18 BIOLOGICAL EFFECTS OF ATOMIC RADIATION 



On the Appraisal of Genetic Effects of Radiation in Man 



By Sewall Wright 



Any attempt at appraisal of genetic damage to man from ionizing radiations must begin 

 with the problems of defining what is meant by genetic damage in the human case and of 

 measuring it. Only when these are answered can there be any concrete estimate of the current 

 genetic burden. Finally there must be an estimate of the amount of increase in radiation re- 

 quired to double the mutation rate in man before the impact of a given increase in exposure 

 to radiation for one generation or for a succession of generations on the immediate and 

 ultimate genetic burden can be appraised. 



There is no question that all that can be learned about mutation and the effect of muta- 

 tion in lower forms is pertinent. Without the evidence already obtained from such research on 

 the relation of dosage to mutation rate in organisms in general, there would be no possibility of 

 an early appraisal of the effects in man. It is to be hoped that further fundamental research 

 will strengthen our knowledge of this relation and perhaps also lead to ways of mitigating the 

 effects. This presentation will, however, be restricted to needed research in man. 



Some of the researches needed are unfortunately in the unpopular and scientifically 

 somewhat unrewarding borderline fields of genetics and the social sciences. Progress in this 

 aspect of the general problem is likely to be slower than that in the scientifically more at- 

 tractive fields that have a less direct bearing on it. 



There is one point of view under which the appraisal of genetic damage from increased 

 radiation is a relatively simple matter. If we assume that there is one best genotype and that 

 this is homozygous in all type genes, it follows that all mutational changes from this are in- 

 jurious and selected against. For each mutation there will be on the average one elimination 

 (or "genetic death") to restore the status quo (in a static population; more than one in a 

 growing population). If we define damage in terms of number of genetic deaths, it follows 

 that all mutations produce equal damage in the long run and it merely becomes necessary to 

 estimate the number of mutations produced by a given amount of radiation to appraise the 

 damage. 



There are, however, several considerations that make this point of view unsatisfactory. 



In the first place, the concept of a single type genotype probably does not apply to any 

 organism and particularly not to human populations in which extreme diversity is itself es- 

 sential to a healthy state of society. It is probable that the optimal state of any population is 

 one in which many alleles with slight differential effect are carried at a large proportion of all 

 loci at more or less equal frequencies. Even conspicuously unfavorable effects of mutations in 

 particular combinations may be balanced by favorable effects in others. 



In the next place, the equating of all unequivocally injurious mutations is very unrealistic 

 without consideration of the personal and social impact. It will perhaps suffice here to note 

 that the occurrence of a dominant mutation, lethal in the first week of development, will pro- 

 duce no appreciable damage to the population or to any one in it. There will be no appre- 

 ciable damage to society and little to any person from a mutation that causes a slight reduction 

 in fecundity of otherwise wholly normal carriers in a population that is in balance with its 

 natural resources, and there may be some advantage to a society that is suffering from over- 

 population. On the other hand, a dominant mutation that gives rise to a distressing and inca- 



