Genetic Loads and Their Population Effects 



219 



load. When the heterozygote is inferior to 

 one homozygote, the heterozygous condition 

 increases the rate at which the mutant is 

 eliminated from the gene pool, and the pop- 

 ulation shows unbalanced polymorphism and 

 tends to become genotypically and pheno- 

 typically monomorphic. This component of 

 the genetic load, called the mutational load, 

 is maintained in the population chiefly by 

 recurrent mutation. Experimental evidence 

 in Drosophila 5 and a statistical analysis of 

 data for man ,; support the view that the 

 great majority of point mutants are detri- 

 mental when heterozygous. We shall, there- 

 fore, consider most of the genetic load to 

 be a mutational load. 



Genetic Death 



How is a mutant gene eliminated from the 

 population? It need not be eliminated by 

 the death of an individual, although some- 

 times it is. A more general way to express 

 the removal of a mutant gene from the gene 

 pool is by genetic death — the failure of a 

 mutant-carrying individual to produce de- 

 scendants carrying the mutant. Thus, all an 

 individual's genes, whether normal or mu- 

 tant, suffer genetic death if that individual 

 fails to produce children. Since mutants 

 are stable, they are usually removed from 

 the gene pool by genetic death and only 

 occasionally by mutation. 



A person carrying a dominant lethal like 

 retinoblastoma suffers genetic death (as well 

 as physical death). In this case the mutant 

 gene is eliminated from the population the 

 generation in which it arises; it has, there- 

 fore, only one generation of persistence. A 

 dominant detrimental mutant with a selec- 

 tion coefficient of .2 and, therefore, an adap- 

 tive value of .8 as compared to normal, will 



n Based upon works of H. J. Muller and co-work- 

 ers, C. Stern and co-workers, J. F. Crow and co- 

 workers, I. H. Herskowitz and R. C. Baumiller, 

 and others. 

 11 Based upon an analysis by N. E. Morton. 



persist for five generations, on the average, 

 before suffering genetic death; that is, given 

 a population approximately the same in size 

 for successive generations, in each genera- 

 tion the mutant-containing individual has a 

 20% chance of not transmitting the mutant. 

 After this mutant arises, it sometimes fails 

 to be transmitted the very first generation; 

 it may suffer genetic death at the fifth gen- 

 eration or at the tenth, but. on the average, 

 the mutant persists five generations. The 

 principle of persistence holds even though 

 genetic drift, migration, or other factors 

 cause fluctuations in the frequency of the 

 mutant. 



Consider the fate in the population of a 

 rare recessive lethal gene like the one pro- 

 ducing juvenile amaurotic idiocy. Each 

 time homozygosis for this gene occurs, it 

 results in genetic death, and two mutant 

 genes are eliminated from the gene pool. 

 But consider the fate of heterozygotes which 

 are 600 times more frequent (Chapter 15) 

 and carry 300 times as many of these genes 

 as do homozygotes. Since it is generally 

 true that heterozygotes for a recessive lethal 

 suffer genetic death about two per cent of 

 the time (see p. 195), approximately .02 

 times 600, or twelve, heterozygous people 

 would suffer genetic death, thus involving 

 the removal of 24 genes, twelve of them 

 being the recessive lethal alleles. Accord- 

 ingly, six times as many of these particular 

 recessive lethal genes suffer genetic death 

 in the heterozygote than in the mutant homo- 

 zygote, even though the reduction in repro- 

 ductive potential in the former type is only 

 '.-o of that in the latter. 



It is apparent that the rarer a mutant is, 

 the smaller will be the proportion of all 

 genetic deaths it causes in homozygotes and 

 the larger the proportion in heterozygotes. 

 For rare mutants, then, natural selection re- 

 moves mutant genes primarily via the ge- 

 netic death of heterozygotes, the small 

 amount of detriment being more important 



