Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 117

Selection basically concerns the differential rate of reproduction by different genotypes in a population. The concept of fitness describes the absolute or relative reproductive rate of genotypes. The contribution of genotypes to the next generation is called the fitness (or adaptive value or selective value). The relative fitness of genotypes in a population may depend on its frequency relative to others. Selection occurs at different levels in the plant – phenotype, genotype, zygote, and gamete – making it possible to distinguish between haploid and diploid selections. The coefficient of selection is designated s, and has values between 0 and 1. Generally, the contribution of a favorable genotype is given a score of 1, while a less favorable (less fit) genotype is scored 1 − s.

An s = 0.1 means for every 100 zygotes produced with the favorable genotype, there will be 90 individuals with the unfavorable genotype. Fitness can exhibit complete dominance, partial dominance, no dominance, or overdominance. Consider a case of complete dominance of the A allele. The relative fitness of genotypes will be:

Genotypes	AA	Aa	aa	Total
Initial frequency	p 2	2pq	q 2	1
Relative fitness	1	1	1 − s
After selecting	p 2	2pq	q 2 (1 − s)	1 − sq 2

The total after selection is given by:

To obtain the gene frequency in the next generation, use

where

The relationship between any two generations may be generalized as:

Similarly, the difference in gene frequency, Δq, between any two generations can be shown to be:

Other scenarios of change in gene frequency are possible.

Plant breeders use artificial selection to impose new fitness values on genes that control traits of interest in a breeding program.