GENETIC FACTORS IN THE ORIGIN OF DIVERSITY 389 



y^ (black) + U (white) 



3| (short) + I4 (lonff) 



51 6 (black, short) + Ke (white, short) + ^f e (black, long) + Ke (white, long) 



When additional pairs of contrasting characteristics are considered, still 

 more complicated ratios are obtained. Another pair of characteristics in 

 guinea pigs is rough hair versus smooth hair. The gene for rough {R) is 

 dominant. Thus when heterozygous rough guinea pigs are mated together 

 % of the offspring are expected to be rough-haired, ^/^ smooth. What will 

 be expected when triply; heterozygous black, short, rough (BbSsRr) 

 guinea pigs are mated together? We may answer the question by the 

 checkerboard method (c.f.. Fig. 17.6), noting that in this case three pairs 

 of chromosomes are involved, and that the females will produce eight 

 types of ova, the males eight types of sperms. Or we may answer the 

 question by multiplying the 9:3:3:1 ratio already obtained by another 

 3:1 ratio: 



%6 (black, short) + ^i^ (white, short) + ^le (black, long) + ife (white, long) 

 % (rough) + K (smooth) 



The reader will find working this out completely an instructive exercise. 

 We may note that the first and largest item will consist of offspring showing 

 all three dominants (black, short, rough) and that %y; • % or -%4 of the 

 offspring will be expected to be of this type. 



Still more complicated ratios result when more than three pairs of 

 contrasting characteristics are being considered. Since in actuality every 

 individual is heterozygous for many pairs of genes (p. 466) the amount 

 of genetic diversity produced by the "shuffling" of chromosomes, with their 

 contained genes, is enormous. This is part of the raw material for evolu- 

 tionary change. 



Lethal Genes 



We note that these more complicated ratios are elaborations of the 1:2:1 

 ratio (and its modification the 3:1 ratio). These ratios may be modified in 

 various ways. For example, one homozygote or the other may be lethal. 

 A lethal gene is one which kills fertilized ova or embryos homozygous 

 for it. Such homozygotes are not hatched or born, or at best they die 

 young. Yellow mice, for example, are always heterozygous (for present 

 purposes we may designate their genotype as "Fv"). When yellow mice 

 are mated together the following results are obtained: 



