158 MENDELISM chap. 



occasional appearance of a sport in a supposedly 

 pure strain is often due to the reappearance of a 

 recessive character. Thus even in the most highly 

 pedigreed strains of polled cattle such as the Aber- 

 deen-Angus, occasional individuals with horns appear. 

 The polled character is dominant to the horned, and 

 the occasional reappearance of the horned animal 

 is due to the fact 'that some of the polled herd are 

 heterozygous in this character. When two such indi- 

 viduals are mated, the chances are i in 4 that the 

 offspring will be horned. Though the heterozygous 

 individuals may be indistinguishable in appearance 

 from the pure dominant, they can be readily separated 

 by the breeding test. For when crossed by the 

 recessive, in this case horned animals, the pure 

 dominant gives only polled beasts, while the hetero- 

 zygous individual gives equal numbers of polled and 

 horned ones. In this particular instance it would 

 probably be impracticable to test all the cows by 

 crossing with a horned bull. For in each case it 

 would be necessary to have several polled calves 

 from each before they could with reasonable certainty 

 be regarded as pure dominants. But to ensure that 

 no horned calves should come, it is enough to use a 

 bull which is pure for that character. This can 

 easily be tested by crossing him with a dozen or so 

 horned cows. If he gets no horned calves out of 

 these he may be regarded as a pure dominant and 

 thenceforward put to his own cows, whether horned 

 or polled, with the certainty that all his calves will 

 be polled. 



Or, again, suppose that a breeder has a chestnut 

 mare and wishes to make certain of a bay foal from 



