Mutational Loads and Their Consequences 



243 



genes are eliminated from the gene pool. 

 But consider the fate of the heterozygotes 

 who are 600 times more frequent (Chapter 

 27), and carry 300 times as many of these 

 genes as do the homozygotes. Since it is 

 generally true that heterozygotes for a reces- 

 sive lethal sufier genetic death about four 

 per cent of the time (see p. 211), approxi- 

 mately .04 X 600, or 24, heterozygous people 

 would suffer genetic death, involving the re- 

 moval of 48 genes, of which 24 would be the 

 recessive lethal allele. Accordingly, 12 times 

 as many of these particular recessive lethal 

 genes suffer genetic death in the heterozy- 

 gote than in the homozygote, even though 

 the reduction in reproductive potential in 

 a group of the former type of individuals is 

 only Yiv, of what it is in a group of the latter 

 type. 



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 that it causes in 

 heterozygotes. For rare mutants, then, 

 natural selection removes mutant genes 

 primarily via the genetic death of heterozy- 

 gotes, the small amount of detriment when 

 heterozygous being more important from the 

 population or gene pool standpoint than the 

 greater detrimental effect when homozygous. 

 However, each mutant is harmful to the 

 population to the same degree, in terms of 

 its effect on reproductive potential, in that 

 each is eventually the cause of a genetic 

 death, at which time it is removed from the 

 gene pool. Thus, hypoploidy which acts as 

 a dominant lethal persists only one genera- 

 tion before it causes a genetic death. A 

 point mutation which produces a reproductive 

 disadvantage of only Ko% will persist, on the 

 average, 1000 generations, at which time it 

 will be the cause of genetic death. 



As a matter of fact, speaking not in terms 

 of biological fitness, but in terms of the total 

 amount of suffering to which a human popu- 

 lation is subject, point mutants with the 



smallest heterozygous detriment are the most 

 harmful type of mutant. Consider, on 

 one hand, the gross chromosomal abnor- 

 mality which kills in utero. This destroys a 

 life early, so that the individual involved has 

 not suffered very long timewise. In this 

 case, also, the parents may suffer relatively 

 httle, for such deaths may occur so early as 

 to result in abortions which pass unnoticed. 

 Consider, on the other hand, the effect of 

 heterozygous point mutants in individuals 

 who are past the reproductive age. These 

 people already have or have not suffered 

 genetic death, but continue, nevertheless, to 

 be subjected to the previously produced, and 

 newly produced, small phenotypic detriment 

 of heterozygosity which adds to their aches, 

 pains, and disease susceptibility. In this 

 respect, then, the mutant with a smaller 

 effect on reproductive potential causes more 

 suffering than one with a larger effect, for the 

 longer the persistence, the more damage that 

 is done in postreproductive life. 



You might at first suppose that the amount 

 of gene-caused human suffering can be re- 

 duced through the practice of medicine. This 

 is true, in terms of the individual being 

 treated. For there is no doubt that the in- 

 dividual who is diabetic for genetic reasons, 

 and who is given insulin, is better off than he 

 would be without this medicine. But re- 

 member that this medicine does not cure 

 the genetic defect. Moreover, the medicine, 

 by increasing the diabetic's reproductive 

 potential, serves to increase the persistence 

 of the mutants involved, so that the genetic 

 death which must eventually occur to remove 

 the mutant is only postponed to a later 

 generation, each additional generation re- 

 quiring the same medication. It would be 

 true that the total amount of human suffering 

 also would be reduced, until the time of 

 genetic extinction of the mutant, if in fact 

 the medicine completely normalized the 

 genetic defect. But to do this would require 

 that the medicine replace the primary prod- 



