266 HEREDITY AND ENVIRONMENT 



this case red is not completely dominant over 

 white, but the hybrid is more or less interme- 

 diate between the two parents (Fig. 50). 



It has long been known that the breed of 

 fowls called Blue Andalusian does not breed 

 true, but in each generation produces a certain 

 number of blacks and whites as well as blues. 

 Bateson found that the blues are really hybrids 

 between blacks and whites in which neither of 

 the latter is completely dominant. Black and 

 white appear only when they are pure (homo- 

 zygous), blue only when both black and white 

 are present (heterozygous). 



Again a cross of red and white cattle pro- 

 duces roan offspring, but the latter when in- 

 terbred give rise to reds, roans and whites in 

 the proportion of 1 :2 :1, showing that the roans 

 are heterozygotes in which red is not com- 

 pletely dominant over white, while the reds 

 and whites are homozygotes and consequently 

 breed true. 



Lang found that when snails with uniformly 

 colored shells were crossed with snails having 

 bands of color on the shells the hybrids were 

 faintly banded, thus being more or less inter- 



