Il6 AGRICULTURAL RESEARCH AND THE FARMER : 



eliminate the red germ cells, and these may be carried by black 

 animals as well as by red ; in other words he must weed out the 

 impure dominants (black, concealing red) as well as the red 

 recessives. Thus a knowledge of the principles of heredity 

 supplies a definite test which can be applied to determine whether 

 a black animal is pure or not, and that test is to mate it with 

 a red animal, that is, with a recessive. So mated, the true- 

 breeding black will produce only blacks, while the impure 

 dominant will produce both blacks and reds. This, of course, is 

 a policy of perfection, and cows in a pedigree herd are too valuable 

 to devote an appreciable proportion of their progeny to testing 

 operations. But since red calves are never born of blacks, 

 unless both parents are impure dominants, it is clear that the 

 use of a bull which has been tested by mating to red cows, and 

 shown to produce only black progeny, would be sufficient to 

 prevent the appearance of red calves in a herd, whatever the 

 proportion of impure dominants among the cows. If the breeder 

 makes use only of tested bulls, this proportion 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, 

 a considerable increase must be expected in the number of red 

 calves in later generations, for such a bull introduces as many 

 " red " germ cells as " black," and must necessarily increase the 

 proportion of impure dominants in the herd. 



The case of the black-red cattle illustrates a type of problem 

 in which an important external character depends upon a single 

 pair of germ " factors," and in such cases the procedure for 

 ensuring purity is simple. But the characters that breeders are 

 concerned with are rarely so simple and distinct. The cases 

 are usually far more complicated, where a cross seems to result 

 merely in a muddled blend, and at first sight there appears no 

 suggestion of clear cut alternative pairs of factors. The question 

 thus arises as to whether such complicated cases can be resolved 

 in terms of a few definite factors, each of which is transmitted 

 according to the accepted laws of inheritance. It is such 

 problems as these, applied to rabbits and poultry, that have 

 been engaging the attention of the Cambridge workers for some 

 years past, for it was realised that if complex problems in heredity 

 in these small animals could be resolved into simple hereditary 

 units, it was fairly certain that the same thing would apply 

 to the larger animals, and experiments on the latter could then 

 be initiated with every assurance of success. It may here be 

 said that in the case of every problem that has so far been 

 investigated, and which it has been practicable to carry through 



