THE METHOD OF EXPERIMENTAL BREEDING 



469 



the hybrid individuals, or those that will not breed true, can be 

 distinguished superficially from those that will give rise to offspring 

 like themselves. Dominance is not an essential feature of Mendelian 

 theory, although almost universal in practical breeding. The 

 purity of the germplasm is completely established whether or not 

 dominance is involved. 



O^ 



BLACK, SHORT-HAIRED, SMOOTH' 

 COATED GUINEA PIG 



WHITE, LONG-HAIRED, ROUGH - 

 COATED GUINEA PIG 



i^ BLACK. SHORT-HAIRED, ROUGH- 

 COATED GUINEA PIGS 



INTERCROSSED 



Fi 



BLACK, 



SHORT-HAIRED. 

 ROUGH -COATED 



3 WHITE, 



SHORT-HAIRED, 



SMOOTH -COATED 



9 BLACK. 



SHORT-HAIRED. 



SMOOTH -COATED 



3 BLACK, 



LONG-HAIRED, 



SMOOTH -COATED 



9 WHITE, 

 SHORT-HAIRED. 

 ROUGH -COATED 



■ ■ . V 



■ ■ 3 WHITE, 

 LONG-HAIRED, 

 ROUGH -COATED 



9 BLACK. 



LONG-HAIRED. 



ROUGH -COATED 



'Im._.'^ 



1 WHITE. 

 LONG-HAIRED. 

 SMOOTH -COATED 



i'"i<;. 248. — Re.sults of tri-hybridization in guinea pigs which differ with respect 



to color and length of hair and quality of coat. Black hair is dominant to 



white hair, sho: t hair to long hair, and rough coat to smooth coat. 



(Rearrangement of figures from Castle, "Genetics and Eugenics," copyright, 1916, by 

 Harvard University Press, printed by permission.) 



If individuals that differ with respect to two characteristics are 

 ciossed, the process is known as di-hyhridization. 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. 247). Whether these characteristics 

 have been contributed by the male or the female parent, the com- 

 bination always gives individuals that exhibit both dominant char- 



