BIRDS AND EVOLUTION 



395 



the following clear case. If a member of a true-breeding 

 race of Black Rose Comb Bantams be paired with another, 

 the offspring will be like the parents. The germ-cells will be 

 all alike in having a double dose of the factor for blackness. 

 Similarly if two members of a true-breeding race of White 



'^ x 



Fig. 59. — Combs of fowls, s., simple serrated comb ; p., pea comb ; 

 R., rose comb. 



Rose Combs are paired, the offspring will be all alike in 

 having no pigment. It is usual to call such pure-bred 

 offspring homozygous, which means that their germ- cells are 

 all alike as regards the character in question. 



But what will happen when White and Black Rose Combs 

 are mated ? The fertilised egg-cells will have the factors 



