Heredity — Genetics 687 



DiHYBRiD Cross (Guinea Pig) (See Fig. 336) 



Parents (P) 



Gametes produced 



Offspring (Fi) 



Gametes in equal numbers in 



both sperms and eggs 

 Offspring (F2) (shown by the 



Punnet square) 



Black-smooth (BBrr) 



i 



Br 



X White-rough (bbRR) 



bR 



Black-rough (BbRr) 



i i i i 



BR Br bR br 



Sperms 



E 



g 



s 



BR 



Br 



bR 



br 



From the above squares there are the following: 

 9 black-rough 

 3 black-smooth 

 3 white-rough 

 1 white-smooth 



This is the phenotypic ratio of 9-3-3-1 for a di- 

 hybrid cross in the F2 generation, when the 

 above parents are used. 



Parents (P) 



Gametes produced 

 Offspring (Fi) 



Trihybrid Cross (Guinea Pig) 



Black-short-smooth 

 (BBSSrr) 



BSr 



X White-long-rough 

 (bbssRR) 



bsR 



1/ 



Gametes (equal numbers 

 of each kind in both 

 sperms and eggs) 



Offspring (F2) 



Black-short-rough 



(BbSsRr) 



BSR BSr BsR Bsr bSR bSr bsR bsr 



By the Punnet square as in the monohybrid 

 and dihybrid crosses there will be: 

 27 black-short-rough 

 9 black-short-smooth 

 9 black-long-rough 

 9 white-short-rough 

 3 black-long-smooth 

 3 white-short-smooth 

 3 white-short-smooth 

 3 white-long-rough 

 1 white-long-smooth 



Thus a typical F2 trihybrid, phenotype ratio is 27-9-9-9-3-3-3-1. By means of 

 the Punnet square or checkerboard, it is apparent that all of the 27 which ap- 



