DOMINANCE 



which he gave was the best that the knowl- 

 edge of his time afforded, but it leaves much 

 to be desired. This lack, however, has been 

 completely supplied by the Mendelian princi- 

 ples. An illustration or two may now be 

 cited. 



When pure-bred black guinea-pigs are mated 

 with red ones, only black offspring are as a 

 rule obtained. The hairs of the offspring do 

 indeed contain some red pigment, but the black 

 pigment is so much darker that it largely 

 obscures the red. In other words, black be- 

 haves as an ordinary Mendelian dominant. In 

 the next generation black and red segregate 

 in ordinary Mendelian fashion, and the young 

 produced are in the usual proportions, three 

 black to one red, or 1 : 1 in back-crosses of the 

 heterozygous black with red. All black races 

 behave alike in crosses with the same red in- 

 dividual, but among red animals individual 

 differences exist. Some, instead of behaving 

 like Mendelian recessives, produce in crosses 

 with a black race a third apparently new con- 

 dition, but in reality a very old one, the agouti 

 type of coat found in all wild guinea-pigs, as 



63 



