476 



GENETICS 



of the gray parent possess a single gene for gray coat-color, and 

 those of the white parent a single gene for white coat-color. When 

 fertilization occurs only one type of offspring can result. This 

 zygote contains two genes for coat-color, but one is for whiteness 

 and one for grayness. A zygote of this kind is called a hetero- 

 zygote, in contrast with zygotes like those from which the parents 

 developed, which are known as homozygotes, because the genes of 

 the pair are alike. The Fi individuals are spoken of as hetero- 



Zygotes from 

 which Pj 

 developed 



Gametes 

 of Pi 



Zygotes from 

 which Fi 

 develop 



1 Heterozygous Gray • 1 Homozygous White 



Fig. 253. — Diagram illustrating independent segregation and possible recom- 

 binations of chromosomes bearing genes when a heterozygous gray mouse is 

 crossed with a homozygous white mouse. 



G, gene for gray coat-eolor; W, gene for white coat-color. 



zygous, while the parent generation, in this case, is homozygous. 

 As can be seen from the diagram, two kinds of gametes are formed 

 by individuals of the Fi generation, and three recombinations are 

 possible in the Fo generation. According to the law of chance, one 

 of these classes, the GW combination, will occur twice as fre- 

 quently as either the GG or the WW. The effect of the dominance 

 of the gene for grayness is such that both homozygous and hetero- 

 zygous gray mice appear alike, and the apparent ratio in the F2 

 generation is 3 gray: 1 white. It is, therefore, necessary to obtain 

 offspring from particular cases in order to differentiate genetically 



