MULTIPLE FACTORS 177 



itself does not change the triangular type, as shown by 

 the fact that both single recessives are triangular in 

 type and are identical in appearance with the double 

 dominant. Only where the two recessives occur in 

 the same individual does the type change to round. 



Six families w^ere bred from the Fi, and gave the 

 following counts: 



The actual ratios range from 16 :1 to 36.5 :1, which 

 exceed the expected ratio of 15 : 1. Nevertheless, the 

 deficiency in the round class is probably due to the 

 lower viability of the round-capsuled type, for in 

 later cultures where the conditions were more 

 favorable the expected 15:1 ratios are more nearly 

 realized. That 15:1 is the true ratio is shown by 

 tests that were applied to these F2 plants. In Fig. 

 54, the 16 classes (15 :1) of F2 individuals are repre- 

 sented. Within each square is also given the genetic 

 composition of the class. The letter ''c" stands for 

 one of the recessive factors, and the letter "d" for 

 the other factor. Both of these recessive factors 

 acting in conjunction produce the round capsules ccdd. 

 Beneath each figure is given the expected ratio for 



12 



