Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 118

 Selection is most effective at intermediate gene frequency (q = 0.5) and least effective at very large or very small frequencies (q = 0.99 or q = 0.01). Further, selection for or against a rare allele is ineffective. This is so because a rare allele in a population will invariably occur in the heterozygote and be protected (heterozygote advantage).

 Migration increases variation of a population. Variation of a population can be expanded in a breeding program through introductions (impact of germplasm). Migration also minimizes the effects of inbreeding.

 In the absence of the other factors or processes, any one of the frequency altering forces will eventually lead to fixation of one allele or the other.

 The forces that alter gene frequencies are usually balanced against each other (e.g. mutation to a deleterious allele is balanced by selection).

 Gene frequencies attain stable values called equilibrium points.

 In both natural and breeding populations, there appears to be a selective advantage for the heterozygote (hybrid). Alleles with low selection pressure may persist in the population in heterozygote state for many generations.

 As population size decreases, the effect of random drift increases. This effect is of importance in germplasm collection and maintenance. The original collection can be genetically changed if a small sample is taken for growing to maintain the accession.