224 GENETICS 



A. Lack of dominance: heterozygotes intermediate. — In 

 some cases, as we have seen on earlier pages, wlien two par- 

 ents differing in a single gene pair are mated, neither charac- 

 ter shows dominance. The offspring (Fi) from the mating 

 are intermediate between the two parents, or show some 

 blend of the two differing characteristics. That is, if the genes 

 of the two parents are AA and aa, the heterozygotes Aa are 

 not like either parent, but show a blend of their characteris- 

 tics. 



This is the usual case, as before seen (page 219), when 

 parents showing two diverse recessive alleles are mated in 

 Drosophila. Thus if eosin eye is mated with buff eye, the 

 progeny (Fi) show eyes of intermediate color; if vestigial 

 wings are crossed with antlered, the progeny (Fi) show 

 wings of intermediate type. 



Many cases of this sort are known in various organisms. 

 One of the best known is that of the Andalusian fowl. Here 

 a black individual (AA) mated to a white one (aa) gives 

 in Fi individuals (Aa) that show a somewhat intermediate 

 color known as blue. Similar cases are known for plant and 

 animal colors of various sorts. The heterozygote in such 

 cases is frequently not closely intermediate between the par- 

 ents; in different cases it shows various intergradations of 

 the parental characters. 



In such cases of lack of dominance, the ratios in which the 

 different characters appear in later generations are changed; 

 the familiar 3 to i ratio does not appear. If the parents are 

 designated AA and aa, then the heterozygotes Aa of Fi arc 

 all intermediate. When these are bred together, they yield 

 the usual proportions : 



AA + 2Aa +aa 



Thus there are in F2 three grades, one (AA) like one 

 parent, one (Aa) intermediate, and the third (aa) like the 

 other parent. And these three grades occur In the propor- 



