372 



Inbreeding, Selection, and Heterosis 



lines would differ only in the type of progeny they produce. If 

 there is dominance, then, the important effect of inbreeding is 

 to segregate out homozygous recessives. 



The number of homozygous lines which will segregate out 

 from inbreeding depends upon the number of genes for which 



TABLE 22 



Homozygous Lines That Separate Out from Inbreeding from an 

 Organism Heterozygous for Different Numbers of Pairs 



OF Genes 



the original organism is heterozygous. If the heterozygote is a 

 monohybrid, two homozygous lines, AA and aa, can be produced. 

 If it is a dihybrid, four homozygous lines {AA BB, AA bb, aa BB, 

 and aabb) will separate out (Table 22). If the genes are not 

 duplicate or cumulative, 2" different homozygous lines are to be 

 expected when the original organism is heterozygous for n pairs 

 of genes; but if the genes are duplicate and cumulatiA-e and show 

 no dominance, the number of different homozygous lines is n + 1. 

 Thus, in our hypothetical problem where duplicate, cumulative, 

 nondominant genes are involved at three loci, four different true- 

 breeding phenotypes are produced. 



