THE FACTORIAL HYPOTHESIS 



271 



three rose: three pea: one single. Since single comb 

 was not present in either of the grandparental strains, 



H'P'uR. ^% 



Fig. 64. — Combs of fowls. «, Single; h, pea; c, rose; d, wiilnut; e, Breda. 



how then can its appearance in this cross be explained? 

 The difficulty was met as follows: The ratio shows 



