170 GENETICS 



black or the splashed white should be regarded as the 

 homozygous dominant, since dominance is imperfect. 

 In either case the heterozygous blue Andalusian is at 

 once evident in the first filial generation without further 

 crossing. 



A similar case of imperfect dominance is furnished 

 by the roan color of cattle which results when red and 

 white are crossed. If two roans are mated, they pro- 

 duce red, roan, and white offspring in the proportion 

 of 1 : 2 : 1, thus showing that roan is a heterozygous 

 character in which the dominance of red is imperfect. 



Even in cases of apparently perfect dominance it is 

 sometimes possible by close inspection to detect differ- 

 ences between a pure dominant (DD), Figure 17, and 

 a heterozygous dominant (DR) when a superficial ex- 

 amination is not sufficient to distinguish them. 



Morgan cites a Drosophila cross between "ebony" 

 and "sooty" wings wherein the F 2 generation ranges 

 from ebony to sooty in an inseparable transition but 

 it proves, nevertheless, to be of three classes in the 

 proportion of 1 : 2 : 1, as further breeding tests show. 



3. DELAYED DOMINANCE 



A character which is really dominant is sometimes 

 so late in manifesting itself in the individual growth 

 of the offspring that it may properly be termed a 

 delayed dominant. 



Dark-haired individuals often do not acquire their 

 definitive hair color until adult life, and it is common 

 knowledge that the eyes of an infant for a consider- 



