Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 177

Correlation is a measure of the degree of association between traits as previously discussed. This association may be on the basis of genetics or may be non‐genetic. In terms of response to selection, genetic correlation is what is useful. When it exists, selection for one trait will cause a corresponding change in other traits that are correlated. This response to change by genetic association is called correlated response. Correlated response may be caused by pleiotropism or linkage disequilibrium. Pleiotropism is the multiple effect of a single gene (i.e. a single simultaneously affects several physiological pathways). In a random mating population, the role of linkage disequilibrium in correlated response is only important if the traits of interest are closely linked.

In calculating correlated response, genetic correlations should be used. However, the breeder often has access to phenotypic correlation and can use them if they were estimated from values averaged over several environments. Such data tend to be in agreement with genetic correlation. In a breeding program the breeder, even while selecting simultaneously for multiple traits, has a primary trait of interest and secondary traits. The correlated response (CRy) to selection in the primary trait (y) for a secondary trait (x) is given by

where h_x and h_y are square roots of the heritabilities of the two respective traits and ρg is the genetic correlation between traits. This relationship may be reduced to

since h_y = √(V_Gy/Vpy)

It is clear that the effectiveness of indirect selection depends on the magnitude of genetic correlation and the heritability of the secondary traits being selected. Further, given the same selection intensity and a high genetic correlation between the traits, indirect selection for the primary trait will be more effective than directional selection, if heritability of the secondary trait is high (ρ_gh_x > h_y). Such a scenario would occur when the secondary trait is less sensitive to environmental change (or can be measured under controlled conditions). Also, when the secondary trait is easier and more economic to measure, the breeder may apply a higher selection pressure to it.

Correlated response has wider breeding application in homozygous, self‐fertilizing species and apomicts. Additive genetic correlation is important in selection in plant breeding. As previously discussed, the additive breeding value is what is transferred to offspring and can be changed by selection. Hence, where traits are additively genetically correlated, selection for one trait will produce a correlated response in another.