Wrio-ht: Analysis of Livestock Breeding Methods 



341 



different inbred lines in characters of 

 the above kinds in which it has been 

 almost impossible to demonstrate 

 heredity otherwise. Another result has 

 been the recovery of full vigor on 

 crossing different inbred lines, ex- 

 plained as due to the complementary 

 nature of these lines, each in general, 

 supplying the particular dominant fac- 

 tors for vigor which had been lost in 

 the other. 



Characteristics differ greatly in the 

 extent to which they are determined 

 by heredity. Some, like most coat 

 colors, are almost wholly hereditary. 

 Others, like fertility and length of life 

 are largely environmental as far as the 

 individual' is concerned. Inbreeding is 

 as effective in making homozygous 

 such heredity as there is in the latter 

 case as in the former. Selection, on 

 the other hand, is effective only in pro- 

 portion to the importance of the hered- 

 itary element. The number of inde- 

 pendent factors which effect the char- 

 acter also play a part in determining 

 the effectiveness of selection method -^ 

 With knowledge of the effects to be 

 expected theoretically from the various 

 methods of breeding, in dealing with 

 characters determined to any given ex- 

 tent by heredity, and showing any de- 

 gree of segregation in the second 

 hybrid generation as compared with 

 the first generation, and showing any 

 degree of dominance, it should become 

 possible to lay i^lans on a sound basis 

 for the best system of mating to fol- 

 low to obtain the results which arc de- 

 sired with any particular character. 



The application of even these meth- 

 ods to livestock improvement must 

 necessarily, however, be a rather slow 

 and expensive process. Meanwhile it 

 is important to check them as far as 

 possible by study of the methods ac- 

 tually used by those breeders who 

 have been recognized as having been 

 most successful and find out in terms 

 of modern genetics just what it was 

 that thev did. 



Importance of Skill in Judging 



That the breeders who laid the foun- 

 dations for the pure breeds were ex- 

 ceptional among their contemporaries 

 as judges of livestock and thus ex- 

 ceptionally skillful in making matings 

 between animals which were really 

 superior, we may take for granted as 

 a big element in their success. The 

 importance which Thomas Bates at- 

 tributed to his ability in this direction 

 mav be inferred from a quotation from 

 a letter. "A hundred men may be 

 found to make a Prime IMinister to 

 one fit to judge of the real merits of 

 Shorthorns." We can form some 

 idea of the ideals which these men 

 strove for from the descriptions and 

 pictures of the noted animals which 

 they bred. Recognizing the importance 

 of their ability as judges, it is never- 

 theless difficult to put a quantitative 

 measure on what they accomplished by 

 selection. 



Measurement of Inbreeding 



W'e can, however, discover by study 

 of pedigrees how far they practiced in- 

 breeding and how far they maintained 

 a general resemblance to particular 

 worthy animals by concentrating their 

 blood. We need to use measures of 

 inbreeding and relationship which shall 

 make it possible to relate the methods 

 of these breeders directly to the modern 

 theory of genetics. Such coefficients 

 have been described in a previous 

 paper.' 



The coefficient of inbreeding (F) 

 depends primarily on the number and 

 closeness of the ancestral connections 

 between the sire and dam and second- 

 arily on the degree of inbreeding of 

 the common ancestors of the latter. 

 Every chain of generations in the 

 pedigree by which one may trace back 

 from the sire to a common ancestor 

 and then forward to the dam. passing 

 through no animal more than once 

 (within the given chain), contributes 

 to the inbreeding an amount equal to 

 one-half used as a factor one more 

 time than there are generations in the 



