66 The Breda Comb [ch. 



and P were allelomorphic to S, I came to regard them as 

 also allelomorphic to each other. This idea led to con- 

 fusion, but we know now that no case justifies such an 

 application of the principle of allelomorphism. A rose 

 comb is not due to an elemental factor which can segregate 

 from the pea comb factor. The two factors belong to 

 distinct allelomorphic pairs and each in the gametogenesis 

 of the heterozygote segregates from its own allelomorph, 

 which is simply the absence of the factor in question. The 

 single comb contains neither R nor P The rose comb is 

 a single comb modified by the presence of R, while the pea 

 comb is produced by the presence of P. We may therefore 

 describe the rose as R no P, and the pea 2iS P no R. It is 

 convenient to use capital letters for dominants and small 

 letters for recessives, the rose being thus written Rp, and 

 the Pea, rP, The walnut comb is the RP^ while rp gives 

 the single. 



The allelomorphism of the elements which go to the 

 constitution of the shapes of combs in fowls may without 

 doubt be carried very much further. For example there 

 are indications that the size of the comb depends to some 

 extent at least on other pairs of factors. Another curious 

 set of phenomena, perhaps worth investigating further, may 

 be studied in a cross between a single comb breed and the 

 "Breda" fowl. The Breda is usually said to have ''no 

 comb." As a matter of fact it has two very minute 

 tubercles which represent the comb. When this breed is 

 crossed with the single comb, P-^ has what may be called a i 

 "double" single comb. It consists of two large lobes or 

 leaves diverging outwards from a common base^. Such a 

 comb is evidently due to the introduction by the Breda of a 

 factor which may be called ** bifidity." This factor acts on ' 

 the large comb brought in by the single-combed parent and 

 the result of the combination is a large, double comb. P^ 

 from this cross has not yet been raised, but there can be no 

 doubt that it will contain members having the '' no comb " 

 of the Breda and the "absence of bifidity" of the single- 

 combed breed. Such birds probably have only a minute 

 tubercle at the posterior end of the comb region. 



* The two lobes sometimes unite anteriorly to a greater or less extent. 



