, 10 to 20 times as much biological effect is produced -as there would be in the absorption of 1 rad of -X rays, gamma rays, or beta particles.</p> -<p>Sometimes, then, one speaks of the <i>relative biological -effectiveness</i> (RBE) of radiation, or the <i>roentgen equivalent, -man</i> (rem). A rad of X rays, gamma rays, or beta -particles has a rem of 1, while a rad of alpha particles -has a rem of 10 to 20.</p> -<p>If we allow for the effect of the larger particles (which -are not very common under ordinary conditions) we can -estimate that the gonads of the average human being receive -a total dose of natural radiation of about 3 rems per -30 years. This is just about an irreducible minimum.</p> -<div class="pb" id="Page_30">30</div> -<h3 id="c14">Man-made Radiation</h3> -<p>Man began to add to the background radiation in the -1890s. In 1895, X rays were discovered and since then have -become increasingly useful in medical diagnosis and therapy -and in industry. In 1896, radioactivity was discovered -and radioactive substances were concentrated in laboratories -in order that they might be studied. In 1934, it was -found that radioactive forms of nonradioactive elements -(<i>radioisotopes</i>) could be formed and their use came to be -widespread in universities, hospitals, and industries.<a class="fn" id="fr_4" href="#fn_4">[4]</a></p> -<p>Then, in 1945, the nuclear bomb was developed. With the -uranium or plutonium fission that produces a nuclear explosion, -there is an accompaniment of intense gamma -radiation. In addition, a variety of radioisotopes are left -behind in the form of the residue (<i>fission fragments</i>) of -the fissioning atoms. These fission fragments are distributed -widely in the atmosphere. Some rise high into the -stratosphere and descend (as <i>fallout</i>) over the succeeding -months and years.<a class="fn" id="fr_5" href="#fn_5">[5]</a></p> -<p>It is hard to try to estimate how much additional radiation -is being absorbed by human beings out of these man-made -sources. Fallout is not uniformly spread over the -earth but is higher in those latitudes where nuclear bombs -have been most frequently tested. Then, too, people in -industries and research who are involved with the use of -radioisotopes, and people in medical centers who constantly -deal with X rays, are likely to get more exposure than -others.</p> -<p>These adjuncts of modern science and medicine are more -common and widespread in technologically advanced countries -than elsewhere, and nuclear bombs have most often -been exploded in just those latitudes where the advanced -countries are to be found.</p> -<p>Attempts have been made to work out estimates of this -exposure. One estimate, involving a number of technologically -advanced countries (including the United States) -<span class="pb" id="Page_31">31</span> -showed that an average of somewhere between 0.02 and -0.18 rem per year was absorbed, as a result of radiations -(usually X rays) used in medical diagnosis and therapy. -Occupational exposure added, on the average, not more -than 0.003 rem, though the individuals constantly exposed -in the course of their work would naturally absorb considerably -more than this overall average.</p> -<div class="img" id="pic_18"> -<img src="images/p13.jpg" alt="" width="705" height="600" /> -<p class="caption small"><i>Man-made radioactivity in the atmosphere produced this nuclear-emulsion -photograph. This radiation source is a fission product -produced in a nuclear explosion. The enlargement is 1200 diameters. -Compare this with the natural radioactivity depicted on <a href="#Page_28">page 28</a>.</i></p> -</div> -<p>On the whole, the highest absorption was found, as was -to be expected, in the United States.</p> -<p>If these findings are expanded to cover a 30-year period, -assuming the absorption will remain the same from year -to year, it turns out that the average absorption of man-made -radiation in the nations studied varies from 0.6 rem -to 5.5 rems per 30 years per individual.</p> -<div class="pb" id="Page_32">32</div> -<p>Considering the higher figure to be applicable to the -United States, it would seem that man-made radiation from -all sources is now being absorbed at nearly twice the rate -that natural radiation is. To put it another way, Americans -are just about tripling their radiation dosage by reason of -the human activities that are now adding man-made radiation -to the natural supply. By far the major part of this -additional dosage is the result of the use of X rays in -searching for decayed teeth, broken bones, lung lesions, -swallowed objects, and so on.</p> -<h2 id="c15">DOSE AND CONSEQUENCE</h2> -<h3 id="c16">Radiation Sickness</h3> -<p>The danger to the individual as a result of overexposure -to high-energy radiation was understood fairly soon but not -before some tragic experiences were recorded.</p> -<p>One of the early workers with radioactive materials, -Pierre Curie, deliberately exposed a patch of his skin to -the action of radioactive radiations and obtained a serious -and slow-healing burn. His wife, Marie Curie, and their -daughter, Irène Joliot-Curie, who spent their lives working -with radioactive materials, both died of leukemia, very -possibly as the result of cumulative exposure to radiation. -Other research workers in the field died of cancer before -the full necessity of extreme caution was understood.</p> -<p>The damage done to human beings by radiation could -first be studied on a large scale among the survivors of -the nuclear bombings of Hiroshima and Nagasaki in 1945. -Here marked symptoms of <i>radiation sickness</i> were observed. -This sickness often leads to death, though a slow -recovery is sometimes possible.</p> -<p>In general, high-energy radiation damages the complex -molecules within a cell, interfering with its chemical -machinery to the point, in extreme cases, of killing it. -(Thus, cancers, which cannot safely be reached with the -surgeon’s knife, are sometimes exposed to high-energy -radiation in the hope that the cancer cells will be effectively -killed in that manner.)</p> -<div class="pb" id="Page_33">33</div> -<p>The delicate structure of the genes and chromosomes is -particularly vulnerable to the impact of high-energy radiation. -Chromosomes can be broken by such radiation and -this is the main cause of actual cell death. A cell that is -not killed outright by radiation may nevertheless be so -damaged as to be unable to undergo replication and mitosis.