PRINCIPLES OF LIVESTOCK BREEDING. 39 
one exception, however, which should be pointed out. The mating 
of sire with daughter is in a sense as close inbreeding as brother with 
sister. Yet a male may be bred successively with his daughters, 
granddaughters, great-granddaughters, etc., concentrating his blood 
to any extent, without coming any closer to fixing his type than at 
first if the type were not fixed in himself. This will be clear from 
an illustration. A bay stallion of formula BbHJi produces four kinds 
of reproductive cells (BH, Bh, hH, and bh). Half of these transmit 
the factor for black (b) and half that for chestnut Qi). It is obvious 
that he will sire numerous black foals and chestnut foals, no matter 
how much his blood is concentrated. On the other hand, if it is 
possible to obtain a bay stallion which is known to be of formula 
BBHH, there is no quicker way of fixing a true-breeding race of bays 
than by repeated crosses with his female descendants. Such a 
stallion is prepotent, since in crosses with blacks and chestnuts he 
sires only bay foals. 
Speaking generally, the continued use of a sire of proved prepo- 
tency is the most rapid method of fixing his type, while the use of a 
sire which is not prepotent has no tendency toward fixation, but 
rather the reverse. 
ISOLATION OF GENETIC DIFFERENCES BY INBREEDING. 
It was noted in the section on variation that characteristics differ 
greatly in the degrees to which they are determined by heredity, 
outside conditions which are controllable, and by uncontrollable con- 
ditions, such as chance irregularity in development. Thus, in some 
characteristics, such as quality of coat color and, to a less extent, 
type, consistent close breeding, and uniform conditions, result in a 
highly uniform stock. In the case of functional characteristics, espe- 
cially fertility, there remains much variation even under apparently 
uniform conditions and any amount of inbreeding. Color pattern is 
also often of this kind, as we have seen in the case of guinea pigs. 
The Bureau of Animal Industry has a stock of guinea pigs which is 
descended wholly from a single mating in the twelfth generation of 
brother-sister mating. Variability has been reduced only 25 per 
cent by this inbreeding. There is still variation from nearly solid 
black to solid white, but none of it is now hereditary. The progeny 
of the blackest parents produce progeny of the same average grade 
as the whitest parents. Figure 9 shows the variation in pattern in 
four generations of guinea pigs from this inbred family. 
While inbreeding is of little use in bringing about uniformity in 
such cases, it does something else which, perhaps, is even more 
important. When there is a lot of variation which is not hereditary, 
straight selection is especially apt to be at fault. There can be no 
assured progress, since a single unfortunate mating with an animal 
