INSEPARABLE EFFECTS 31 



one recessive and each recessive only one dominant. It 

 raised the question : how do the two dominants stand 

 to each other : which is the dominant and which the 

 recessive ? But when roses and peas were mated, it 

 was found that neither was dominant to the other, for 

 the hybrids' combs were neither roses nor peas, but a 

 new kind different from both, shaped hke a half walnut. 

 This again was another new phenomenon having 

 no parallel in Mendel's experiments ; but the signifi- 

 cance of both phenomena was brought out when the 

 hybrid walnut combs were bred together, for their 

 progeny consisted of the four kinds, walnut, rose, pea, 

 and single, in numbers approximating to the ratio 

 9:3:3:1 (actually, walnut 279, rose 99, pea 132, and 

 single 45). It will be remembered that, if the hybrids' 

 progeny consist of four groups numerically in the ratio 

 9:3:3:1, the original parents must have differed in 

 two pairs of characters. At the same time the differen- 

 tiating characters borne by each group must be the 

 result of two factors. In this case, however, the effects 

 of the factors operating in each of these combs cannot 

 be identified. The two effects which combine to pro- 

 duce single cannot be separated, and still less can 

 either of these effects be traced the one in pea and the 

 other in rose. Therefore we can only represent the 

 two-pair set of walnut, rose, pea, and single by 

 " unknowns," thus : 



Walnut Rose Pea Single 



X X X X 



Y y Y y 



