Genetic Loads and Their Population Effects 



217 



MUTANT TYPE PER CENT OF CHROMOSOMES 



II III IV 



Lethal or Semilethal 25 25 26 



Sobvital 93 41 95 



Female Sterile 11 14 4 



Male Sterile 8 11 12 



figure 16-2. Genetic load in natural popula- 

 tions of D. pseudoobscura. {After Th. Dob- 

 zhansky.) 



allelic (in which case up to 6.25% of zy- 

 gotes in nature would be mutant homozy- 

 gotes and fail to become adults), or whether 

 all the mutants involve different loci (in 

 which case 6.25% of zygotes would be hy- 

 brid for linked mutants of this kind), or 

 whether some combination of these alterna- 

 tives is obtained. In any case, the chance 

 that both members of a given chromosome 

 pair are free of lethals or semilethals is 

 (0.75)- or 56%. 



What portion of individuals in the pop- 

 ulation carry no lethal or semilethal on any 

 member of autosomes II, III, and IV? This 

 percentage is calculated as (0.75) 2 times 

 (0.75)- times (0.75)-' or about 17%. 

 However, if one considers the X and V 

 chromosomes which can also carry such 

 mutants, the frequency of lethal-semilethal- 

 free individuals in nature is still lower. 

 Moreover, when the subvital mutants (which 

 comprise the most frequent mutant class de- 

 tected) and the sterility mutants are also 

 considered, it becomes clear that very few, 

 if any, flies in natural populations are free 

 of a detrimental mutant load. 



Genetic Loads in Man 



What is the genetic load in man? The vast 

 majority of mutants are detrimental in homo- 

 zygous condition (as already noted in Chap- 

 ter 15). Since inbreeding increases the fre- 



quency of homozygosis, a comparison of the 

 detriment produced in an inbreeding segment 

 with that in a noninbreeding segment of a 

 human population may provide us with an 

 estimate of the genetic load present in het- 

 erozygous condition. From the population 

 records of a rural French population during 

 the last century listing fetal deaths and all 

 childhood and very early adult deaths we 

 can compare the frequency of death to off- 

 spring of unrelated parents with that of 

 cousin marriages.-' The frequency of death 

 to progeny from unrelated parents was .12, 

 whereas it was .25 from cousin marriages. 

 We are not concerned here with establishing 

 the genetic or nongenetic cause of death in 

 the normal outcrossed human population; 

 however, it can be assumed that the extra 

 mortality of .13 (.25 minus .12) has a ge- 

 netic basis in the extra homozygosity result- 

 ing from cousin marriage. This assumption 

 is reasonable in the absence of any known 

 nongenetic factor that tends to cause death 

 to more or fewer offspring from marriages 

 between cousins than from marriages be- 

 tween unrelated parents. (These data would 

 have a nongenetic bias if, for example, it 

 were the custom — which it was not — that 

 all children from cousin marriages are pur- 

 posely starved.) 



Apparently, then, 13% more offspring 

 died because their parents were cousins. 

 The total amount of recessive lethal effect 

 present in the population in heterozygous 

 condition can be calculated as follows: recall 

 (Chapter 15) that of all heterozygous genes, 

 an extra l / 16 become homozygous in off- 

 spring of cousin marriages. In the model 

 half of the l / 16 , or y 32 , must have become 

 homozygous for the normal genes and half 

 of Y 1G , or y 32 , for their abnormal alleles. 

 Therefore, to estimate the total heterozy- 

 gous content of mutants which would have 

 been lethal if homozygous, it is necessary to 



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

 Crow, and H. J. Muller. 



