240 



CHAPTER 28 



FIGURE 28-2. Genetic load in natural 

 populations of D. pseudoobscura. 

 {After Til. Dobzhansky.) 



MUTANT TYPE PER CENT OF CHROMOSOMES 



II III IV 



Lethal or Semilethal 25 25 26 



Subvital / 93 41 95 



Female Sterile 11 ^ ^ 14 4 



Male Sterile 



12 



a particular pair of autosomes are allelic 

 (in which case nearly 6% of zygotes in nature 

 would fail to become adults because of homo- 

 zygosity for the mutants), or whether the 

 mutants involve different loci (in which case 

 6.25% of zygotes would bedihybrid for linked 

 mutants of this kind), or whether some com- 

 bination of these two alternatives obtains. In 

 any case, the chance that both members of a 

 given chromosome pair would be free of le- 

 thals or semilethals would be (0.75)'-, or 56%. 



What portion of the population would 

 carry no lethal or semilethal on either mem- 

 ber of autosomes II, III, and IV? This is 

 calculated to be (0.75)^ X (0.75)^ X (0.75)^, 

 or about 17%. If we consider the fact that 

 the X and V chromosomes can also carry 

 such mutants, the frequency of lethal- 

 semilethal-free individuals in nature is still 

 lower. When, now, the subvital mutants 

 (which are an even more frequent class of 

 mutant) and also sterility mutants are con- 

 sidered, it becomes clear that very few, if any, 

 flies in natural populations are free of a load 

 of detrimental mutants. 



What is the mutant load in man? We 

 have already discussed the fact that the vast 

 majority of mutants are detrimental in 

 homozygous condition (Chapter 24). Death 

 rate data are available for a population in 

 rural France in the last century which include 

 fetal death, and all childhood and very early 

 adult deaths. What we want to compare is 



