THE FACTORIAL HYPOTHESIS 271 
three rose: three pea: one single. Since single comb 
was not present in either of the grandparental strains, 
Fic. 64.—Combs of fowls. a, Single; b, pea; ¢, rose; d, walnut; e, Breda. 
how then can its appearance in this cross be explained? 
The difficulty was met as follows: The ratio shows 
