HOW CHARACTERS BEHAVE 147 
the 50Dr; 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 D?’s will produce only D's, while the 
real Dr's will produce back again the characteristic D? + 2 Dr 
+7? with its 25 per cent of pure 7's. As has been already 
Fic. 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 Joo’ 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 
