MENDELISM 135 



a single comb. The difference from the previous case lies 

 in the fact that both recessive characters, r as well as p, 

 are identical, representing the single comb. (It will be 

 noted also that, whilst up to now in the production of a 

 dihybrid one parent had both dominant, the other both 

 recessive, characters, here each parent has one dominant 

 and one recessive character.) We get, accordingly, the 

 following scheme, which explains all the possible com- 

 binations : 



Rose Variety x Pea Variety. P 



Two allelomorphic pairs : 



I. Rose — R 



Absence of Rose — v (Single) 



2. Pea— P 



Absence of Pea — f> (Single) 



Rosel 

 Pea J 



9 RP + 3 R/) + 3 ^P + I y/' 

 9 Walnut 3 Rose 3 Pta i Single F2 



Fig. 57. — Combs of Fowls. (After Bateson.) 



We have thus in both cases just described definite 

 characters — i.e., the various colours of flowers and the 

 different shapes of the combs respectively produced " by 

 the mutual interaction of factors belonging to distinct 

 allelomorphic systems." 



One more point coming out in the consideration of the 

 phenomena just described needs mentioning here. We 

 have taken the absence of the dominant factor as the 

 recessive character, and found that on that hypothesis 

 we could work out satisfactorily on Mendelian lines the 

 combinations given. Whatever the absence of a character 

 may actually mean as a characteristic of the organism 

 itself, this assumption is in the present stage of our know- 

 ledge the only means of dealing with such cases. 



