THE FACTORIAL HYPOTHESIS 



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

 was not present in either of the grandparental strains, 



FIG. 64. Combs of fowls, a, Single; b, pea; c, rose; d, walnut; e, Breda. 



how then can its appearance in this cross be explained? 

 The difficulty was met as follows: The ratio shows 



