Incomplete Dominance 237 



types in certain definite proportions: one-half are blue, one- 

 fourth are black, and one- fourth are white (Fig. 58). 

 When the whites and blacks are crossed, the offspring are 100 

 per cent blues. That is to say, the offspring all contain pig- 

 ment in their feathers, resembling the black parent; but 



J^ 



X 



^ 



X 



X 



t 



X 



^^|k iflHI^ 4^A^ 



1^ 1^ 



Fig. 59. Color Inheritance in the Four O'Clock, 



MiRABILIS JaLAPA 



When a red-flowered plant is crossed with a white-flowered plant (P), the 

 seeds yield plants bearing pink flowers (Fi). When pink-flowered plants arc 

 mated (Fi), the offspring is of three distinct kinds (Fj): one half bear pink 

 flowers, like their parents; one quarter bear red flowers like their red-flowered 

 grandparents (P) ; and one quarter bear white flowers, like their white-flowered 

 grandparents. The white-flowered and the red-flowered both breed true. 



they also differ from the black parent in showing to a degree 

 the whiteness of the white parent. 



Among the species of four-o'clock, Mirabilis jalapa, 

 some plants have white flowers and some have red flowers. 

 When a red and white are crossed, the hybrid generation 

 bears pink flowers (Fig. 59). In other pairs of contrasting 

 characters studied in the last twenty-five years among many 

 plants and animals, such incomplete dominance has been 

 found repeatedly. 