</p> -<p>If a cell is of a type that will not, in the course of nature, -undergo division, the destruction of the mitosis machinery -is not in itself fatal to the organism. A creature like -<i>Drosophila</i>, which, in its adult stage, has very few cell -divisions going on among the ordinary cells of its body, -can survive radiation doses a hundred times as great as -would suffice to kill a man.</p> -<p>In a human being, however—even in an adult who is no -longer experiencing overall growth—there are many tissues -whose cells must undergo division throughout life. -Hair and fingernails grow constantly, as a result of cell -division at their roots. The outer layers of skin are steadily -lost through abrasion and are replaced through constant -cell division in the deeper layers. The same is true of the -lining of the mouth, throat, stomach, and intestines. Too, -blood cells are continually breaking up and must be replaced -in vast numbers.</p> -<p>If radiation kills the mechanism of division in only some -of these cells, it is possible that those that remain reasonably -intact can divide and eventually replace or do the -work of those that can no longer divide. In that case, the -symptoms of radiation sickness are relatively mild in the -first place and eventually disappear.</p> -<p>Past a certain critical point, when too many cells are -made incapable of division, this is no longer possible. The -symptoms, which show up in the growing tissues particularly -(as in the loss of hair, the misshaping or loss of -fingernails, the reddening and hemorrhaging of skin, the -ulceration of the mouth, and the lowering of the blood cell -count), grow steadily more severe and death follows.</p> -<h3 id="c17">Radiation and Mutation</h3> -<p>Where radiation is insufficient to render a cell incapable -of division, it may still induce mutations, and it is in this -<span class="pb" id="Page_34">34</span> -fashion that skin cancer, leukemia, and other disorders -may be brought about.<a class="fn" id="fr_6" href="#fn_6">[6]</a></p> -<div class="img" id="pic_19"> -<img src="images/p14.jpg" alt="" width="565" height="400" /> -<p class="caption small"><i>Studies at the California Institute of Technology furnish -information on the nature of radiation effects on genes. -The experiments produced fruit flies with three or four -wings and double or partially doubled thoraxes by causing -gene mutation through X-irradiation and chromosome -rearrangements. A is a normal male</i> Drosophila; -<i>B is a four-winged male with a double thorax; and C and -D are three-winged flies with partial double thoraxes.</i></p> -</div> -<div class="img" id="pic_20"> -<img src="images/p14b.jpg" alt="Four-winged male with a double thorax" width="629" height="400" /> -</div> -<div class="img" id="pic_21"> -<img src="images/p14c.jpg" alt="Three-winged fly with partial double thoraxes" width="571" height="400" /> -</div> -<div class="img" id="pic_22"> -<img src="images/p14d.jpg" alt="Three-winged fly with partial double thoraxes" width="614" height="400" /> -</div> -<p>Mutations can be brought about in the sex cells, too, of -course, and when this happens it is succeeding generations -that are affected and not merely the exposed individual. -Indeed, where the sex cells are concerned, the relatively -mild effect of mutation is more serious than the drastic -one of nondivision. A fertilized ovum that cannot divide -eventually dies and does no harm; one that can divide but -is altered, may give rise to an individual with one of the -usual kinds of major or minor physical defects.</p> -<p>The effect of high-energy radiation on the genetic mechanism -was first demonstrated experimentally in 1927 by -Muller. Using <i>Drosophila</i> he showed that after large doses -of X rays, flies experienced many more lethal mutations -per chromosome than did similar flies not exposed to radiation. -The drastic differences he observed proved the -connection between radiation and mutation at once.</p> -<p>Later experiments, by Muller and by others, showed that -the number of mutations was directly proportional to the -quantity of radiation absorbed. Doubling the quantity of -radiation absorbed doubled the number of mutations, tripling -the one tripled the other, and so on. This means that -if the number of mutations is plotted against the amount of -radiation absorbed, a straight line can be drawn.</p> -<div class="pb" id="Page_35">35</div> -<p>It is generally believed that the straight line continues -all the way down without deviation to very low radiation -absorptions. This means there is no “threshold” for the -mutational effect of radiation. No matter how small a -dosage of radiation the gonads receive, this will be reflected -in a proportionately increased likelihood of mutated -sex cells with effects that will show up in succeeding -generations.</p> -<p>In this respect, the genetic effect of radiation is quite -different from the somatic effect. A small dose of radiation -may affect growing tissues and prevent a small proportion -of the cells of those tissues from dividing. The -remaining, unaffected cells take up the slack, however, -and if the proportion of affected cells is small enough, -symptoms are not visible and never become visible. There -is thus a threshold effect: The radiation absorbed must be -more than a certain amount before any somatic symptoms -are manifest.</p> -<p>Matters are quite different where the genetic effect is -concerned. If a sex cell is damaged and if that sex cell is -one of the pair that goes into the production of a fertilized -ovum, a damaged organism results. There is no margin -for correction. There is no unaffected cell that can take -over the work of the damaged sex cell once fertilization -has taken place.</p> -<p>Suppose only one sex cell out of a million is damaged. -If so, a damaged sex cell will, on the average, take part in -one out of every million fertilizations. And when it is used, -<span class="pb" id="Page_36">36</span> -it will not matter that there are 999,999 perfectly good sex -cells that might have been used—it was the damaged cell -that <i>was</i> used. That is why there is no threshold in the -genetic effect of radiation and why there is no “safe” -amount of radiations insofar as genetic effects are concerned. -However small the quantity of radiation absorbed, -mankind must be prepared to pay the price in a corresponding -increase of the genetic load.