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. 
(b) 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. 
(b) 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. 
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