506 



THE PRINCIPLES OF HEREDITY 



and one for white. In this case the single black gene is able to produce 

 the black pigment in the hairs, and the offspring are all black. These 

 coats are just as black as if they had two genes for black, but since 

 each cell carries a white gene in a dormant condition, we say that these 

 guinea pigs are impure black, or heterozygous. 



Parents 



Black male 

 carrying 2 

 black genes 



White female 

 carrying 2 

 white genes 



Gametes 



'Sperm 



/carrying 



. one black 



gene 



First 



Generation 



Gametes 



warn 



Cross these two 



(#)^(o) 



Second 

 Generation 



All black 

 carrying 

 one black 

 and one 

 white gene 



Ratio: 

 S*a\ 3 Black 

 to 



I White 



Fig. 32.2. A monohybrid cross in the guinea pig. The genes for black coat color are 

 represented by the black circles, and the genes for white coat color are represented by 

 white circles. Note how the gene for black dominates over the recessive gene for white. 



When meiosis takes place within the gonads of these heterozygous 

 guinea pigs, the genes for black and white are separated, so that one 

 half of the gametes will contain a gene for black and the other half 

 will contain the gene for white. Fig. 32.2 shows the various combina- 

 tions of genes which result when these heterozygous guinea pigs are 

 crossed with one another. It can be seen from this diagram that about 

 one fourth of the offspring will receive two black genes, about one half 