</p> -<div class="img" id="pic_23"> -<img src="images/p15.jpg" alt="Percent lethal chromosomes vs. Amount of x radiation, r" width="500" height="448" /> -</div> -<p>If the straight line obtained by plotting mutation rate -against radiation dose is followed down to a radiation dose -of zero, it is found that -the line strikes the vertical -axis slightly above the -origin. The mutation rate -is more than zero even -when the radiation dose is -zero. The reason for this -is that it is the dose of -man-made radiation that -is being considered. Even -when man-made radiation -is completely absent there -still remains the natural -background radiation.</p> -<p>It is possible in this manner to determine that background -radiation accounts for considerably less than 1% of the -spontaneous mutations that take place. The other mutations -must arise out of chemical misadventures, out of the random -heat-jiggling of molecules, and so on. These, it can be -presumed, will remain constant when the radiation dose is -increased.</p> -<p>This is a hopeful aspect of the situation for it means that, -if the background radiation is doubled or tripled for mankind -as a whole, only that small portion of the spontaneous -mutation rate that is due to the background radiation will -be doubled or tripled.</p> -<p>Let us suppose, for instance, that fully 1% of the spontaneous -mutations occurring in mankind is due to background -radiation. In that case, the tripling of the background -radiation produced in the United States by man-made -causes (see <a href="#table1">Table</a>) would triple that 1%. In place of 99 non-radiational -<span class="pb" id="Page_37">37</span> -mutations plus 1 radiational, we would have -99 plus 3. The total number of mutations would increase -from 100 to 102—an increase of 2%, not an increase of -200% that one would expect if all spontaneous mutations -were caused by background radiation.</p> -<table class="center" summary=""> -<tr class="th"><th id="table1" colspan="4">RADIATION EXPOSURES IN THE UNITED STATES<a class="fn" id="fr_7" href="#fn_7">[7]</a></th></tr> -<tr class="th"><th> </th><th> </th><th> </th><th>Millirems<a class="fn" id="fr_8" href="#fn_8">[8]</a></th></tr> -<tr><td colspan="3" class="l">Natural Sources </td><td class="r"> </td><td></td></tr> -<tr><td class="l"> </td><td colspan="2" class="l">A. External to the body </td><td class="r"> </td><td></td></tr> -<tr><td class="l"> </td><td class="l"> </td><td class="l">1. From cosmic radiation </td><td class="r">50.0</td></tr> -<tr><td class="l"> </td><td class="l"> </td><td class="l">2. From the earth </td><td class="r">47.0</td></tr> -<tr><td class="l"> </td><td class="l"> </td><td class="l">3. From building materials </td><td class="r">3.0</td></tr> -<tr><td class="l"> </td><td colspan="2" class="l">B. Inside the body </td><td class="r"> </td><td></td></tr> -<tr><td class="l"> </td><td class="l"> </td><td class="l">1. Inhalation of air </td><td class="r">5.0</td></tr> -<tr><td class="l"> </td><td class="l"> </td><td class="l">2. Elements found naturally in human tissues </td><td class="r">21.0</td></tr> -<tr><td class="l"> </td><td colspan="2" class="l">Total, Natural sources </td><td class="r">126.0</td></tr> -<tr><td colspan="3" class="l">Man-made Sources </td><td class="r"> </td><td></td></tr> -<tr><td class="l"> </td><td colspan="2" class="l">A. Medical Procedures </td><td class="r"> </td><td></td></tr> -<tr><td class="l"> </td><td class="l"> </td><td class="l">1. Diagnostic X rays </td><td class="r">50.0</td></tr> -<tr><td class="l"> </td><td class="l"> </td><td class="l">2. Radiotherapy X ray, radioisotopes </td><td class="r">10.0</td></tr> -<tr><td class="l"> </td><td class="l"> </td><td class="l">3. Internal diagnosis, therapy </td><td class="r">1.0</td></tr> -<tr><td class="l"> </td><td class="l"> </td><td class="l">Subtotal </td><td class="r">61.0</td></tr> -<tr><td class="l"> </td><td colspan="2" class="l">B. Atomic energy industry, laboratories </td><td class="r">0.2</td></tr> -<tr><td class="l"> </td><td colspan="2" class="l">C. Luminous watch dials, television tubes, radioactive industrial wastes, etc. </td><td class="r">2.0</td></tr> -<tr><td class="l"> </td><td colspan="2" class="l">D. Radioactive fallout </td><td class="r">4.0</td></tr> -<tr><td class="l"> </td><td class="l"> </td><td class="l">Subtotal </td><td class="r">6.2</td></tr> -<tr><td class="l"> </td><td colspan="2" class="l">Total, man-made sources </td><td class="r">67.2</td></tr> -<tr><td class="l"> </td><td class="l"> </td><td class="l">Overall total </td><td class="r">193.2</td></tr> -</table> -<h3 id="c18">Dosage Rates</h3> -<p>Another difference between the genetic and somatic effects -of radiation rests in the response to changes in the -rate at which radiation is absorbed. It makes a considerable -difference to the body whether a large dose of radiation -is absorbed over the space of a few minutes or a few -years.</p> -<div class="pb" id="Page_38">38</div> -<p>When a large dose is absorbed over a short interval of -time, so many of the growing tissues lose the capacity for -cell division that death may follow. If the same dose is -delivered over years, only a small bit of radiation is absorbed -on any given day and only small proportions of -growing cells lose the capacity for division at any one -time. The unaffected cells will continually make up for this -and will replace the affected ones. The body is, so to speak, -continually repairing the radiation damage and no serious -symptoms will develop.</p> -<p>Then, too, if a moderate dose is delivered, the body may -show visible symptoms of radiation sickness but can recover. -It will then be capable of withstanding another -moderate dose, and so on.</p> -<p>The situation is quite different with respect to the genetic -effects, at least as far as experiments with <i>Drosophila</i> and -bacteria seem to show. Even the smallest doses will produce -a few mutations in the chromosomes of those cells in -the gonads that eventually develop into sex cells. The -affected gonad cells will continue to produce sex cells with -those mutations for the rest of the life of the organism. -Every tiny bit of radiation adds to the number of mutated -sex cells being constantly produced. There is no recovery, -because the sex cells, after formation, do not work in -cooperation, and affected cells are not replaced by those -that are unaffected.