Mutational Loads and Their Consequences 



241 



the death rate of offspring from unrelated 

 parents with that found from cousin mar- 

 riages.^ The death rate of progeny from un- 

 related parents is .12, while it is .25 from 

 cousin marriages. We shall not be con- 

 cerned here with establishing the genetic or 

 nongenetic cause of death in the normal 

 outcrossed human population. It can be 

 assumed, however, that the excess mortality 

 of .13 (.25 — .12) has a genetic basis in the 

 excess homozygosity which is consequent to 

 cousin marriage. This is a reasonable as- 

 sumption in the absence of any other known 

 nongenetic factor which would tend to cause 

 more or fewer offspring to die from marriages 

 between cousins than from marriages be- 

 tween unrelated parents. (These data would 

 have a nongenetic bias, if, for example, it 

 was a custom — which it is not — that all 

 children from cousin marriages are purposely 

 starved.) 



Apparently, then, 13% more offspring 

 died because their parents were cousins than 

 would have died normally. What is the 

 total amount of recessive lethal effect present 

 in the population in heterozygous condition? 

 This can be calculated as follows. Recall 

 (from Chapter 27) that, of all heterozygous 

 genes, an extra ]u become homozygous in 

 offspring of cousin marriages. Half of the 

 Me, or }^2, must have become homozygous 

 for the normal genes and half of }i6, or )i2, 

 for their abnormal alleles. So, to estimate 

 the total heterozygous content of mutants 

 which would kill if they were homozygous, it 

 is necessary to multiply .13 by 32. The result- 

 ant value of about 4 represents a 400% 

 chance that the ordinary individual carried 

 in heterozygous condition a genetic load of 

 detrimental mutants which would be lethal 

 if homozygous. In other words, on the 

 average, each person carried four lethal 

 equivalents in heterozygous condition, or, 

 four times the amount of detriment it would 



2 Based upon an analysis of N. E. Morton, J. F. Crow, 

 and H. J. Muller. 



take to kill if the genes involved were made 

 homozygous. 



This analysis does not reveal how many 

 genes are involved in the production of the 

 four lethal equivalents. In some individuals 

 these might be due to the presence in het- 

 erozygous condition of 4 recessive lethals, or 

 8 mutants having 50% viability, or 16 mu- 

 tants with 25% viability, etc., or any combi- 

 nation of detrimental mutants whose total 

 is four lethal equivalents. It should be 

 realized that, since the last century, improve- 

 ments in the environment (in housing, 

 nutrition, and medical care) make it most 

 hkely that the effect of these same mutants 

 in present-day society would be expressed by 

 somewhat less than four lethal equivalents. 

 Similarly, in the light of such progress, the 

 detrimental effects of these mutants in het- 

 erozygous condition would be expected to 

 be somewhat less at present than they were 

 a century ago. Accordingly, in the last 

 century, a hypothetical homozygous combi- 

 nation which, because of variable penetrance 

 and expressivity, produced no detectable ef- 

 fect 25% of the time, detrimental effect (but 

 not lethality prior to maturity) 15% of the 

 time, and lethality before maturity 60% of 

 the time, might at the present time have the 

 respective values of 50%, 10%, 40%. In 

 the earlier period this combination would 

 have produced .6 of a lethal equivalent and 

 at present .4. Notice also that the detriment 

 which is not lethal before maturity would 

 also be reduced, changing from 15% to 10%, 

 or, speaking in terms of detrimental equiva- 

 lents, what was .15 would now be .10. It is 

 clear that the genes responsible for lethal 

 equivalents and for detrimental equivalents 

 must be the same, at least in part. 



It is equally clear that present-day man 

 also carries a load of mutants. Some of 

 those transmitted to him arose in his parents 

 (probably two of each five zygotes carry a 

 newly arisen mutant, as mentioned on p. 200), 

 and others arose in his more remote an- 



