NATURE OF THE GENETIC EFFECTS 429 



tically all individuals in any population are having their ability to live 

 and reproduce reduced considerably below that of hypothetical indi- 

 viduals homozygous for normal genes exclusively, and that the greater 

 part of this reduction is usually caused by the collective action of multiple, 

 individually minute, fractional "loads," most of them occasioned by 

 heterozygous mutant genes. Yet even the relatively small part of the 

 total load on an individual which is caused by homozygous genes (again 

 usually minute and unrecognizable in their individual action, even 

 though homozygous) is often quite sizeable. This is shown by the 

 marked increase in size, vigor, fertility, and general viability (including 

 even, as shown in unpublished work of Russell's on mice, resistance to 

 the lethal effects of radiation), which often results from crossing widely 

 different strains — a phenomenon known as heterosis and utilized exten- 

 sively in the production of hybrid corn, poultry, swine, etc. If, then, 

 the mutant genes could have been removed even in their heterozygous 

 state and replaced by normal ones, the results would have been far more 

 remarkable, in view of the fact that much the greater part of the load is 

 usually in this form. Since now these very large differences are mainly 

 due to the cumulative action of numerous mutant gene effects, each of 

 which is so tiny that it cannot be detected individually, it follows that 

 the individual effects are real, significant, and cumulative. 



It is of interest to estimate the total collective magnitude of these efTects 

 per individual in an ordinary population, and the risk of genetic extinc- 

 tion therewith entailed, and then to compare these figures with those, to 

 be superposed upon them, which represent the genetic effects produced by 

 a given amount of radiation. The theoretical calculation is basically a 

 simple one. It has been noted in the foregoing that each mutant gene 

 which passes into the population, no matter how slight its detrimental 

 effect on an individual possessing it, is finally eliminated by reason of 

 that detrimental effect, usually in a heterozygous individual. ■ In con- 

 sequence of this, there must in the long run be about as many genetic 

 deaths per generation in a population as there are mutations arising in it, 

 i.e., 2jLi (if )u represents the mutation frequency in the germ cells), minus the 

 frequency of excess mutant genes contained in cases of what may be 

 termed "overlapping" genetic deaths, explained in the next paragraph. 

 The factor 2 here arises from the fact that a heterozygous individual 

 (homozygotes being here considered to be of negligible frequency) can be 

 affected by a mutant gene received from either one of the two germ cells 

 from which that individual originated. Moreover, looking at the matter 

 conversely, the genetic death of the heterozygote, since he comprises 

 two genomes (sets of genes), one of which has the normal allele, reduces 

 the per-genome and therefore the per-germ-cell frecjuency of the mutant 

 gene by only half as much as it reduces the frequency of individuals bear- 

 ing the mutant gene; hence, in order to effect sufficient gene elimination 



