MULTIPLE FACTORS 179 



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 white 

 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 white 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 



