19-21.] 



Research in Animal Briieding. 



15 



which, though Hke the pure dominant in appearance, differs 

 constitutionally from it in producing both " dominant " and 



recessive " germ cells in equal numbers. The true test of the 

 pure bred animal is that it breeds true, and this we cannot tell 

 from its appearance, but only from the nature of the germ cells 

 that it produces. Mendel's advice to the breeder is : Think in 

 terms of germ cells." 



If the breeder wishes to prevent the appearance of reds in 

 his herd he must eliminate the red germ cells, as these may be 

 carried by blacks as well as by reds. The impure dominant 

 blacks must be weeded out in order to be sure that red calves 

 will not appear. -Through Mendelism there is now a definite 

 test that can be applied to determine whether the black is pure 

 or not, and that test is to mate with the recessive; so mated, the 

 true-breeding black will produce only blacks, while the impure 

 dominant will produce an equal number of blacks and reds. This 

 of course is a policy of perfection, and unlikely to be put into 

 practice. Cows in a pedigree herd are too valuable to devote 

 an appreciable proportion of their progeny to testing operations. 

 Rut since red calves are never born of blacks, unless both parents 

 are impure dominants, it is clear that the use of a bull which 

 had been tested by mating to red cows, and shown to produce 

 only black progeny, would be sufficient to prevent the appear- 

 ance of red calves in a herd, whatever the proportion of impure 

 dominants among the cows. In practice, therefore, the breeder 

 would be well advised to make sure of the bulls by testing them, 

 even though he did not trouble about the cow^s. Rut although 

 nothing but blacks will be produced, the red germ cells will still 

 be scattered about in some of the cows. He cannot be vsure, 

 without testing, that an animal sold out of the herd will be a pure 

 black. Nevertheless if he makes use only of tested l)ulls the 

 proportion of inrpure dominants among the cows will gradually 

 decrease, and the possibilities of any beast sold being a true 

 black will increase correspondingly. If, however, he uses a new 

 bull without testing it, and it happens to be an impure dominant, 

 t\ considerable increase must be expected in the number of red 

 ( lives in later generations, for such a bull introduces as many 



'•ed " germ cells as " black," and nmst necessarily increase 

 the proportion of impure dominants in the herd. 



Th ^ breeder may reason that, provided the animal brought 

 into tli^ herd had a good pedigree, why should further trouble be 

 taken? If its ancestry shows an unbroken line of ])lacks for, 

 say. the last 10 generations, is it not practically certain that 

 iK^ne but b' :ck calves will ho thrown? The answer is tliat \hh]]- 



