APPLICATIONS OF PRINCIPLES OF HEREDITY 451 



By selecting individuals in this third generation and grow- 

 ing from them, and by keeping records of their behavior, he 

 succeeded in finding the strains that had the necessary elements 

 in their germ cells and so transmitted the desired combination 

 of characters, in accordance with the Law of Combinations (see 

 p. 448). In this way it was possible to combine in one variety of 

 wheat the strong stem, the closed head, and the winter-resisting 

 qualities needed for successful wheat farming in this region. 

 By similar methods it has been possible to combine three or 

 viore characters desired in a plant or an anitnal from as many 

 different varieties of ancestors. 



474. Breeding for immunity. The chief problem of those 

 who have to do with plants and animals is to get organisms 

 that combine desirable qualities and show none of the unde- 

 sirable qualities. Thus, there are American breeds of cattle 

 raised for beef on the large prairie ranches ; they have good 

 beef qualities and are easily handled in large herds. But most 

 of our cattle are very susceptible to the destructive Texas fever, 

 which has caused the death of vast numbers. It had been ob- 

 served that the " Brahmin " cattle of India were immune to 

 Texas fever. On mating one of these immune animals with 

 one of a susceptible variety the immunity appears as dominant. 

 Brahmin cattle were accordingly imported for crossing with our 

 native cattle, in order to establish a variety having the beef qual- 

 ities of the American cattle and the immunity to Texas fever.^ 

 Not every combination of desirable characters appearing in a 

 hybrid plant or animal can be transmitted. It is necessary that the 

 two genes, or elements from the parental germ plasm which 

 determine a given character, be both dominant or both recessive. 



1 In the meantime it has been found out that the Texas fever is transmitted 

 by a Httle animal known as the tick, which sucks the blood from the diseased 

 cattle. By suitable quarantine it has been possible to restrict the Texas fever; 

 and by applying to the bodies of the cattle something that will either kill the 

 ticks or prevent their biting the cattle, it may be possible to eradicate this costly 

 disease. But if we could replace our present herds of cattle with a type that is 

 quite immune, the added cost would no doubt be made up in a very short time. 