</p> -<p>This means (judging by the experiments on lower creatures) -that what counts, where genetic damage is in question, -is not the rate at which radiation is absorbed but the -total sum of radiation. Every exposure an organism experiences, -however small, adds its bit of damage.</p> -<p>Accepting this hard view, it would seem important to -make every effort to minimize radiation exposure for the -population generally.</p> -<p>Since most of the man-made increase in background -radiation is the result of the use of X rays in medical -diagnosis and therapy, many geneticists are looking at this -with suspicion and concern. No one suggests that their use -be abandoned, for certainly such techniques are important -<span class="pb" id="Page_39">39</span> -in the saving of life and the mitigation of suffering. Still, -X rays ought not to be used lightly, or routinely as a matter -of course.</p> -<p>It might seem that X rays applied to the jaw or the chest -would not affect the gonads, and this might be so if all the -X rays could indeed be confined to the portion of the body -at which they are aimed. Unfortunately, X rays do not -uniformly travel a straight line in passing through matter. -They are scattered to a certain extent; if a stream of -X rays passes through the body anywhere, or even through -objects near the body, some X rays will be scattered -through the gonads.</p> -<p>It is for this reason that some geneticists suggest that -the history of exposure to X rays be kept carefully for each -person. A decision on a new exposure would then be determined -not only by the current situation but by the individual’s -past history.</p> -<p>Such considerations were also an important part of the -driving force behind the movement to end atmospheric -testing of nuclear bombs. While the total addition to the -background radiation resulting from such tests is small, -the prospect of continued accumulation is unpleasant.</p> -<p>What’s more, whereas X rays used in diagnosis and therapy -have a humane purpose and chiefly affect the patient -who hopes to be helped in the process, nuclear fallout affects -all of humanity without distinction and seems, to many -people, to have as its end only the promise of a totally -destructive nuclear war.</p> -<p>It is not to be expected that the large majority of humanity -that makes up the populations outside the United States, -Great Britain, France, China, and the Soviet Union can be -expected to accept stoically the risk of even limited quantities -of genetic damage, out of any feeling of loyalty to -nations not their own. Even within the populations of the -three major nuclear powers there are strong feelings that -the possible benefits of nuclear testing do not balance the -certain dangers.</p> -<p>Public opinion throughout the world is a key factor, then, -in enforcing the Nuclear Test Ban Treaty, signed by the -governments of the United States, Great Britain, and the -Soviet Union on October 10, 1963.</p> -<div class="pb" id="Page_40">40</div> -<h3 id="c19">Effects on Mammals</h3> -<p>Although genetic findings on such comparatively simple -creatures as fruit flies and bacteria seem to apply generally -to all forms of life, it seems unsafe to rely on these -findings completely in anything as important as possible -genetic damage to man through radiation. During the -1950s and 1960s, therefore, there have been important -studies on mice, particularly by W. L. Russell at Oak Ridge -National Laboratory, Oak Ridge, Tennessee.</p> -<p>While not as short-lived or as fecund as fruit flies, mice -can nevertheless produce enough young over a reasonable -period of time to yield statistically useful results. Experimenters -have worked with hundreds of thousands of offspring -born of mice that have been irradiated with gamma -rays and X rays in different amounts and at different intensities, -as well as with additional hundreds of thousands -born to mice that were not irradiated.</p> -<p>Since mice, like men, are mammals, results gained by -such experiments are particularly significant. Mice are -far closer to man in the scheme of life than is any other -creature that has been studied genetically on a large scale, -and their reactions (one might cautiously assume) are -likely to be closer to those that would be found in man.</p> -<p>Almost at once, when the studies began, it turned out that -mice were more susceptible to genetic damage than fruit -flies were. The induced mutation rate per gene seems to be -about fifteen times that found in <i>Drosophila</i> for comparable -X ray doses. The only safe course for mankind then is to -err, if it must, strongly on the side of conservatism. Once -we have decided what might be safe on the basis of <i>Drosophila</i> -studies, we ought then to tighten precautions several -notches by remembering that we are very likely more -vulnerable than fruit flies are.</p> -<p>Counteracting the depressing nature of this finding was -that of a later, quite unexpected discovery. It was well -established that in fruit flies and other simple organisms, -it was the total dosage of absorbed radiation that counted -and that whether this was delivered quickly or slowly did -not matter.</p> -<div class="pb" id="Page_41">41</div> -<div class="img" id="pic_24"> -<img src="images/p16.jpg" alt="" width="600" height="687" /> -<p class="caption small"><i>Arrangement for long-term -low-dose-rate irradiation -of mice used for mutation-rate -studies at Oak Ridge -National Laboratory. The -cages are arranged at -equal distances from a -cesium-137 gamma-ray -source in the lead pot on -the floor. The horizontal -rod rotates the source.</i></p> -</div> -<p>This proved to be <i>not</i> so in the case of mice. In male -mice, a radiation dose delivered at the rate of 0.009 rad -per minute produced only from one-quarter to one-third -as many mutations as did the same total dose delivered at -90 rads per minute.