324 GENETICS IN RELATION TO AGRICULTURE 



mated. On the other hand, the general principles derived from the 

 mathematical study of the composition of populations are of universal 

 application. These principles may be summarized as follows: 



1. (a) Continued self-fertilization tends to eliminate all heterozygotes 

 from the population. 



(6) The number of homozygous genotypes to which a self-fertilized 

 population will be reduced depends upon the number of pairs of factors 

 involved. 



(c) Such a population after a few generations will consist entirely of 

 pure lines. 



2. (a) With a given amount of natural crossing in the absence of any 

 disturbing effects there will be an approximation toward a definite pro- 

 portion of heterozygotes in the population. 



(6) Such a population approaches very nearly a condition of equilib- 

 rium within a few generations. 



(c) Under the influence of disturbing elements the proportion of 

 heterozygotes may be increased or decreased, but the condition of 

 equilibrium will be rapidly approached if the disturbing elements remain 

 fairly constant. 



