168 GENETICS 



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. 

 C = complementary color factor acting with P to produce 



color. 

 P = complementary pigment factor acting with C to 

 produce color. 

 (The factors C and P may be omitted for the sake of 

 simplicity, 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, shown in Figure 50. 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 numerical agreement with this expectation. 

 The action of extension and restriction factors is, 

 therefore, plainly a case of Mendelian dihybridism 

 in which two independent pairs of alternative char- 

 acters are concerned. 



