236 Experimental Transformation of Species 



Incomplete Dominance 



This first important fact, while found to hold for hun- 

 dreds of characters in plants and animals, is not the universal 

 rule. There are many traits that cannot be in this way 



y 



X 



y 



X 



¥ 



¥ V y 



X 



Fig. 58. Heredity of Pigmentation in the Blue 



Andalusian Fowl 



When a black hen is mated with a white cock, or vice versa (P), the offspring 

 (Fi ) is always " blue " — really a mixture of black and white speckles in the feathers. 

 When two blues are mated (F,^ and F2) the offspring appears in three types; one half 

 are blues, like the (hybrid) parents; one quarter are blacks, like the black grand- 

 parent (P) ; and one quarter are white, like the white grandparent (P). When blacks 

 are mated with blacks they breed" true, whether their parents were hybrids or of pure 

 black stock. The whites will also breed true, whether of hybrid or of pure stock. 

 The blues, however, always break up in the way described, whether they are of 

 hybrid parentage or themselves the result of crossing whites and blacks. 



separated from their alternatives. In a variety of fowl 

 known as the Andalusian blues, there appear regularly three 

 types — a white, a black, and a " blue " (really speckled 

 white and black) . When the whites are bred among them- 

 selves the offspring are all white. When the blacks are bred 

 among themselves the offspring are all black. But when the 

 blues are bred together the offspring appears in the three 



