IO4 White Fowls [CH. 



factors which must be together present in the zygote in 

 order that colour should be produced. The colour of the 

 F^ birds is practically Black-red, approaching closely to the 

 plumage of Gallus bankiva (see Plate IV). 



It might a priori be expected that since the White 

 Rosecomb (3) has very distinct though light pigmentation 

 in its down, a cross between this and the Silky (4) would 

 be more likely to give a coloured F^. This, nevertheless, 

 is not the case. The cross between these two breeds is an 

 ordinary white, showing that no reliance whatever can be 

 placed on such considerations, and that experiment alone 

 can decide what properties the several types possess. 



The whiteness of the recessive whites is thus due to 

 the absence of one of the elements needed for the de- 

 velopment of colour. Analytical experiments show on the 

 contrary that the white of the dominant white (i) is not 

 due essentially to the absence of an element but to the 

 presence of a factor which prevents the development of 

 colour. Thus, if X and Y are the two complementary 

 factors which produce colour, the one recessive white is 

 Xy and the other is xY. The dominant white (i) has the 

 suppressing factor S, and as the result of many experiments 

 it appears that the individuals of that breed have various 

 compositions in respect of X and Y, though always 

 homozygous for S. 



Hence it follows that when Dominant white (i) is 

 crossed with Recessive white (2), F l is white; but F z may, 

 and in our experience always does, give some coloured 

 birds. The proportion in which the coloured F z birds 

 appear must of course depend on the composition of the 

 individual dominant white which was originally introduced, 

 and various ratios occur. The numbers obtained show 

 that of the dominant whites some are Xx Yy SS, another 

 contained only one of the colour-factors and so may be 

 written Xxyy SS. Probably all the possible combinations 

 occur, but the experiment has not been carried far enough 

 to prove that they do*. 



* The difference in size between Recessive white (2) and the White 

 Rosecomb is so considerable that the direct cross between these two 

 cannot be made. But F from White Rosecomb x Silky has been crossed 

 with (2) and the offspring were in about equal numbers whites and browns. 

 There is therefore no reasonable doubt that the factor carried by the 

 Silky and by the Rosecomb is the same. Either both have X, or both Y. 



