THE PRINCIPLES OF HEREDITY 



511 



both are recessive, and only those guinea pigs which are homozygous 

 for recessive genes will express the effects of these genes. The first 

 o-eneration will all have black short hair, but will be heterozygous for 

 both pairs of genes. When these are crossed among themselves we 

 obtain the interesting ratio of 9 black short : 3 black long : 3 white 

 short : 1 white long. Fig. 32.5 shows how the gametes are assorted to 

 Sfive this result. 



Parents 



White long 

 haired female 

 (homozygous) 



Gametes 



First Jf#0 

 Generation Sj I 



Gametes 



Second 

 Generation 



All black, 

 short haired 

 (heterozygous) 



Ratio: 



9 Black short 

 3 Black long 

 3 White shon 

 I White long 



Fig 32.5. A dihybrid cross. A homozygous, black, short-haired guinea pig is mated 

 with a homozygous, white, long-haired guinea pig. All of the first generation are black 

 and short because these two genes are dominant over the white and long. When two 

 of these first-generation guinea pigs are mated, the gametes combine to give 9 black- 

 short, 3 black long, 3 white long, and 1 white long in the second generation. This 

 type of cross illustrates the principle of independent assortment of genes. 



