254 MR. J. T. CUNNINGHAM ON 



stronger hereditary tendency in the Brown Leghorn to the 

 absence of pigmentation, and therefore it is not surprising that 

 this character should be dominant in the cross with the Silky. It 

 seems to me, then, more probable that the un pigmented character 

 in Bateson and Punnett's experiments is directly alternative or 

 allelomorphic to the pigmented ; and I shall endeavour to show 

 that their results can be as well, if not better, explained on this 

 hjrpothesis as on their own. It will be evident from this paper 

 that I am far from denying the occurrence of segregation, which 

 is an obvious fact, although my own results show that it is not 

 necessarily absolute or complete. It is further evident that a 

 sexual difference could not appear in the F^'s if there were not 

 already some sexual diflerence in one or both of the two breeds 

 crossed. The pigmentation tends to appear in the female sex 

 more than in the male : this not only occurs in the F^'s of the 

 cross Silky (^ X Brown Leghorn 5 > but a similar tendency is 

 seen, according to Bateson and Punnett, in the shanks of other 

 breeds. It is not, then, an unreasonable hypothesis that the 

 female Brown Leghorn has a greater tendency to the develop- 

 ment of pigmentation, or that the hereditary tendency towards 

 the tinpigmented character is weaker in this sex and therefore 

 not dominant. We may also assume that when fertilisation 

 ta,kes plaoe, whether we regard femaleness as segregated from 

 maleness or each of them as alternatively dominant, the tendency 

 to pigmentation accompanies the female character. To express 

 these assumptions in Mendelian fashion, we may write W for the 

 unpigmented character which is dominant, and Wp for the 

 weaker unpigmented character, retaining P for the pigmentation 

 of the Silky. Then, instead of the comj)licated formula? of Bateson 

 and Punnett we should have the two reciprocal crosses expressed 

 as follows : — 



(A.) Brown Leghorn c5* x Silky $ , 



WW d PP 5 . 



GametesW + W P c? + P $ . 



F, WP 6 + WP ?. 



(B.) Silky J X Brown Leghorn $ . 

 PP c? WWp 2 ■ 



Gametes P + P W c? + Wp $ . 



F^ WP cJ + WpP$. 



The above formulae are not to be taken as implying that sex is 

 determined entii'ely by the female, or that either sex does not 

 contain the other in a latent condition, but merely that, as half 

 the ova in fertilisation must actually give rise to females, the 

 tendency to pigmentation passes only into these females. 



We have next to see how this works out for the F,'s. 

 According to Bateson and Punnett's formiilse, the F^'s from cross 

 A are for the males 6 slightly pigmented to 2 unpigmented. 



