Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 159

Epistasis is the interaction of alleles at different loci. It complicates gene action in that the value of a genotype or allele at one locus depends on the genotype at other epistatically interacting loci. In other words, the allelic effects at one locus depend on the genotype at a second locus. An effect of epistasis is that an allele may be deemed “favorable” at one locus and then deemed “unfavorable” under a different genetic background. In the absence of epistasis, the total genetic value of an individual is simply the sum total of the individual genotype values because the loci are independent. Epistasis is sometimes described as the masking effect of the expression of one gene by another at a different locus.

Estimation of gene action or genetic variance requires the use of large populations and a mating design. The effect of the environment on polygenes makes estimations more challenging. As N.W. Simmonds observed, at the end of the day, what qualitative genetic analysis allows the breeder to conclude from partitioning variance in an experiment is to say that a portion of the variance behaves as though it could be attributed to additive gene action or dominance effect, and so forth.