Incomplete Dominance 



87 



Another striking case of incomplete dominance is the often- 

 cited blue Andalusian fowl. This variety is a heterozygote and 

 can be produced only by crossing a black with a white. There 

 is no dominance, and the Fi from such a cross is neither black 

 nor white but a peculiar intermediate shade called "blue" (Fig. 



Fig. 29. A blue Andalusian female. This t3^pe is the heterozygote from 

 a cross between certain black and white types. Therefore it never breeds 

 true when crossed with a blue Andalusian male, but produces a ratio of 

 1 black : 2 blue : 1 white. In the female the neck feathers are dark 

 blue. In the male the back and saddle feathers, in addition to the neck 

 feathers, are almost a solid blue. (Courtesy of Dr. M. A. Jull.) 



29). Since the blue fowls are always heterozygotes, they cannot 

 possibly be made to breed true. When two blue fowls are mated, 

 the offspring are in the ratio of 1 black : 2 blue : 1 white. When 

 a blue is mated with a white the offspring are 1 blue : 1 white, 

 and when a blue and a black are crossed the resulting ratio is 1 

 black : 1 blue. 



An interesting dominance relationship is found in certain 

 crosses between horned and hornless sheep. In Dorset Horn 

 sheep, the males have very large horns and the horns of the 



