Chapter *16 



GENETIC LOADS AND 

 THEIR POPULATION EFFECTS 



Genetic Loads in Drosophila 



The fruit fly, Drosophila pseudoobscura, is 

 commonly found in northern Mexico and 

 the western United States. When collected 

 in the wild, almost all its individuals are 

 phenotypically alike, except for the sex 

 differences, appearing wild-type or normal. 

 We cannot accept this phenotypic uniform- 

 ity as evidence of genotypic uniformity, how- 

 ever, since a Drosophila population appear- 

 ing wild-type can conceal considerable 

 genetic variability in the form of isoalleles, 

 recessive point mutants, reciprocal transloca- 

 tions, paracentric inversions, and so on. We 

 would like to estimate the genetic load — the 

 total amount of this genetic variability pres- 

 ent in a natural population of D. pseudo- 

 obscura. 1 



D. pseudoobscura has five pairs of chro- 

 mosomes — the usual X and Y sex chromo- 

 somes, three pairs of large rod-shaped auto- 

 somes (II, III, IV), and a dotlike pair of 

 autosomes (V) (Figure 16-1). Numerous 

 laboratory strains of this species are avail- 

 able whose autosomes are marked by various 

 point and rearrangement mutants. We can, 

 therefore, make a suitable series of crosses 

 between laboratory strains and flies collected 

 in the wild which will yield information on 

 the presence of autosomal mutants in the 

 wild-type flies. In practice, autosomes II, 

 III, and IV of individual wild-type flies are 

 made homozygous to detect the presence of 



the following recessive mutants (see Figure 

 6-2): 



1. Lethal (causing death to all individuals 

 before adulthood) or semilethal (caus- 

 ing more than ninety and less than one 

 hundred per cent mortality before 

 adulthood) 



2. Subvital (causing significantly less than 

 normal but greater than ten per cent 

 survival to adulthood ) 



3. Female sterile (sterile to females) 



4. Male sterile (sterile to males). 



The results of this study are summarized 

 in Figure 16-2. About 25% of all auto- 

 somes tested this way carry a recessive lethal 

 or semilethal mutant. Recessive subvital 

 mutants are found in about 40% of III chro- 

 mosomes tested and in more than 90% of 

 II's and IV's tested; mutants causing sterility 

 are present in 4 to 14% of tested chromo- 

 somes. Obviously the natural population 

 carries a tremendous load of detrimental 

 mutants. 



How is this load of mutants distributed 

 in the fly population? Consider first one 

 pair of the autosomes tested. Each member 

 has a 25% chance of carrying a lethal or 

 semilethal and a 75% chance of being free 

 of such mutants. The chance that both 

 members of a pair of chromosomes will 

 carry a lethal or semilethal is (0.25 )- or 

 6.25% . From the data presented we cannot 

 tell whether all the lethals and semilethals 

 found in a particular pair of autosomes are 



1 The following is based upon 

 Dobzhansky and collaborators. 



216 



work of Th. 



FIGURE 16-1. 

 Chromosomal 

 complement of 

 D. pseudoobscura. 



