Where radiation is insufficient to render a cell incapable of division,
it may still induce mutations, and it is in this fashion that skin
cancer, leukemia, and other disorders may be brought about.[6]
[Illustration: _Studies at the California Institute of Technology
furnish information on the nature of radiation effects on genes. The
experiments produced fruit flies with three or four wings and double or
partially doubled thoraxes by causing gene mutation through
X-irradiation and chromosome rearrangements. A is a normal male_
Drosophila; _B is a four-winged male with a double thorax; and C and D
are three-winged flies with partial double thoraxes._]
[Illustration: Four-winged male with a double thorax]
[Illustration: Three-winged fly with partial double thoraxes]
[Illustration: Three-winged fly with partial double thoraxes]
Mutations can be brought about in the sex cells, too, of course, and
when this happens it is succeeding generations that are affected and not
merely the exposed individual. Indeed, where the sex cells are
concerned, the relatively mild effect of mutation is more serious than
the drastic one of nondivision. A fertilized ovum that cannot divide
eventually dies and does no harm; one that can divide but is altered,
may give rise to an individual with one of the usual kinds of major or
minor physical defects.
The effect of high-energy radiation on the genetic mechanism was first
demonstrated experimentally in 1927 by Muller. Using _Drosophila_ he
showed that after large doses of X rays, flies experienced many more
lethal mutations per chromosome than did similar flies not exposed to
radiation. The drastic differences he observed proved the connection
between radiation and mutation at once.
Later experiments, by Muller and by others, showed that the number of
mutations was directly proportional to the quantity of radiation
absorbed. Doubling the quantity of radiation absorbed doubled the number
of mutations, tripling the one tripled the other, and so on. This means
that if the number of mutations is plotted against the amount of
radiation absorbed, a straight line can be drawn.
It is generally believed that the straight line continues all the way
down without deviation to very low radiation absorptions. This means
there is no “threshold” for the mutational effect of radiation. No
matter how small a dosage of radiation the gonads receive, this will be
reflected in a proportionately increased likelihood of mutated sex cells
with effects that will show up in succeeding generations.
In this respect, the genetic effect of radiation is quite different from
the somatic effect. A small dose of radiation may affect growing tissues
and prevent a small proportion of the cells of those tissues from
dividing. The remaining, unaffected cells take up the slack, however,
and if the proportion of affected cells is small enough, symptoms are
not visible and never become visible. There is thus a threshold effect:
The radiation absorbed must be more than a certain amount before any
somatic symptoms are manifest.
Matters are quite different where the genetic effect is concerned. If a
sex cell is damaged and if that sex cell is one of the pair that goes
into the production of a fertilized ovum, a damaged organism results.
There is no margin for correction. There is no unaffected cell that can
take over the work of the damaged sex cell once fertilization has taken
place.
Suppose only one sex cell out of a million is damaged. If so, a damaged
sex cell will, on the average, take part in one out of every million
fertilizations. And when it is used, it will not matter that there are
999,999 perfectly good sex cells that might have been used—it was the
damaged cell that _was_ used. That is why there is no threshold in the
genetic effect of radiation and why there is no “safe” amount of
radiations insofar as genetic effects are concerned. However small the
quantity of radiation absorbed, mankind must be prepared to pay the
price in a corresponding increase of the genetic load.
[Illustration: Percent lethal chromosomes vs. Amount of x radiation, r]
If the straight line obtained by plotting mutation rate against
radiation dose is followed down to a radiation dose of zero, it is found
that the line strikes the vertical axis slightly above the origin. The
mutation rate is more than zero even when the radiation dose is zero.
The reason for this is that it is the dose of man-made radiation that is
being considered. Even when man-made radiation is completely absent
there still remains the natural background radiation.
It is possible in this manner to determine that background radiation
accounts for considerably less than 1% of the spontaneous mutations that
take place. The other mutations must arise out of chemical
misadventures, out of the random heat-jiggling of molecules, and so on.
These, it can be presumed, will remain constant when the radiation dose
is increased.
This is a hopeful aspect of the situation for it means that, if the
background radiation is doubled or tripled for mankind as a whole, only
that small portion of the spontaneous mutation rate that is due to the
background radiation will be doubled or tripled.
Let us suppose, for instance, that fully 1% of the spontaneous mutations
occurring in mankind is due to background radiation. In that case, the
tripling of the background radiation produced in the United States by
man-made causes (see Table) would triple that 1%. In place of 99
non-radiational mutations plus 1 radiational, we would have 99 plus 3.
The total number of mutations would increase from 100 to 102—an increase
of 2%, not an increase of 200% that one would expect if all spontaneous
mutations were caused by background radiation.
RADIATION EXPOSURES IN THE UNITED STATES[7]
Millirems[8]
Natural Sources
A. External to the body
1. From cosmic radiation 50.0
2. From the earth 47.0
3. From building materials 3.0
B. Inside the body
1. Inhalation of air 5.0
2. Elements found naturally in human tissues 21.0
Total, Natural sources 126.0
Man-made Sources
A. Medical Procedures
1. Diagnostic X rays 50.0
2. Radiotherapy X ray, radioisotopes 10.0
3. Internal diagnosis, therapy 1.0
Subtotal 61.0
B. Atomic energy industry, laboratories 0.2
C. Luminous watch dials, television tubes, 2.0
radioactive industrial wastes, etc.
D. Radioactive fallout 4.0
Subtotal 6.2
Total, man-made sources 67.2
Overall total 193.2