428 RADIATION BIOLOGY 



sophila. Hence for detrimental mutant genes also it usually turns out 

 that their effect in heterozygotes is the important factor in determining 

 the damage they do in individuals, and the amount of their persistence 

 in the population. For these reasons we may usually, for practical pur- 

 poses, simplify our calculation by using for the value of i the amount of 

 impairment in heterozygotes alone, and by taking p as the reciprocal of 

 this value of i. 



Individual mutant genes that have been produced by irradiation of 

 individuals of a given generation are very seldom to be detected by 

 means of recognizable effects in the offspring, or indeed in any later gen- 

 eration descendants, in any mixed population like a human one, which 

 undergoes relatively little inbreeding. This is because of a combination 

 of reasons. First is the rarity with which the mutant genes become 

 homozygous and thereby more marked in their effects; in fact, those 

 whose effects when homozygous would be especially marked and recog- 

 nizable are the very ones which tend to be eliminated sooner as hetero- 

 zygotes and therefore to occur least often as homozygotes. Second is the 

 fact that, even as homozygotes, most mutant genes have effects that are 

 not readily detected. Third, their effects in the heterozygous state, that 

 in which they usually exist, are much weaker and less recognizable still, 

 commonly involving, as far as the ordinary observer is concerned, only a 

 small quantitative difference from the phenotype that would otherwise be 

 present. Fourth, these slight effects are superimposed upon the innumer- 

 able variations which would be present anyhow in any such population. 

 These other variations are caused both by the segregation and recombina- 

 tion of the many mutant genes accumulated from scores of past genera- 

 tions during which spontaneous mutations have been occurring, and also 

 by the operation of environmental factors, such as disease, nutritional 

 differences, mode of life, etc. For these reasons it is doubtful whether it 

 would be practicable, even by the large-scale study of such populations, 

 to demonstrate that mutations had been induced in them, even if these 

 mutations had been relatively abundant and, in their collective effect 

 over the course of many generations, highly damaging. 



It is sometimes assumed that a "unit" genetic load which is dis- 

 tributed in very small fractions among very many individuals may be 

 regarded as of negligible consequence, in comparison, for example, with a 

 single case of major affliction leading to premature death. However, this 

 idea is an erroneous one. It is true that the amount of- actual damage 

 done to life or reproduction in cases of genetic impairment is often subject 

 to a high statistical variation, so that in cases of slight impairment the 

 damage is for some individuals nil, but such cases are compensated for by 

 others in which the individual incurs more damage than that calculated, 

 so that the average amount of damage is maintained. The reality of the 

 small effects may be better realized when the fact is considered that prac- 



