A very rapid decrease in heterozygosis is naturally indicated in the formative period of the breed (studied in Shorthorn cattle7'8).But even after large numbers are reached the average rate of decrease per generation in 6 breeds including cattle,8"l0 horses,9 sheep'0 and swine'0 is about 0.5%, equivalent to 25 as the effective number of sires per generation.It is probably also true in nature that the effective population number may be very much smaller than the actual number, making the effect considered here an important one in the life of the species.
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Sewall Wright (1933) studied this question.
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