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 
young 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, which 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 
