MEN■DELIA^^ EXPERIMENT ON FOWLS. 181 



were both of the light- coloured type, and the brood consisted of 

 6 coloured birds and one pile. The 6 coloured were all of the 

 liglit type, the charMcter showing in the chicks in the down as 

 well as in the mature plumage. In lioth these broods there were 

 recessive piles, 3 in F-III,"l in F.Y. Therefore the parents 

 must have been carrying the recessive white or pile, and in the 

 parents of F. Ill there was segregation not only between colour 

 and pile, but between light and dark colour. The brood F. YI 

 was the offspring of F, V 1 $ pile and F^ YIII 7 S dark silky. In 

 this brood there were five coloured birds and four pile. Of the 

 former two were accidentally killed during -my absence from 

 home, so that I was unable to examine their characters, one was 

 a male, concerning which there is no entry in my notes to show 

 whether it was of the light or dark type, one was a dark female 

 with black head, and one was a female of the light-coloured type. 

 It is evident therefore that in this case segregation of light and 

 dark took place in the coloured male parent «hen mated with 

 the recessive pile — in other words, the dark type was carrying 

 the recessive light colour, as well as the recessive pile. On the 

 other liand, F,. YII and F^, X are broods in which a light-coloured 

 parent (female in one case, male in the other) ma.ted with pile 

 produced only the light type in the coloured offspring. 



We may conclude, then, that in the dark type segregation may 

 take place, while the lighter tji^e is recessive and breeds true. 

 Whether the original black-red cock of this cross was heterozygous 

 with respect to darkness of colour, or the lighter type arose de 

 novo from the dark type, is another question. Mendelians would, 

 of course, assume that the light type was a unit or factor which 

 always existed, as they do not admit the origin of new factors, 

 but their assumption seems to me unjustified. The facts as 

 described in my experiments may be explained on Mendelian 

 principles by the hypothesis of epistatic and hypostatic factors as 

 applied by Bateson to the colours of mice*. We may assume 

 that in the dark-coloured type there is a factor, D, which causes 

 the development of the dark colour and is. absent in the light 

 type. If we write D for the factor which determines the darker 

 colour and for that which determines colour as distinguished 

 from absence of colour, using as usual the small letters for the 

 absence of these factors, then a, dark-coloured bird carrying both 

 light colour and white (or pile) may be represented as 

 DdCc and the pile as ddcc. 



The gametes of the dark bird will be 



DC, dC, Dc, dc, while those of the recessive will be all dc. 



The fertilizations will be 



DC dc dark-coloured offspring. 



dCdc light 



Dc dc recessive white or pile. 



dc dc ,, ,, 



* ' Mendel's Principles of Heredity,' 1909, p. 78. 



