THE FACTOR HYPOTHESIS 159 



Symbols 



B =* black pigment, hiding brown or chocolate. 

 b = absence of B, or chocolate. 

 Y = yellow pigment, hidden by B. 

 E = extension of B over entire body, hiding Y. 

 e = restriction of B to eyes alone, thus exposing Y over the 



entire body. 



O = complementary color gene acting with P to produce color. 

 P = complementary pigment gene acting with C to produce color. 

 (The genes C and P may be omitted for the sake of sim- 

 plicity, since they are present in each instance.) 



First Cross 



"Extended" chocolate (bEY) X black-eyed yellow 

 (BeY) = black (BbEeYY). 



Second Cross 



When these cross-breds are mated with each other, 

 they each form four kinds of gametes, BEY, BeY, 

 bEY, and beY, which unite into sixteen theoretical 

 genotypic possibilities, some of which are unlike (Fig. 34). 

 These fall into four phenotypes, nine black (BEY), three 

 black-eyed yellow (BeY), three chocolate (bEY), and 

 one brown-eyed yellow (beY). The actual results in 

 Castle's experiments gave all four kinds in close nu- 

 merical agreement with this expectation. The action 

 of extension and restriction genes is, therefore, plainly 

 a case of Mendelian dihybridism in which two inde- 

 pendent pairs of alternative characters are concerned. 



E. BABBIT PHENOTYPES 



Perhaps no better application of the factor hy- 

 pothesis, so far as supplementary genes are concerned, 

 may be found than in the case of the color of rabbits. 



