34 BULLETIN 905, U. S. DEPARTMENT OF AGRICULTURE,. 



HEREDITY OF FORM AND FUNCTION IN LIVESTOCK. 



RELATIONS OF THEORY TO PRACTICE. 



In the preceding sections we have attempted to present a brief 

 outline of the present theory of heredity as it has been developed in 

 the main from experiments with small animals and plants. Illus- 

 trations were given which show that these principles apply to farm 

 livestock, at least in the case of coat color and a number of other 

 rather superficial characteristics, such as the presence of horns in 

 cattle and shape of comb in poultry. There is every reason for 

 believing that these principles are of general application, and it is 

 hardly too much to say that the normal method of inheritance is 

 now clearly understood. Unfortunately, however , a thorough 

 understanding does not necessarily mean easy control. The peculi- 

 arities of form and function appear to be so complex in their mode 

 of inheritance that an understanding of the fundamental laws of 

 heredity is at present valuable to the stock breeder largely from the 

 light which they throw on such long-known methods of breeding as 

 inbreeding, crossbreeding, selection, etc. In the present section 

 most attention will accordingly be paid to these secondary principles. 



EQUALITY OF INHERITANCE FROM THE SEXES. 



As already noted, there is, in general, equal inheritance from the 

 sexes with respect to all kinds of characteristics. There is, for 

 example, no scientific foundation for such beliefs as that the dam 

 controls the external form, the sire the constitution of the internal 

 organs, or the reverse. The only known exceptions are the rather 

 small class of sex-linked characteristics, which have already been dis- 

 cussed. For reasons other than heredity, the dam naturally has 

 more influence in the birth weight and some other characteristics of 

 yoimg animals, but these effects seem to be outgrown. 



The rule that there is in general equal inheritance from the sexes 

 must not be taken as meaning that the sire or dam may not be 

 prepotent in a particular cross on account of reasons other than sex. 



PREPOTENCY. 



An ideally prepotent animal is one that impresses his character- 

 istics on all his progeny, however mated. There are many unsound 

 beliefs connected with prepotency. It is often believed that it is a 

 characteristic of an animal as a whole, closely related to vigor. Thus it 

 is often held that a strongly masculine type in a male is an indication 

 of general prepotency. This type is desirable in itself as an indication 

 of vigor, winch is of the utmost importance in all breeding, but there 

 is no good evidence that prepotency in any other special characteristic 

 is indicated in this way. The idea of general prepotency has also led 



