~iéand one lost in each generation. *
UCRL- 3644
Thus it has been pointed out that if, through
increase in radiation exposure, the genetic gain of undesirable genes increased
from one per generation to two per generation, there would be a relatively
great reduction in the quality of the best 25% of individuals (assuming that
reduction in quality of offspring is proportional to the number of undesirable
mutations per individual). Because of speculative--but reasonable and
cautious--arguments of this nature, geneticists have uniformly cautioned
against allowing any major proportion of the population to accumulate radiation
as high as 50 r, which is the amount estimated to double the human mutation
rate.
Life-Shortening Effects
Life-shortening effects of radiation have been observed under a
variety of experimental conditions. An experiment of particular significance
because of the large numbers of animals and the range of exposure was the
exposure of mice to nuclear detonation at ''Operation Greenhouse " (Furth
et al.). The fraction of life span lost per unit of radiation exposure appears
to be essentially the same for a number of species, including the mouse, the
rat, the guinea pig, the rabbit, and man. The largest number of experimental
observations concerns the mouse. In the mouse, the fraction of the life span
lost per unit of whole-body radiation exposure is acceptably constant over a
wide range of variation in radiation exposure. The tentative conclusion is
that radiation effect simulates aging itself, and that a unit of radiation exposure,
regardless of the intensity and duration of exposure, produces approximately
the same relative disturbance to body structure in adults of all mammalian
species. On the human life-span scale, these effects of radiation summarized
from small-animal data suggest that 1 roentgen of radiation exposure is equivalent to 5 to 15 days of physiologic aging. This prediction is confirmed directly in man (with reasonable technical reservations) by Doctor Shields
Warren's recent investigation of life span of radiologists compared with
physicians not using radiation in their practice of medicine. The average
age at death is approximately 6 years less for radiologists than for physicians in general practice or for pathologists, both selected as being relatively
unexposed to radiation. The estimation of accumulated radiation exposure
in radiologists is uncertain, but has been approximated as 300 to 500 r. Thus,
The average mutation frequency of 1.5 spontaneous mutations of human
genes per generation, as summarized by Penrose, corresponds to 30 mutations per million genes per generation, assuming that humans have about
50,000 genes:
30 mutations per generation x 50,000 genes per individual _ 1.5 mutations
1,000,000 genes
per individual per generation.
The average mutation rate may be less than this estimate, since one may
suspect that the genes usually observed to mutate are perhaps ten times as
mutable as the average gene.