Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 129

As previously indicated, plant breeding is a special case of evolution, whereby a mixture of natural and especially artificial selection operates, rather than natural selection alone. The Hardy‐Weinberg equilibrium is not satisfied in plant breeding because of factors including non‐random mating. Outcrossing promotes random mating, but breeding methods impose certain mating schemes that encourage non‐random mating, especially inbreeding. Inbreeding is measured by the coefficient of inbreeding (F), which is the probability of identity of alleles by descent. The range of F is 0 (no inbreeding; random mating) to 1 (prolonged selfing). It can be shown mathematically that

If F = 0, then the equation reduces to the familiar p ² + 2pq + q ² . However, if F = 1, it becomes p : 0 : q. The results show that any inbreeding leads to homozygosis (all or nearly all loci homozygous), with extreme inbreeding leading to a complete absence of heterozygosis (all or nearly all loci heterozygous).

Differential fitness is a factor that mitigates against the realization of the Hardy‐Weinberg equilibrium. According to Darwin, the more progeny left, on average, by a genotype in relation to the progeny left by other genotypes, the fitter it is. It can be shown that the persistence of alleles in the population depends on whether they are dominant, intermediate, or recessive in gene action. An unfit (deleterious) recessive allele is fairly quickly reduced in frequency but declines slowly thereafter. On the other hand, an unfit dominant allele is rapidly eliminated from the population, while an intermediate allele is reduced more rapidly than a recessive allele because the former is open to selection in the heterozygote. The consequence of these outcomes is that unfit dominant or intermediate alleles are rare in cross‐breeding populations, while unfit recessive alleles persist because they are protected by their recessiveness. The point that will be made later but is worth noting here is that inbreeding exposes unfit recessive alleles (they become homozygous and are expressed) to selection and potential elimination from the population. It follows that inbreeding will expose any unfit allele, dominant or recessive. Consequently, species that are inbreeding would have opportunity to purge out unfit alleles and hence carry less genetic unfitness load (i.e. have more allele fitness) than outcrossing species. Furthermore, inbreeders (self‐pollinated species) are more tolerant of inbreeding whereas outcrossing species are intolerant of inbreeding.

Whereas outcrossing species have more heterozygous loci and carry more unfitness load, there are cases in which the heterozygote is fitter than either homozygote. Called overdominance, this phenomenon is exploited in hybrid breeding (see Chapter 18).