Maintenance of quantitative genetic variation in animal breeding programmes

Factors influencing the maintenance of genetic variation in quantitative traits in populations undergoing artificial selection are reviewed. Formulae are given for simple cases, in particular for the infinitesimal model where variation is lost by genetic drift and gained by mutation and therefore the minimum population size to maintain genetic variation is a function of mutation rate. For genes with effects too large for the infinitesimal model assumptions to apply, it is suggested that a criterion for minimum population size is that large enough to give high probabilities of fixation of segregating genes. Those most at risk are at low frequency, and likely to be recent mutations. If the population is subdivided with low rates of migration, variation over the totality of populations may be maintained or increased, but cannot necessarily be readily utilised. For example, if populations are conserved without selection alongside selected populations, variation among the populations rises but is ever harder to utilise as the conserved populations fall behind. With the introduction of new molecular genetic technology, it is likely to be possible to introduce new variation into populations in a number of ways. In view of the large generation interval of some species, the discount rate per generation that should be applied is very high, so long term gains have low net present value.  2000 Elsevier Science B.V. All rights reserved.