Chance as a Cause of Changes in Gene Frequency Within Pure Breeds of Livestock

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Abstract
Chance deviations in gene frequency from one generation to the next, as a result of Mendelian sampling and the elimination of potential parents, are important in the pure breeds of livestock . Studies of inbreeding and relationship within 9 breeds of cattle, 4 breeds of horses, 2 of sheep, and 2 of swine indicate that these chance changes in gene frequency from one generation to the next Usually have a magnitude of around 0.02 to 0.05. The major cause for these chance changes being so much larger than would be required automatically by the finite size of the population seems to be inequality in the number of gametes which the various potential parents actually contribute to the next generation. Attention to family and pedigree when selecting breeding stock accentuate this. Economic incidents, which make whole herds more important or less important as sources of breeding stock for other purebred herds, also increase the correlation between the fates of relatives. Presumably these circumstances make chance fluctuations in gene frequency more important in purebred livestock than in most wild spp., but that remains debatable in the absence of exact information about the actual pedigrees of wild animals.

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