HETEROZYGOTE FORMS 181 



almost exactly intermediate, or the appearance of the 

 cross-bred form may be nearer to that of one parent 

 than to that of the other. Dominance is clearly only 

 an extreme case of this latter phenomenon. The 

 term c dominance ' is applied to those cases in which 

 the appearance of the hybrid offspring is so near to 

 that of one parent as to be no longer clearly distin- 

 guishable from it. 



In other cases, still of a simple Mendelian nature, 

 the appearance of the heterozygote may be quite 

 different from that of either parent homozygote. An 

 excellent example which is almost certainly of this 

 nature is afforded by the Andalusian fowls studied 

 by Messrs. Bateson and Punnett. And this will also 

 serve as our first illustration of the application of these 

 principles to animals as well as to plants. The facts 

 of the case are as follows : 



The ' blue ' type of Andalusian appears to be a 

 heterozygote form which has never been got to breed 

 true. When a pair of these birds are mated together 

 only about half their offspring are like themselves, the 

 remainder being entirely different. Half these re- 

 maining ' wasters ' are black, and half are nearly 

 white, showing only a few black ' splashes.' If, now, 

 a pair of the black wasters are mated together, they 

 breed perfectly true, yielding only black offspring like 

 themselves. Similarly the splashed whites mated 

 together give rise to splashed white, and nothing else. 

 Both these forms, then, the black and the splashed 

 white, are clearly pure homozygotes. On mating a 

 black and a splashed white together, black-bearing 



