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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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