PRINCIPLES OF HEREDITY 



653 



genes themselves have no color; they are neither brown nor black. 

 The brown gene controls certain chemical reactions which lead to the 

 formation of a brown pigment in the hair cells, whereas the black gene 

 directs the chemical reactions toward the formation of black pigment 

 in the hair cells. In working genetic problems, letters are conventionally 

 used as symbols for the genes. A pair of genes for black pigment is 

 represented as BB, and a pair of genes for brown pigment by bb. A 

 capital letter is used for one gene and the corresponding lower case letter 

 is used to represent the gene for the contrasting trait, the allele. 



270. A Monohybrid Cross 



The events of a hypothetical mating of a pure-bred brown male 

 guinea pig (bb) with a pure black female (BB) are given in Figure 32.1. 

 During meiosis in the male the two bb genes separate and each sperm 



Brown 



Black 



Parents 



§) Gometes (W) 



Block 



Black 



Black 



Brown 



1 v.*/^.«j?" 



Figure 32.1. An example of a monohybrid cross: the mating of a brown with a 

 black guinea pig. (Villee: Biology.) 



