NATURE OF THE GENETIC EFFECTS 425 



certain proportion of the offspring of individuals heterozygous for them 

 receiving aneuploid chromosome combinations which kill off these off- 

 spring in embryonic stages, and this lowered productivity reduces, from 

 generation to generation, the relative number of individuals carrying such 

 aberrations, until those aberrations which arose in any given generation 

 have finally become eliminated from the population. Although in 

 primates and especially in civilized man this elimination is, as previously 

 explained, much slower than in most organisms, it is even in them prob- 

 ably more rapid, especially for translocations, which constitute the great 

 majority of these aberrations, than the rate of elimination (discussed in 

 the following pages) of most mutant genes. At the same time, in mam- 

 mals the heterozygous carriers of the translocations and inversions prob- 

 ably do not suffer from any somatic ill effects caused by their aberrations, 

 since in mammals, as in most organisms studied, chromosome changes are 

 not likely to be associated with position effects at all, much less with 

 dominant position effects. 



Elimination of the above type, occurring only through death of 

 embryos, constitutes less of a burden on any population than when, as in 

 the case of most mutant genes, the elimination occurs in the later stages 

 of the abnormal individuals. For the older abnormals prior to their 

 elimination engage in a competition with the normals which is more 

 detrimental to the latter. Moreover, from a human viewpoint, the 

 ehmination of embryos is also less objectionable than that of older indi- 

 viduals because of the fact that in such cases the abnormals die before 

 they themselves have had a chance to suffer consciously from the effects 

 of their abnormality. 



Gene mutations are not only individually more damaging and objec- 

 tionable than gross chromosome aberrations, for the reasons given, but 

 they are also far more frequent and more diverse in their phenotypic 

 expressions, both when they occur spontaneously and when they are 

 produced by irradiation — except in the special case of irradiation of 

 mature spermatozoa, when chromosome aberrations arise with a fre- 

 quency comparable to theirs. It is therefore appropriate to consider 

 in some detail the manner in which the resulting mutant genes affect the 

 individuals inheriting them and the population in general throughout the 

 course of any number of generations. 



There are usually many circumstances, both nongenetic and genetic, 

 besides the possession of some given mutant gene which decide how many 

 offspring an individual produces, and each offspring has only a 50 per 

 cent chance of receiving from its parent any mutant gene for which the 

 latter is heterozygous. Since this remains true generation after genera- 

 tion, a given mutant may in the course of time become more or less multi- 

 phed in the population or may become eliminated from it, quite apart 

 from any detriment or benefit conferred by the gene. However, those 



