MENDELIAN ANALYSIS OF THE PURE 

 BREEDS OF LIVESTOCK 



I. The Measurement of Inbreeding and Relationship 



Sewall Wright 



Bureau of Animal Industry, United States Department of Agriculture, 



IVasJiington, D. C. 



THE pure breeds of livestock 

 which we have today are the re- 

 sult of many years of patient ef- 

 fort. It seems likely that they will fur- 

 nish the material for further improve- 

 ment for many years to come. It shovild 

 thus be of value to study the methods 

 used in their development and attempt 

 to express in terms of modern genetics 

 what these methods should have ac- 

 complished, and what, in consequence, 

 is the present genetic status of the 

 breeds. Before dealing- directly with 

 the work of certain of the leading- 

 breeders it will be well to consider 

 briefly the successive phases in the 

 history of livestock breeding, the 

 light which discoveries in genetics 

 has thrown on the earlier breeding- 

 methods, and finally, the analytic 

 methods by which the data in live- 

 stock pedigrees can be related to 

 Mendelian theory. 



Successive Phases in Livestock 

 Breeding 



Livestock breeders did not have to 

 wait for the development of a science 

 of breeding to accomplish a great deal 

 in the improvement of livestock. From 

 the first, indeed, there must have been 

 modification of the wild types through 

 the retention of those animals which 

 were most tractable, and it was mere- 

 ly necessary for the early shepherds 

 and herdsmen to come to a realization 

 of the great fact of heredity, for the 

 conscious molding of animal forms and 

 function toward greater usefulness to 

 man to commence. 



Primitive man naturally made no 

 distinction between innate differences 

 between animals and those due to 

 dififerences in care and feeding, and 

 believing that all were equally trans- 

 missible, concluded that good care 

 and liberal feeding were short cuts to 

 livestock improvement. Numerous other 

 beliefs, partially true or wholly false, 

 such as those concerning the injurious 

 efifects of matings between close rela- 

 tives, the efifect of maternal im- 

 P'ressions, telegony, and so forth, 

 contributed to the traditional lore of 

 breeding. The realization of the fact 

 of heredity, aided indirectly by some of 

 the other beliefs, was undoubtedly 

 enough to lead to a very considerable 

 improvement. 



Intensive efiforts toward the im- 

 provement of local types of cattle and 

 sheep began in England early in the 

 eighteenth century. After the leaders 

 in this movement had reached a cer- 

 tain degree of success, they began to 

 find difficulty in securing animals from 

 other herds and flocks which did not 

 have a detrimental influence. They 

 began cautiously to breed within their 

 own stocks. Such experiments were 

 conducted most boldly by Robert Bake- 

 well,^* who discovered that he could 

 practice close and continued inbreeding 

 not only without necessarily causing 

 degeneration of his stock, but with a 

 rapid fixation of the types for which 

 he was selecting. Those seeing his 

 Longhorn cattle and Leicester sheep 

 could not but see the difference be- 

 tween them and the stock of his neigh- 

 bors. Their uniformity of type brought 



*For numbered references, see Literature Cited at end of article. 



339 



