222 GENETICS 



verse (and not alleles), the same rule holds: purple-eyed 

 flies mated with vestigial wings yield offspring (Fi ) that are 

 normal both for eyes and wings. 



//. Duplicate genes. — In certain cases a dominant char- 

 acter is produced in case a dominant gene of either of two 

 pairs, AA or BB, is present, while the corresponding reces- 

 sive character occurs only when both these dominant genes 

 are lacking, so that in their place are two recessive pairs 

 aa and bb. Such cases give special inheritance ratios; the 

 proportion of dominants to recessives is 15 to i instead of 

 3 to I. These are known as cases of duplicate genes. 



We have before mentioned a case of this sort, described 

 by G. H. ShuU (1914) in the common weed Bursa bursa- 

 pastoris, or shepherd's purse. Some races have dominant tri- 

 angular pods, while others have recessive ovate pods. It ap- 

 pears that the race with ovate pods carries two recessive 

 gene pairs, aa and bb, present in different autosomes. The 

 race with triangular pods has two corresponding dominant 

 pairs of genes, AA and BB, and the presence of a dominant 

 gene of either of these pairs suffices to cause the production 

 of the triangular pods. When these two races are crossed, 

 therefore, the following is the situation: 



Parents: AABB aabb 



Triangular Ovate 



Fi AaBb: Triangular 



When these Fi individuals are bred together they of 

 course produce in F2 the sixteen sets of individuals of the 

 constitutions shown in table 2, page 117. As examination 

 of this table shows, 15 out of the 16 contain either A or B or 

 both; hence these 15 have triangular pods. Only one of the 

 16, that with the constitution aabb, contains neither A nor 

 B ; this one has therefore ovate pods. Thus in such cases the 

 ratio in F2 is 15 dominant to i recessive. 



A considerable number of such cases are known, yielding 