</p> -<p>In the male, cells in the gonads are constantly dividing -to produce sex cells. The latter are produced by the billions. -It might be, then, that at low radiation dose rates, a -few of the gonad cells are damaged but that the undamaged -ones produce a flood of sperm cells, “drowning out” the few -produced by the damaged gonad cells. The same radiation -dose delivered in a short time might, however, damage so -many of the gonad cells as to make the damaged sex cells -much more difficult to “flood out”.</p> -<p>A second possible explanation is that there is present -within the cells themselves some process that tends to -repair damage to the genes and to counteract mutations. It -might be a slow-working, laborious process that could -keep up with the damage inflicted at low dosage rates but -not at high ones. High dosage rates might even damage the -repair mechanism itself. That, too, would account for the -fewer mutations at low dosage rates than at high ones.</p> -<div class="pb" id="Page_42">42</div> -<p>To check which of the two possible explanations was -nearer the truth, Russell performed similar tests on female -mice. In the female mouse (or the female human -being, for that matter) the egg cells have completed almost -all their divisions before the female is born. There are -only so many cells in the female gonads that can give rise -to egg cells, and each one gives rise to only a single egg -cell. There is no possibility of damaged egg cells being -drowned out by floods of undamaged ones because there -are no floods.</p> -<p>Yet it was found that in the female mouse the mutation -rate also dropped when the radiation dose rate was decreased. -In fact, it dropped even more drastically than was -the case in the male mouse.</p> -<p>Apparently, then, there must be actual repair within the -cell. There must be some chemical mechanism inside the -cell capable of counteracting radiation damage to some -extent. In the female mouse, the mutation rate drops very -low as the radiation dose rate drops, so that it would seem -that almost all mutations might be repaired, given enough -time. In the male, the mutation rate drops only so far and -no farther, so that some mutations (about one-third is the -best estimate so far) cannot be repaired.</p> -<p>If this is also true in the human being (and it is at least -reasonably likely that it is), then the greater vulnerability -of our genes as compared with those of fruit flies is at -least partially made up for by our greater ability to repair -the damage.</p> -<p>This opens a door for the future, too. The workings of the -gene-repair mechanism ought (it is to be hoped) eventually -to be puzzled out. When it is, methods may be discovered -for reinforcing that mechanism, speeding it, and increasing -its effectiveness. We may then find ourselves no longer -completely helpless in the face of genetic damage, or even -of radiation sickness.</p> -<p>On the other hand, it is only fair to point out that the -foregoing appraisal may be an over-optimistic view. Russell’s -experiments involved just 7 genes and it is possible -that these are not representative of the thousands that -exist altogether. While the work done so far is most suggestive -<span class="pb" id="Page_43">43</span> -and interesting, much research remains to be -carried out.</p> -<p>If, then, we cannot help hoping that natural devices for -counteracting radiation damage may be developed in the -future, we must, for the present, remain rigidly cautious.</p> -<h3 id="c20">Conclusion</h3> -<p>It is unrealistic to suppose that all sources of man-made -radiation should be abolished. The good they do now, the -greater good they will do in the future, cannot be abandoned. -It is, however, reasonable to expect that the present -Nuclear Test Ban Treaty will continue and that nations, -such as France and China, which have nuclear capabilities -but are not signatories of the Treaty will eventually sign. -It is also reasonable to expect that X ray diagnosis and -therapy will be carried on with the greatest circumspection, -and that the use of radiation in industry and research -will be carried on with great care and with the use of ample -shielding.</p> -<div class="img" id="pic_25"> -<img src="images/p17.jpg" alt="" width="600" height="699" /> -<p class="caption small"><i>A film badge (left) and a personal radiation monitor -(right) record the amount of radiation absorbed by -the wearer. These safety devices, worn by persons -working in radiation environments, are designed to -keep a constant check on each individual’s absorbed -dose and to prevent overexposure.</i></p> -</div> -<div class="pb" id="Page_44">44</div> -<p>As long as man-made radiation exists, there will be some -absorption of it by human beings. The advantages of its use -in our modern society are such that we must be prepared to -pay some price. This is not a matter of callousness. We -have come to depend a great deal for comfort and even for -extended life, upon the achievements of our technology, and -any serious crippling of that technology will cost us lives. -An attempt must be made to balance the values of radiation -against its dangers; we must balance lives against lives. -This involves hard judgments.</p> -<p>Those working under conditions of greatest radiation -risk—in atomic research, in industrial plants using isotopes, -and so on—can be allowed to set relatively high -limits for total radiation dosages and dose rates that they -may absorb (with time) with reasonable safety, but such -rates will never do for the population generally. A relative -few can voluntarily endure risks, both somatic and genetic, -that we cannot sanely expect of mankind as a whole.<a class="fn" id="fr_9" href="#fn_9">[9]</a></p> -<p>From fruit fly experiments it would seem that a total -exposure of 30 to 100 rads of radiation will double the spontaneous -mutation rate. So much radiation and such a doubling -of the rate would be considered intolerable for humanity.</p> -<p>Some geneticists have recommended that the average -total exposure of human beings in the first 30 years of life -be set at 10 rads. Note that this figure is set as a <i>maximum</i>. -Every reasonable method, it is expected, will be -used to allow mankind to fall as far short of this figure as -possible. Note also that the 10-rad figure is an <i>average</i> -maximum. The exposure of some individuals to a greater -total dose would be viewed as tolerable for society if it -were balanced by the exposure of other individuals to a -lesser total dose.