ganism into the many structures we can distinguish and finally into its par- 

 ticular characteristics, or variations. We analyze the chromosome-action, 

 trying to find the smallest possible units, in the hope of explaining very com- 

 plex processes. But however far we may carry our analysis, the problem of 

 heredity remains the problem of life itself: (1) a living organism builds itself 

 out of foreign materials; (2) it passes through a cycle of change which ends 

 in its death; but (3) it perpetuates its own distinct qualities in the living 

 processes of other objects — the immediate offspring or later descendants. 



What Are the Practical Applications of Genetics? 



Need for Better Types of Organisms Fanciers, commercial breeders, 

 and seedsmen are constantly looking for interesting novelties, both among 

 their own growths and the world over. Occasionally there appears a ''sport", 

 or an exceptional individual, with valuable characteristics (see page 509). 

 Furthermore, breeders of plants and animals have not been content with find- 

 ing desirable individuals or strains by chance, but have attempted to bring 

 about variations of a kind that are both useful and permanent. But it is only 

 since the beginning of the present century that we have known the biological 

 principles for combining systematically in a race or variety a number of 

 desirable qualities, and avoiding undesirable ones. 



Among the most serious of the "undesirable" qualities in domestic plants 

 and animals is susceptibility to disease. The late blight of the potato causes 

 an annual loss of about nine million bushels. In the poultry industry the loss 

 of pullets runs from thirty to forty per cent. It is not possible, as we have seen, 

 to transmit all the characters that appear in a hybrid or even in a combination 

 that results from segregation. It is necessary that those factors or "genes''' in 

 the two parental gametes which determine a desired character shall be either 

 both dominant or both recessive. If only one of the germ cells is dominant, a 

 particular individual may have the quality in which we are interested, but its 

 offspring will be of two kinds (see illustration, p. 486). 



Breeding for Immunity Certain American breeds of good beef cattle 

 that could be handled in great herds on large prairie ranches were susceptible 

 to the destructive Texas fever. The "Brahman" cattle of India were immune 

 to Texas fever. On mating these immune animals with a susceptible variety 

 the immunity appears as dominant. Brahman cattle were accordingly im- 

 ported for crossing with our native cattle. A new variety was established; 

 this combined the beef quaUties of the American cattle with the immunity of 

 the Hindu type. In this case, breeding for immunity ceased to be important 

 when we learned to prevent the disease (see page 617). But in other cases this 

 principle has been of great value. 



In the case of wheat, immunity to "rust" is recessive. It has nevertheless 



496 



