Multiple Alleles; Multigenic Traits 



65 



gression in polygenic situations. When no 

 dominance occurs, the average offspring 

 from parents at A, B, and C will be at the 

 corresponding points A'. B\ C\ respectively, 

 in the offspring curve. (The environment 

 will cause some fluctuation around these 

 phenotypic mean points in the offspring 

 curve.) In the case of dominance, how- 

 ever, the offspring of A will be, on the aver- 

 age, to the right of A, as shown by arrows, 

 whereas the offspring of C will generally be 

 to the left of C. Contrary to what one might 

 expect, the loss of extreme individuals gen- 

 eration after generation will not make the 

 entire population more and more homo- 

 geneous phenotypically; there will be a 

 closely counterbalancing tendency for the 

 average members, B, of the population to 

 produce offspring more extreme than them- 

 selves in either direction. The result is that, 

 as in cases of no dominance, the distribution 

 curve for the offspring will be the same as 

 for the parent population. 



PARENT 

 GENERATION 



Group Selected 

 as Parents 



Mean 



Mean of 

 selected group 



OFFSPRING 

 GENERATION 



FIGURE 5- 



character. 



Mean 



8. Selection for a quantitative 



PARENTS 



7^TS<7\ 



OFFSPRING 



figure 5-7. The principle of regression. 



To obtain a line of phenotypically extreme 

 individuals from a population showing a 

 quantitative trait, one would choose the ex- 

 treme individuals as parents (Figure 5-8). 

 If dominance were absent, the very first off- 

 spring generation would have the same mean 

 as the group selected as parents. Some de- 

 gree of dominance usually occurs, and hence 

 regression will usually occur; and the mean 

 size of the first generation offspring will be 

 somewhat less extreme than that of the se- 

 lected parents, but somewhat more extreme 

 than the original mean. As one continues 

 to select appropriately extreme individuals 

 as parents, the offspring in successive gen- 

 erations will, on the average, approach more 

 and more closely the extreme phenotype 

 desired. 



