182 



PHYSIOLOGIC GENETICS 



paper, the probable universality of pleiotropy and its consequences. Pleiotropy 

 makes it unlikely that any allele is the most favorable in all respects even in a given 

 combination with other genes. High selective value in a genotype requires a great 

 deal of compensatory interaction among the genes. The consequence is a tendency 

 toward overdominance and interaction among loci of the fourth type. 



Fig. 29. Selective values of alleles. 



_W 



1.0000 

 0.9375 



0.7500 



0.4375 



abed 



_W 



0.2500 

 0.1250 

 



ABCD 



aabbAabbAAbbAABbAABB 



oaBbAaBbAaBB. 



aaBB 



Above: Selective values assigned homozygous combinations of four pairs of alleles, 

 assumed to have equivalent effects on a quantitative character, but intermediate optimum. 



Below : Selective values assigned to additive pleiotropic effects of two of the pairs of 

 alleles. 



In the case of an intermediate optimum illustrated in the upper part of figure 29, 

 the six optimal genotypes all have the same value of W so that it would be a matter of 

 indifference which one is established in evolution. This ceases to be the case if there 

 are small pleiotropic effects. It is assumed in the lower part of figure 29 that genes 

 A and B have additive pleiotropic effects while C and D do not. Figure 30 shows the 

 selective values ( W) of all homozygous genotypes except for those at the extremes of 

 the quantitative character {abed with W = and ABCD with W = 0.25). The six 

 peak genotypes are now at three levels: one low, four intermediate, and one high. 



In the case of a character that is dependent on cumulative action of multiple 

 factors above a threshold or below a ceiling, even selection directed toward one of these 



