PRACTICAL IMPORTANCE OF MENDEL'S LAW 373 



mentioned of the varieties of wheat susceptible and immune 

 to " rust " is in itself very suggestive. 



A case like that of the Andalusian fowls shows how immediate 

 may be the practical utility of Mendelism. The pairing of two 

 Andalusians yields only six Andalusians to the dozen ; the crossing 

 of a black and a white yields twelve Andalusians to the dozen. 



The impossibility of fixing the Andalusian characters into 

 a stable race is simply due to the fact that the Andalusians are 

 hybrids, and the same is probably true of cases like the sugar- 

 beet, where selection seems to have ceased to produce further 

 improvement. 



The breeder who wishes to get a stable and pure strain rapidly, 

 has obviously a clue in the behaviour of the extracted recessives 

 and the extracted dominants of the F 2 generation. There are 

 many similar practical applications of Mendelian results. 



Inbreeding. — Breeders who have with carefulness evolved a 

 fine herd are often very loath to introduce fresh blood, even when 

 they suspect that they are approaching the limits of safe in- 

 breeding. But if Mendelism applies to the organisms bred, 

 then it does not seem as if the introduction of fresh blood need 

 affect the purity of the stock. A cross is effected to secure 

 reinvigoration ; when the results of the cross are inbred, forms 

 like the original parent will reappear. 



Old-established form. " Fresh blood." 

 A B 



If A be dominant, A(B) ; or B(A), if B be dominant, 

 x x 



A A 



n A + n A(B) n A + «B(A) 



Or if A(B) be inbred the result will be n A + 2 n A(B) + n B. 

 Or if B(A) be inbred the result will be n A + 211 B(A) + n B. 

 There is obviously no theoretical danger of losing A, 