</p> -<p>A total exposure of 10 rads might increase the overall -mutation rate, it is roughly estimated, by 10%. This is -serious enough, but is bearable if we can convince ourselves -<span class="pb" id="Page_45">45</span> -that the alternative of abandoning radiation technology -altogether will cause still greater suffering.</p> -<p>A 10% increase in mutation rate, whatever it might mean -in personal suffering and public expense, is not likely to -threaten the human race with extinction, or even with -serious degeneration.</p> -<p>The human race as a whole may be thought of as somewhat -analogous to a population of dividing cells in a growing -tissue. Those affected by genetic damage drop out and the -slack is taken up by those not affected.</p> -<p>If the number of those affected is increased, there would -come a crucial point, or threshold, where the slack could -no longer be taken up. The genetic load might increase to -the point where the species as a whole would degenerate -and fade toward extinction—a sort of “racial radiation -sickness”.</p> -<p>We are not near this threshold now, however, and can, -therefore, as a species, absorb a moderate increase in -mutation rate without danger of extinction.</p> -<p>On the other hand, it is <i>not</i> correct to argue, as some do, -that an increase in mutation rate might be actually beneficial. -The argument runs that a higher mutation rate might -broaden the gene pool and make it more flexible, thus -speeding up the course of evolution and hastening the -advent of “supermen”—brainier, stronger, healthier than -we ourselves are.</p> -<p>The truth seems to be that the gene pool, as it exists -now, supplies us with all the variability we need for the -effective working of the evolutionary mechanism. That -mechanism is functioning with such efficiency that broadening -the gene pool cannot very well add to it, and if the -hope of increased evolutionary efficiency were the only -reason to tolerate man-made radiation, it would be insufficient.</p> -<p>The situation is rather analogous to that of a man who -owns a good house that is heavily mortgaged. If he were -offered a second house with a similar mortgage, he would -have to refuse. To be sure, he would have twice the number -of houses, but he would not need a second house since -he has all the comfort he can reasonably use in his first -<span class="pb" id="Page_46">46</span> -house—and he would not be able to afford a second -mortgage.</p> -<p>What humanity must do, if additional radiation damage is -absolutely necessary, is to take on as little of that added -damage as possible, and not pretend that any direct benefits -will be involved. Any pretense of that sort may well -lure us into assuming still greater damage—damage we -may not be able to afford under any circumstances and -for any reason.</p> -<p>Actually, as the situation appears right now, it is not -likely that the use of radiation in modern medicine, research, -and industry will overstep the maximum bounds -set by scientists who have weighed the problem carefully. -Only nuclear warfare is likely to do so, and apparently -those governments with large capacities in this direction -are thoroughly aware of the danger and (so far, at least) -have guided their foreign policies accordingly.</p> -<div class="pb" id="Page_47">47</div> -<h2 id="c21">SUGGESTED REFERENCES</h2> -<h3 id="c22">Books</h3> -<p class="book"><i>Radiation, Genes, and Man</i>, Bruce Wallace and Theodosius Dobzhansky, -Holt, Rinehart and Winston, Inc., New York 10017, -1963, 205 pp., $5.00 (hardback); $1.28 (paperback).</p> -<p class="book"><i>Genetics in the Atomic Age</i> (second edition), Charlotte Auerbach, -Oxford University Press, Inc., Fair Lawn, New Jersey 07410, -1965, 111 pp., $2.50.</p> -<p class="book"><i>Atomic Radiation and Life</i> (revised edition), Peter Alexander, Penguin -Books, Inc., Baltimore, Maryland 21211, 1966, 288 pp., -$1.65.</p> -<p class="book"><i>The Genetic Code</i>, Isaac Asimov, Grossman Publishers, Inc., The -Orion Press, New York 10003, 1963, 187 pp., $3.95 (hardback); -$0.60 (paperback) from the New American Library of World -Literature, Inc., New York 10022.</p> -<p class="book"><i>Radiation: What It Is and How It Affects You.</i> Ralph E. Lapp and -Jack Schubert, The Viking Press, New York 10022, 1957, 314 pp., -$4.50 (hardback); $1.45 (paperback).</p> -<p class="book"><i>Report of the United Nations Scientific Committee on the Effects of -Atomic Radiation</i>, General Assembly, 19th Session, Supplement -No. 14 (A/5814), United Nations, International Documents Service, -Columbia University Press, New York 10027, 1964, 120 pp., -$1.50.</p> -<p class="book"><i>The Effects of Nuclear Weapons</i>, Samuel Glasstone (Ed.), U. S. -Atomic Energy Commission, 1962, 730 pp., $3.00. Available -from the Superintendent of Documents, U. S. Government Printing -Office, Washington, D. C. 20402.</p> -<p class="book"><i>Effect of Radiation on Human Heredity</i>, World Health Organization, -International Documents Service, Columbia University Press, -New York 10027, 1957, 168 pp., $4.00.</p> -<p class="book"><i>The Nature of Radioactive Fallout and Its Effects on Man</i>, Hearings -before the Special Subcommittee on Radiation of the Joint Committee -on Atomic Energy, Congress of the United States, 85th -Congress, 1st Session, U. S. Government Printing Office, 1957, -Volume I, 1008 pp., $3.75; Volume II, 1057 pp., $3.50. Available -from the Office of the Joint Committee on Atomic Energy, Congress -of the United States, Senate Post Office, Washington, -D. C. 20510.</p> -<p class="book"><i>Genetics, Radiobiology, and Radiology</i>, Proceedings of the Midwestern -Conference, Wendell G. Scott and Evans Titus, Charles -C. Thomas Publisher, Springfield, Illinois 62703, 1959, 166 pp., -$5.50.</p> -<h3 id="c23">Articles</h3> -<p class="book">Genetic Hazards of Nuclear Radiations, Bentley Glass, <i>Science</i>, -126: 241 (August 9, 1957).</p> -<p class="book">Genetic Loads in Natural Populations, Theodosius Dobzhansky, -<i>Science</i>, 126: 191 (August 2, 1957).</p> -<div class="pb" id="Page_48">48</div> -<p class="book">Radiation Dose Rate and Mutation Frequency, W. L. Russell and -others, <i>Science</i>, 128: 1546 (December 19, 1958).