Heterosis is the advantage of crossbred individuals over the average of purebreds from the breeds used in the cross. Heterosis arises because dominance effects frequently create a situation in which the heterozygote is superior to the average of the two homozygotes. The examples of gene action illustrate this. If a trait is controlled by many pairs of genes in which there is dominance, then we would expect an advantage for crossbred animals, relative to the average of the purebreds that formed the cross. Because, as seen previously, minimally heritable traits are generally influenced by dominance effects, such traits may be expected to show evidence of large amounts of hetero-sis. Such a pattern has been observed.[1,4,7] Minimally heritable traits, such as those involved with reproduction or livability, tend to also show a large advantage for crossbreds over purebreds. Similarly, there is little heterosis for traits associated with carcass merit where heritability tends to be high. Besides the contributions of dominance and heterozygocity, epistasis also affects the amount of observed heterosis. Different types of epistasis
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