330 Interaction of Genes 



hydroxide. This alkah turned the flowers of three-sevenths of 

 the yellow type a reddish color, and the flowers of the other 

 four-sevenths turned a very deep purple. Very probably the 

 plants whose flowers turned reddish were genotypieally the Ab 

 type and the others were aB and ab. By this chemical test, the 

 9 : 7 ratio is resolved into a 9 : 3 : 4 ratio. The difference be- 

 tween them is that in the 9:3:4 ratio A produces a substance 

 different in appearance as well as in chemical nature from that 

 produced by a. 



Duplicate Genes 



In some plants and animals, two identical pairs of genes are 

 present in different chromosomes, so that A is dominant over a^ 

 and B, which is the same as A, is dominant over b, which is the 

 same as a. These are called duplicate genes, and it is some- 

 times believed that the presence of such genes indicates more 

 or less remote polyploidy. It is conventional today to designate 

 two such pairs of genes by the same letter followed by different 

 numerical subscripts, as Aiai and ^2^2. Because Ai produces 

 the same phenotype as A2, the Aiao and ai^2 classes are the same 

 and are different from the aia2 class. In some cases Ai and A2 

 together produce the same phenotype as Aia2 or aiA2; in other 

 cases the two dominants interact to produce something different. 

 For example, in the common shepherd 's-purse. Cay sella bursa- 

 pastoris, which is often found in waste places in America, the 

 seed pod, or capsule, is triangular, but in one of the less well- 

 known species Capsella Heegeri, from Germany, the capsules 

 are spindle-shaped and appear round in cross-section. The 

 triangular-shaped capsule is dominant over the round type, but 

 in typical plants two pairs of genes for capsule shape are pres- 

 ent. These pairs of genes have been designated Cc and Dd. 

 (These symbols were chosen before the present-day system was 

 adopted of designating duplicate genes by the same symbol fol- 

 lowed by a different subscript. These gene pairs would now 

 be designated T^t-^ and ^2^2-) Capsella bur sa-past oris is homo- 

 zygous for the two dominant genes whereas C. Heegeri has the 

 genetic constitution ccdd. The Fi has triangular capsules and 

 is genotypieally CcDd whereas the phenotypic F2 ratio is 15 

 triangular : 1 round. The plants with triangular-shaped cap- 



