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 F. 

 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 9 

 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. 



