146 MUTATION. 



have a gene more than the whites, and that the blues have 

 this colour because they are only half-black. 



We must remember that there is an alternative hypothesis, 

 namely that the blacks have a gene more than the whites, and 

 the whites another gene more than the blacks, and that the 

 blues are blue because they inherit both genes, and the black- 

 making tendency of one gene counteracts the white-making 

 tendency of the other. This would imply that the r61e of the 

 white parent was not a passive one, but that the white is a 

 dominant white. It would permit the explanation of the exis- 

 tence of true-breeding blue fowls. In the case of the Andalusian 

 blue, and the Kraaikop, however, where there are no true-breed- 

 ing blues, we need the secondary hypothesis that there is a 

 mutual repulsion between the gene which the blacks have more 

 than the whites and that which the whites have more them the 

 blacks. 



Is there any way of choosing between the two explana- 

 tions ? We think that there are some facts which make us prefer 

 the hypothesis, that in these cases we are concerned with two 

 genes, which where they are together, counteract each other's 

 effect upon colour. In the first place, as we saw, the theory 

 that the blues are blue because they are heterozygous for a 

 factor for which the blacks are pure, but which is absent from 

 whites, impUes that the white is a recessive white, and contrib- 

 utes nothing. If this were true, we could substitute any other 

 recessive white fowl in the cross-blue Andalusian white, for 

 the Andalusian white, and we would obtain the same results. 

 Now, when in 1911 we mated a blue Andalusian male to a re- 

 cessive white (Wyandotte) hen, we did not obtain blues and 

 whites in equal proportion, as we would have obtained if the 

 hen had been a white Andalusian. (Fig. 17). Half of the num- 

 ber of young were black (12), and half of them were blue (13). 

 We can explain this by saying that the white Wyandotte hen, 

 but for lack of pigment would have been black. But then it 

 becomes clear that we should expect only black young, unless 

 the male, who was blue, transmitted something to half the 



