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 BbHh produces four kinds 

 of reproductive cells (BH, Bh, bH, and bh). Half of these transmit 

 the factor for black (b) and half that for chestnut (7*,). 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 



