346 



The Journal of Heredity 



TWO METHODS OF BREEDING 



Figure 4. Two pedigrees in which the number of ancestors in each generation remains 

 the same, but in which the coefficient of inbreeding (F) of the individual X equals 0, while 

 that of V approaches 100 per cent. This is in accord wil^ the results of experiments in 

 inbreeding, where crossing two weakened inbred lines results in the return of normal vigor. 

 Methods of calculating inbreeding that are based only on the ratio of the number of 

 ancestors in each generation to the greatest possible number would not bring out this 

 difference. 



tween two individuals, each of which 

 comes from a line in which brother- 

 sister matings have been made genera- 

 tion after generation. The other comes 

 from a hne in which double first cousin 

 matings have been made for genera- 

 tions. In each case there are two 

 parents, four grandparents, four great 

 grandparents, and four in every an- 

 cestral generation to the point where 

 inbreeding commenced. They both, 

 therefore, have the same very high 

 coefficient of inbreeding according to 

 Pearl's formula. According to the' 

 writer's formula, on the other hand, 

 they are nearly as dififerent as possible. 

 The former has a coefficient of 0, the 

 latter of nearly 1. The former is 



heterozygous in all respects in which 

 the ancestral lines differ ; the latter ap- 

 proaches perfect homozygosis. 



With respect to vigor, the experi- 

 ments of the Bureau of Animal In- 

 dustry with guinea pjigs, already re- 

 ferred to, demonstrate that the cross- 

 ing of two weakened inbred lines re- 

 sults in a recovery of the vigor that 

 has been lost. Thus generally speak- 

 ing, the animal X (Figure 4), would 

 be a vigorous animal ; the animal }' 

 would be weak in various respects. 

 The experimental evidence is thus in 

 accord with the coefficients and 1 

 while as far as the writer knows there 

 is no experimental evidence to indicate 

 that the niunber of ancesters per sc 



