HOW CHARACTERS BEHAVE 



H7 



the 50ZV; that is to say, where one character is dominant and 

 the other recessive, it is simply impossible to separate the pure 

 dominant from the mixed dominant and recessive by appear- 

 ances merely. It can only be done by a resort to the breeding 

 test, when the really pure Z> 2 's will produce only D's, while the 

 real ZVs will produce back again the characteristic D 1 4- 2 Dr 

 + r 2 with its 25 per cent of pure rs. As has been already 



Fig. 25. Showing albino sire and black dam with their offspring, all black. 

 Below, a pair of the hybrid offspring and their litter (see text). From photo- 

 graphs furnished by W. E. Castle, Harvard University 



explained, no such difficulty exists with regard to the pure reces- 

 sive character, because from the first those that look like re- 

 cessive are recessive. For this reason breeders are always glad 

 when a desired character proves to be recessive, because it can 

 be so much more easily separated from its associated character 

 than can a dominant. 



This behavior of unit characters in hybrids is beautifully illus- 

 trated by the work of Professor Castle with guinea pigs, as 



