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Bulmer Effect

The Bulmer effect was first observed by D. C. Bulmer in 1971, and is often applied to the breeding (artificial selection) of cattle and other animals. In the Bulmer effect, genetic variation is reduced over generations due to selection, and in proportion to how different the phenotype of a specific progeny's parents are in relation to the generation the parents come from.

Hypothetially, a calf from a parent who is very different – say, red as opposed to black and white – is much more likely to be red and different from the other cattle than other calves. But, as generations continue, an equilibrium is reached in the population whereby the red genes are gradually diminished in strength, and may completely disappear, homogenizing the breed.

Selection (natural or artificial) is likely to emphasize this effect, so that genes that are not beneficial are likely to be diluted in the population and possibly disappear.

However, if the unique phenotypic trait is expressed as a network of genes some of which are essential to the population, the trait may have a more pronounced effect on the breed, with a statistically significant shifting of the phenotypic expression – whereby the unique trait remains after settling into the population's genetic equilibrium.

Book Resources On Bulmer Effect

Consanguinity, Inbreeding, and Genetic Drift by Luigi Luca Cavalli-Sforza et al

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