84 MENDELISM chap. 



gold - pencilled Hamburgh in the silver - pencilled 

 Hamburgh ; the gold Sebright bantam in the silver 

 Sebright, and so for many other kinds. The gold 

 and the silver forms each breed true to their own 

 colour. But when they are crossed certain definite 

 and remarkable phenomena make their appearance. 

 If we mate a silver Sebright cock from a pure silver 

 strain with a gold Sebright hen, the resulting chickens 

 are all silvers (see PI. V.). Silver is evidently 

 dominant to gold, and this is confirmed when the 

 silver F^ birds are mated together. The F^ genera- 

 tion consists of silvers and golds in the ratio 3:1. 

 So far the case is simple. A remarkable feature, 

 however, is brought oiit when we come to regard 

 the F.^, generation from the point of view of the 

 distribution of the sexes. As is commonly the case, 

 they are produced in approximately equal numbers. 

 But all of the recessive golds of the F^ generation 

 are females. Not a single gold cock has been 

 known to have arisen from such a mating. Further 

 analysis has shown that the F^ silvers are, as is to 

 be expected, of two kinds, viz. those which are 

 homozygous and those which are heterozygous for 

 the silver factor. When crossed with the recessive 

 gold some of the F^ silvers gave nothing but silvers, 

 while others, the heterozygotes, give silvers and golds 

 in equal numbers. Of the heterozygotes some are 

 cocks and some are hens, but all the homozygotes 

 are cocks. The general results of this experiment 

 are shown graphically in Fig. 1 8. They are capable 

 of a simple explanation if we make the following 

 assumption : 



(i) Silver is dominant to gold. This is evident 



