THE METHOD OF CYTOLOGY 



477 



between Individuals that exhibit a dominant characteristic. The 

 theoretical explanation of the results obtained when a heterozygous 

 gray mouse is crossed with a white mouse, which will always be 

 homozygous since white is recessive, is presented in Fig. 253. It 

 is the difference between the offspring obtained in this cross and 

 those obtained by crossing a homozygous gray with a white (Fig. 

 252), that furnishes the type of breeding test used for differentiating 

 homozygous from heterozygous dominant individuals. The dia- 



Zygotes from 

 which Pj 

 developed 



Gametes 

 of Pi 



Zygotes from 

 which Fi 

 develop 



1 Homozygous Gray 1 Heterozygous Gray 



Fig. 254. — Diagram illustrating independent segregation and possible recom- 

 binations of chromosome.s bearing genes when a homozygous gray mouse is 

 crossed with a heterozygous gray mouse. 



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



gram in Fig. 254 shows why indiscriminate crossing between the 

 gray mice would fail to yield information that would enable one to 

 distinguish homozygous gray mice with certainty. The breeding 

 results are adequately explained in these cases of mono-hybridism 

 by the theory that the genes, or Mendehan units, are carried in 

 the chromosomes and so distributed to gametes and zygote (Fig. 

 255). 



The theory of the chromosomes as the physical basis of heredity 

 can hkewise be used to explain cases of di- and tri-hybridism. In 



