SEGREGATION AND DOMINANCE 141 



(8 X 8) different zygotes, which corresponds to a 

 monohybrid raised to the third power (3 + 1)^ 

 These sixty-four zygotes group together in eight 



RSP 



i 



RsP 



1 



RSb 



i 



Rsp 



■l' 



SP 



i 



rsP 



i 



r Sp 



I 



r »p 



I 



?i 



RSP- 

 RsP- 

 RSp- 

 Rsp- 

 rSP- 

 rsP- 

 r Sp - 



r sp - 



Fig. 47. — Diagram showing the possible combinations in a guinea-pig 

 trihybrid of the F2 generation. R, rosetted coat ; r, non-rosetted coat 

 (absence of R) ; S, short hair ; s, angora hair (absence of S) ; P, pig- 

 mented ; p, albino (absence of pigment). The eight possible triple 

 gametes of each parent are placed in the upper and left hand margins. 

 Each of the sixty-four squares represents a possible zygote or ferti- 

 lized egg, having received a triple gamete from each parent. 



different phenotypes and twenty-seven different 

 genotypes. 



The trihybrid cross with its resulting combinations 

 is well illustrated by Castle's work on guinea-pigs 

 which confirms the Mendelian hypothesis on an 



