GENERAL ZOOLOGY 



Zygotes from 

 which Pi 

 developed 



Gametes of 

 Pi after 

 maturation 



Zygotes from 

 which Fi gray 

 hybrids de- 

 velop 



Gametes 

 of Pi 



Zygotes from 

 which P2 gen ■ 

 eration de- 

 velops 



1 homozygous gray 



2 heterozygous gray 



1 homozygous white 



Fig. 6.11. A cross between a homozygous gray mouse and a homozygous white mouse (cf. 

 Fig. 6.4). The diagram illustrates the segregation of a pair of genes when their carriers, a pair 

 of homologous chromosomes, undergo disjunction; notice the possible combinations that can 

 occur between gametes. U\ gene for gray coat color; w, gene for white coat color. 



the same zygote. A zygote of this kind is called a heterozygote, in contrast 

 to the zygotes like those from which the parents developed, which are known 

 as homozygotes because the genes of the pair are alike. The /^j individuals 

 are spoken of as heterozygous, and the parents in this case are called homo- 

 zygous. As can be seen from Figure 6.11, two kinds of gametes are formed 

 when disjunction occurs in individuals of the F^ generation, and three kinds 

 of combinations of gametes with chromosomes and genes are possible in the 

 F2 generation. According to chance, one of these classes, the Ww combi- 

 nation, will occur twice as frequently as either the IVW or the ww. The 

 eflfect of the dominance of the gene for grayness is such that both homo- 

 zygous and heterozygous gray mice look alike; the apparent or phenotypic 

 ratio in the F.2 generation is 3 gray : 1 white. It is necessary, therefore, 

 to obtain offspring from particular crosses in order to differentiate genetically 

 between individuals that exhibit a dominant characteristic, that is, to separate 

 the genotypes of the dominant phenotype. The theoretical explanation of the 

 results obtained when a heterozygous gray mouse is crossed with a white 



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