166 MENDELISM chap. 



often due to the reappearance of a recessive character. 

 Thus even in the most highly pedigreed strains of polled 

 cattle such as the Aberdeen 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 domi- 

 nant, they can be readily separated by the breeding test. 

 For when crossed by the recessive, in this case horned ani- 

 mals, the pure dominant gives only polled beasts, while 

 the heterozygous 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. 



