226 MULTIPLE FACTORS 



following case, is the production of a white-seeded 

 wheat. A cross between white-seeded and red- 

 seeded wheat gave in F2 one white to sixty-three reds, 

 showing that three independent recessive factors 

 were involved. 



Nilsson-Ehle also found that in oats a type without 

 ligules reappeared in F2 in such a ratio that four 

 recessive factors must have combined to have pro- 

 duced the type without ligules. East found certain 

 kinds of yellow corn that gave in F2 fifteen yellows 

 to one white. We may here also interpret the w^hite 

 as the double recessive. East has pointed out that 

 in crosses of certain strains of red corn white appears 

 in F2 in such a way as to suggest that three or possi- 

 bly four recessive factors combine to produce white. 



In other cases of multiple factors, the two factor- 

 differences differ in the intensity of their effect, and 

 so in F2 the two classes aB and Ab can be distin- 

 guished from each other, and a 9:3:3:1 ratio there- 

 fore results. In some of these cases, however, the 

 factors are in a sense non-cumulative in that one of 

 the factor-differences produces no effect when a given 

 allelomorph of the other pair of factors is present. 

 Thus, in the ratio 9AB:3aB:3Ab:lab if, in the 

 presence of b, a and A produce no different effect 

 there would be a ratio of 9:3:4. This is true in a 

 cross of a black mouse (AB) with a w^hite mouse 

 carrying both the recessive factor (b) for producing 

 an absolutely white color and also the recessive 

 (a) which merely ''dilutes" the black to blue. The 

 "diluter" a of course can not have any visible effect 



