GENERAL ZOOLOGY 



Black, smooth-coated 



White, rough -coated 



Black, rough-coated 

 Interbred 



9 black, rough-coated 



3 white, rough-coated 



3 black, smooth -coated 



1 white, smooth-coated 



Fig. 6.6. Results of dihybridization in guinea pigs which differ in color of hair and quality 

 of coat; black hair is dominant to white hair, and rough coat to smooth coat (cf. Fig. 6.15). 

 (Rearrangement of figures from W. E. Castle, Genetics and Eugenics, copyright 1916 by Harvard 

 University Press, printed by permission.) 



feature of heredity, although it is almost universally encountered in practical 

 breeding. The behavior of the hereditary units in the germ cells is the same 

 whether or not dominance is involved. 



If two individuals that differ in two characteristics are crossed, the process 

 is known as dihybridization. In guinea pigs black hair and rough coat are 

 dominant to white hair and smooth coat. When a black-haired, smooth-coated 

 guinea pig is bred with a white-haired, rough-coated one, the offspring are all 

 black-haired and rough-coated (Fig. 6.6). Whether these characteristics have 

 been present in the male or in the female parent is not important; the combi- 

 nation always produces individuals in which both dominant characters are 

 seen. When these F^ hybrids are crossed, four kinds of offspring (phenotypes) 

 result in the F.^ generation, in the following ratio — 9 black-haired, rough- 

 coated : 3 white-haired, rough-coated : 3 black-haired, smooth-coated : 1 white- 

 haired, smooth-coated. Mendel's original work on dihybrids consisted in 

 crossing wrinkled green peas with smooth yellow peas. The offspring of such 

 a hybridization are smooth yellow peas, a fact which indicates that smooth 

 surface is dominant to wrinkled surface and yellow color is dominant to green 

 color in the seeds. In the F2 generation smooth yellow peas, smooth green 

 peas, wrinkled yellow peas, and wrinkled green peas occur in the ratio of 

 9:3:3:1. 



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