240 



GENETICS AND EUGENICS 



It is nevertheless of value to know what the tendency of a 

 particular system of breeding is, if persistently followed, as 

 regards homozygosity, for homozygosity implies fidelity to 

 type in transmission and is probably what the animal breeder 

 means by "prepotency," so far as he has any clearly defined 



TABLE 32a 



Probable Percentage of Homozygosity, under Different 



Systems of Inbreeding, in Populations at the Outset Entirely 



Heterozygous. (From Data of Jennings and Fish.) 



idea in mind when he uses the term. Inbreeding tends auto- 

 matically to replace heterozygous germinal conditions by 

 homozygous conditions in the inbred population and the 

 *' closer" the degree of inbreeding the stronger is this tend- 

 ency. Jennings has worked out formulae for calculating the 

 probable percentage of homozygosity in populations inbred 

 after a particular system of matings for any number of gen- 

 erations. The results in three systems of matings for a 

 series of from 10 to 25 inbred generations are sho^n in 

 Table 32a. The progress toward homozygosis, it will be ob- 

 served is rapid in self-fertilization, heterozygotes being only 

 one-tenth of one per cent after 10 generations of inbreed- 

 ing. The elimination of heterozygotes is equally rapid when 



