180 MR. J. T. CUNNINGHAM ON RESULTS OF A 



The absent factors being represented by small letters, one of the 

 white parents is Xy, the other xY, or, rather, these symbols 

 represent the constitution of the gametes, the jiarent l)}rds tl)em- 

 selves must be XXyy or xxYY. The Silky in my experiment 

 tlien must contain one factor for colour and its gnnietes must be, 

 say, Xy. The Black-red, on the other hand, must contain both 

 colour-factors, and its constitution must be XXYY and its gametes 

 must be XY. The Fj, then, with colour dominant will be XXYy, 

 and the gametes of this will be XY, Xy. The fertilizations in F^ 

 will therefore be XYXY, XYXy an<l XyXy. 



The result, it will be seen, is much the same as if the colour was 

 due to a single factor, the first combination giving pure dominants, 

 the second dominant (i.e., coloured) birds cai-rying recessive 

 white, and the third recessive white like the origina,l Silk}'. 

 The hypothesis of two coloui--factors and the presence of one of 

 them in the Silky thus in no way helps to explain the appeai-ance 

 of the pile-colour in the later generations of my cross : whether 

 there is one colour-factor or two, the appeaiance of colour in the 

 recessive could not occur if segregation were complete, as Mende- 

 lians assume it to be. Segregation would result in the recessive 

 in F.^ or any other generation having a total absence of one colour- 

 factor, whether X or Y, and the occui'rence of the colour in the 

 pile shows that some portion of the missing colour-factor has 

 passed from the dominant to the recessive — in other Avords, has 

 not been completely segregated. 



Differences in the Coloiired Birds. 



Although the production of the pile coloration is the most 

 important of the results of this cross which I hfU'e to record, I 

 propose to discuss peculiarities in the heredity of other chai'acters 

 and consider how far they depart from expectation according to 

 Mendelian theory. One of these consists of variations in the 

 depth of colour in the coloured dominants. These variations did 

 not form a continuous series, but divided themselves into two 

 distinct types — a dark and a light. I noticed this first in some of 

 the coloured birds of F,, where F, I 6 & 8 $ are noted as having 

 light-coloured heads, F., I 7 as having a black head. In later 

 generations I noticed the whole plumage of hens with black head 

 was darker than in those with light head, but I found it difficult 

 to distinguish the two types in the cocks when adult, though it 

 was evident enough in the first mature plumage in both sexes. 

 In F^ IX, the parents of A^hich were F^ I ] J coloured X F,, III 

 10 5 pile, there were 5 coloured birds to one white, and all the 

 coloured were of the lighter type, but whether the male parent 

 was of this type was not noted. 



I tried to acertain whether the lighter type was recessive to 

 the darker. The parents of F, III were both dark silky, and of 

 the 5 coloured birds in this brood two were of the daiker type 

 Avith black crests, two were light, and one was intermediate in 

 general colour, but without the black crest. The parents of F^ Y 



