INBREEDING AND CROSSBREEDING 239 



made in the production of a particular individual, as a con- 

 sequence of mating together related individuals among his 

 direct ancestors, but this the coefficient of inbreeding can 

 not do with great exactness because even with the closest 

 possible inbreeding (self-fertilization) the approach to honuj- 

 zygosity in individual cases is quite a matter of chance. 

 Thus, East and Jones say, forcefully and quite correctly, 

 *'The rate at which complete homozygosity is approached 

 depends on the constitution of the individuals chosen. 

 Theoretically in any inbred generation the progenitors of 

 the next generation may be either completely heterozygous 

 or completely homozygous or any degree in between, de- 

 pending upon chance. The only condition which must fol- 

 low in self-fertilization is that no individual can ever be more 

 heterozygous than its parent, but may be the same or less. 

 Thus it is seen that artificial inbreeding, as it is practiced, 

 may theoretically never cause any reduction in heterozygos- 

 ity, or it may bring about complete homozygosity in the first 

 inbred generation. In other words the rate at which homo- 

 zygosity is approached may vary greatly in different lines." 

 . . . ** Although nearly complete homozygosis is theoreti- 

 cally brought about by seven generations of self-fertilization, 

 the attainment of absolute homozygosity is a difficult matter 

 and in practice it may never be reached." . . . "Con- 

 tinued selective mating is necessary to bring about homo- 

 zygosity. Intermittent inbreeding alternating with periods 

 of outcrossing, which is the prevailing state of affairs ^^•ith 

 many organisms, cannot maintain any high degree of homo- 

 zygosity." These statements show that the coefficient of in- 

 breeding, though it appears to be very precise, like Galton's 

 law of ancestral heredity, is subject to similar limitations. 

 It indicates what is true of populations in the mass, but has 

 small utility as an indicator of what happens in individual 

 cases. \Mien it is applied to the case of a particular Jersey 

 bull it may be very much less reliable as an index of prob- 

 able performance than the judgment of an experienced cattle 

 breeder. 



