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