452 ELEMENTARY BIOLOGY 



Immunity to disease is not always dominant. In the case of 

 wheat, immunity to the rust is recessive. It is nevertheless 

 possible to establish strains of wheat that combine immunity 

 with other desirable qualities, since, as we have seen, it is only 

 necessary to breed the hybrids into the next generation in 

 order to get a complete segregation of the various characters 

 in all possible combinations, and then select in a third genera- 

 tion the offspring that have the desired character in a pure 

 dominant or pure recessive condition.^ 



475. Breeding for special points. Those who have to handle 

 cows or sheep often find the presence of horns in these animals 

 a nuisance. Many farmers therefore take steps to prevent the 

 development of the horns by destroying the "button" in the 

 young animal by means of alkali or other chemicals. Occasion- 

 ally, however, animals have appeared that failed to develop 

 horns at all. When such a naturally hornless animal is mated 

 with one that has horns, the offspring is found to be without 

 horns. In other words, the polled, or hornless, condition is 

 dominant. It is therefore possible to establish breeds of cattle 

 that never produce horns. 



In raising sheep certain kinds of fleece are found to be 

 more profitable than others. In order to combine the merino 

 wool with hornlessness, for example, it would be necessary 

 to find out by means of breeding experiments which char- 

 acters were dominant and which recessive, and then estab- 

 lish in three generations new breeds possessing the desired 

 characters. 



476. Practical breeding. The failure of their hybrids to 

 breed true has been the despair of agriculturists and breeders 

 in the past. Only those were successful who, like Luther 

 Burbank, were patient enough to try out large numbers and 

 keen enough to detect the rare individuals that would breed the 

 desirable qualities "true." With the discoveries of the biolo- 

 gists, every intelligent fancier of plants or animals can produce 



^ This is not necessary when plants are propagated vegetatively. 



I 