</p> -<p class="book">Ionizing Radiation and the Living Cell, Alexander Hollaender and -George E. Stapleton, <i>Scientific American</i>, 201: 95 (September -1959).</p> -<p class="book">Radiation and Human Mutation, H. J. Muller, <i>Scientific American</i>, -193: 58 (November 1955).</p> -<p class="book">Ionizing Radiation and Evolution, James F. Crow, <i>Scientific American</i>, -201: 138 (September 1959).</p> -<h3 id="c24">Motion Pictures</h3> -<p class="book"><i>Radiation and the Population</i>, 29 minutes, sound, black and white, -1962. Produced by the Argonne National Laboratory. This film -explains how radiation causes mutations and how these mutations -are passed on to succeeding generations. Mutation research -is illustrated with results of experimentation on generations -of mice. A discussion of work with fruit flies and induced -mutations is also included. This film is available for loan without -charge from the AEC Headquarters Film Library, Division -of Public Information, U. S. Atomic Energy Commission, Washington, -D. C. 20545 and from other AEC film libraries.</p> -<p>The following films were produced by the American Institute of -Biological Sciences and may be rented from the Text-Film Division, -McGraw-Hill Book Company, 330 West 42nd Street, New -York 10036.</p> -<p class="book"><i>Mutation</i>, 28 minutes, sound, color, 1962. This film discusses -chromosomal and genetic mutations as applied to man. Muller’s -work in inducing mutations by X rays is described.</p> -<p>These three films are 30 minutes long, have sound, are in black -and white, and were released in 1960. They are part of a 48-film -series that is correlated with the textbook, <i>Principles of Genetics</i>, -(fifth edition), Edmund W. Sinnott, L. C. Dunn, and Theodosius -Dobzhansky, McGraw-Hill Book Company, 1958, 459 pp., $8.50.</p> -<p class="book"><i>Mutagen-Induced Gene Mutation.</i> The narrator of this film is -Hermann J. Muller, who won a Nobel Prize in 1946 for his work -in the field of genetics. The measurement of X-ray dose in -roentgens and the dose required to double the spontaneous mutation -rate in <i>Drosophila</i> and mice are discussed. The magnitude -and meaning of permissible doses of high-energy radiation are -discussed. Other mutagenic agents (ultraviolet light and chemical -substances) are discussed, concluding with comments on the -importance of gene mutation in the present and future.</p> -<p class="book"><i>Selection, Genetic Death and Genetic Radiation Damage.</i> The narrator -of this film is Theodosius Dobzhansky, the coauthor of -this booklet. Genetic death is discussed in detail, as are examples -of how genetic loads are changed subsequent to radiation -exposure. While it is generally agreed that the great majority -of mutants are harmful when homozygous, more evidence is -needed about the beneficial and detrimental effects of mutants -<span class="pb" id="Page_49">49</span> -when heterozygous. In the case of sickle cell anemia, heterozygotes -are adaptively superior to normal homozygotes. This -makes for balanced polymorphism, by which a gene is retained -in the population despite its lethality when homozygous because -of the advantage it confers when heterozygous.</p> -<p class="book"><i>Gene Structure and Gene Action.</i> The lecturer of this film is G. W. -Beadle of Cornell University. The Watson-Crick structure of -DNA is discussed in terms of mutation. Several tests of the -chain separation hypothesis for DNA replication are described -(experiments with heavy DNA, radioactive chromosomes, and -the replication of DNA in vitro). This working hypothesis is -presented: The coded information in DNA is transferred to -RNA, which serves as a template for polypeptide synthesis.</p> -<table class="center" summary=""> -<tr class="th"><th colspan="2">PHOTO CREDITS</th></tr> -<tr><td colspan="2" class="l">Dr. Asimov’s photograph by David R. Phillips, courtesy <i>Chemical and Engineering News</i></td></tr> -<tr class="th"><th>Page</th></tr> -<tr><td class="l"><a href="#Page_4">4</a> </td><td class="l">James German, M.D.</td></tr> -<tr><td class="l"><a href="#Page_6">6</a> </td><td class="l">Bausch & Lomb, Inc.</td></tr> -<tr><td class="l"><a href="#Page_12">12</a> </td><td class="l">James German, M.D.</td></tr> -<tr><td class="l"><a href="#Page_20">20</a> </td><td class="l">Indiana University</td></tr> -<tr><td class="l"><a href="#Page_24">24</a> </td><td class="l">Robert C. Filz, Air Force Cambridge Research Laboratories</td></tr> -<tr><td class="l"><a href="#Page_25">25</a> </td><td class="l">J. K. Boggild, Niels Bohr Institute, Copenhagen University</td></tr> -<tr><td class="l"><a href="#Page_26">26</a> </td><td class="l">Brookhaven National University</td></tr> -<tr><td class="l"><a href="#Page_28">28</a>, <a href="#Page_31">31</a> </td><td class="l">Herman Yagoda, Air Force Cambridge Research Laboratories</td></tr> -<tr><td class="l"><a href="#Page_41">41</a> </td><td class="l">Oak Ridge National Laboratory</td></tr> -</table> -<h2 id="c25">Footnotes</h2> -<div class="fnblock"><div class="fndef"><a class="fn" id="fn_1" href="#fr_1">[1]</a>For more detail about cell division, see <i>Radioisotopes and Life -Processes</i>, another booklet in this series. -</div><div class="fndef"><a class="fn" id="fn_2" href="#fr_2">[2]</a>This is more commonly known as “Mongolism” or “Mongolian -idiocy” though it has nothing to do with the Mongolian people. -</div><div class="fndef"><a class="fn" id="fn_3" href="#fr_3">[3]</a>Actually, all waves have some of the characteristics of particles -and all particles have some of the characteristics of waves. -Usually, however, the radiation is predominantly one or the other -and little confusion arises under ordinary circumstances in speaking -of waves and particles as though they were separate phenomena. -</div><div class="fndef"><a class="fn" id="fn_4" href="#fr_4">[4]</a>For more about this subject, see <i>Radioisotopes in Industry</i> and -<i>Radioisotopes in Medicine</i>, companion booklets in this series. -</div><div class="fndef"><a class="fn" id="fn_5" href="#fr_5">[5]</a>For more about this subject, see <i>Fallout from Nuclear Tests</i>, -another booklet in this series. -</div><div class="fndef"><a class="fn" id="fn_6" href="#fr_6">[6]</a>For details on <i>somatic</i> effects of radiation, see <i>Your Body and -Radiation</i>, a companion booklet in this series. -</div><div class="fndef"><a class="fn" id="fn_7" href="#fr_7">[7]</a>Estimated average exposures to the gonads, based on 1963 report of Federal Radiation Council. -</div><div class="fndef"><a class="fn" id="fn_8" href="#fr_8">[8]</a>One thousandth of a rem. -</div><div class="fndef"><a class="fn" id="fn_9" href="#fr_9">[9]</a>Nevertheless, it should be pointed out that the precautions taken -in the atomic energy industry are such that absorption of radiation -is not as severe a problem as one might suspect. Fully 95% of -those engaged in this work receive less than 1 rem a year. Only -1% receive more than 5 rems. -</div> -</div> -<hr /> -<h3 id="c26"><span class="ss">UNITED STATES ATOMIC ENERGY COMMISSION</span></h3> -<dl class="undent"><dt><i>Dr. Glenn T. Seaborg, Chairman</i></dt> -<dt><i>James T. Ramey</i></dt> -<dt><i>Dr. Gerald F. Tape</i></dt> -<dt><i>Dr. Samuel M. Nabrit</i></dt> -<dt><i>Wilfrid E. Johnson</i></dt></dl> -<h4><i><span class="ss"><span class="small">ONE OF A SERIES ON</span></span></i> -<br /><i><span class="ss">UNDERSTANDING THE ATOM</span></i></h4> -<p>Nuclear energy -is playing a vital role -in the life of -every man, woman, and child -in the United States today. -In the years ahead -it will affect increasingly -all the peoples of the earth. -It is essential -that all Americans -gain an understanding -of this vital force if -they are to discharge thoughtfully -their responsibilities as citizens -and if they are to realize fully -the myriad benefits -that nuclear energy -offers them.</p> -<p>The United States -Atomic Energy Commission -provides this booklet -to help you achieve -such understanding.</p> -<div class="img" id="pic_26"> -<img src="images/p21.jpg" alt="Edward J. Brunenkant" width="300" height="99" /> -</div> -<div class="verse"> -<p class="t0">Edward J. Brunenkant</p> -<p class="t0">Director</p> -<p class="t0">Division of Technical Information</p> -</div> -<p class="tb">This booklet is one of the “Understanding the Atom” -Series. Comments are invited on this booklet and others -in the series; please send them to the Division of Technical -Information, U. S. Atomic Energy Commission, Washington, -D. C. 20545.</p> -<p>Published as part of the AEC’s educational assistance -program, the series includes these titles:</p> -<dl class="undent"><dt>NUCLEAR POWER AND MERCHANT SHIPPING</dt> -<dt>PLUTONIUM</dt> -<dt>OUR ATOMIC WORLD</dt> -<dt>NUCLEAR ENERGY FOR DESALTING</dt> -<dt>CONTROLLED NUCLEAR FUSION</dt> -<dt>WHOLE BODY COUNTERS</dt> -<dt>PLOWSHARE</dt> -<dt>POPULAR BOOKS ON NUCLEAR SCIENCE</dt> -<dt>SNAP, NUCLEAR SPACE REACTORS</dt> -<dt>NUCLEAR REACTORS</dt> -<dt>ATOMS, NATURE, AND MAN</dt> -<dt>MICROSTRUCTURE OF MATTER</dt> -<dt>SYNTHETIC TRANSURANIUM ELEMENTS</dt> -<dt>COMPUTERS</dt> -<dt>RESEARCH REACTORS</dt> -<dt>GENETIC EFFECTS OF RADIATION</dt> -<dt>POWER FROM RADIOISOTOPES</dt> -<dt>NONDESTRUCTIVE TESTING</dt> -<dt>RARE EARTHS</dt> -<dt>FOOD PRESERVATION BY IRRADIATION</dt> -<dt>FALLOUT FROM NUCLEAR TESTS</dt> -<dt>RADIOACTIVE WASTES</dt> -<dt>RADIOISOTOPES IN INDUSTRY</dt> -<dt>ATOMS AT THE SCIENCE FAIR</dt> -<dt>RADIOISOTOPES AND LIFE PROCESSES</dt> -<dt>ATOMIC FUEL</dt> -<dt>ATOMIC POWER SAFETY</dt> -<dt>DIRECT CONVERSION OF ENERGY</dt> -<dt>CAREERS IN ATOMIC ENERGY</dt> -<dt>RADIOISOTOPES IN MEDICINE</dt> -<dt>ACCELERATORS</dt> -<dt>NUCLEAR TERMS, A BRIEF GLOSSARY</dt> -<dt>NEUTRON ACTIVATION ANALYSIS</dt> -<dt>ATOMS IN AGRICULTURE</dt> -<dt>POWER REACTORS IN SMALL PACKAGES</dt></dl> -<p>Single copies of any booklet may be obtained free by -writing to:</p> -<p class="center"><span class="ss">USAEC, P. O. BOX 62, OAK RIDGE, TENNESSEE<span class="hst"> 37830</span></span></p> -<p>Requests for more than three titles generally can not be -honored.</p> -<p>Complete sets of the series are available to school and -public librarians, and to teachers who can make them -available for reference or for use by groups. Requests -should be made on school or library letterheads and indicate -the proposed use.</p> -<p>Students and teachers who need publications on specific -topics related to nuclear science, or references to other -reading material, may also write to the Oak Ridge address. -Requests should state the topic of interest exactly, and the -use intended.</p> -<p><span class="u">IMPORTANT</span>: All requests should include the “Zip Code” -in the address to which the material is to be mailed.</p> -<p class="tbcenter">Printed in the United States of America</p> -<hr /> -<p class="center">USAEC Division of Technical Information Extension, Oak Ridge, Tennessee -<br />September 1966</p> -<h2>Transcriber’s Notes</h2> -<ul> -<li>Retained publication information from the printed edition: this eBook is public-domain in the country of publication.</li> -<li>Where possible, UTF superscript and subscript numbers are used; some e-reader fonts may not support these characters.</li> -<li>In the text version only, underlined or italicized text is delimited by _underscores_.</li> -<li>In the text version only, superscript text is preceded by caret and delimited by ^{brackets}.</li> -<li>In the text version only, subscripted text is preceded by underscore and delimited by _{brackets}.</li> -<li>In the text version only, added a brief label to each illustration; and for graphs, provided tabular summaries of the data where possible.</li> -</ul> - - - - - - - -<pre> - - - - - -End of the Project Gutenberg EBook of The Genetic Effects of Radiation, by -Isaac Asimov and Theodosius Dobzhansky - -*** END OF THIS PROJECT GUTENBERG EBOOK THE GENETIC EFFECTS OF RADIATION *** - -***** This file should be named 55738-h.htm or 55738-h.zip ***** -This and all associated files of various formats will be found in: - http://www.gutenberg.org/5/5/7/3/55738/ - -Produced by Stephen Hutcheson and the Online Distributed -Proofreading Team at http://www.pgdp.net - -Updated editions will replace the previous one--the old editions will -be renamed. - -Creating the works from print editions not protected by U.S. copyright -law means that no one owns a United States copyright in these works, -so the Foundation (and you!) can copy and distribute it in the United -States without permission and without paying copyright -royalties. 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