Selection for Several Traits

In the preceding chapter only selection with regard to a single trait was considered. One may say that, in practice, selection generally involves several traits. An inexperienced breeder might assume that he is selecting with regard to just a single quantitatively varying trait, for instance biomass yield of maize (Example 11.1), whereas (s)he is, in fact, selecting with regard to a set of mutually correlated traits (see end of Section 11.1). Selection, indeed, is often indirect.

With regard to traits with quantitative variation breeders always apply indirect selection. They select among candidates on the basis of observed phenotypic values, whereas the trait of interest concerns the genotypic values underlying the observed phenotypic values. Recently, indirect selection based on molecular markers has become an important new tool to improve the efficiency of selection with regard to traits with quantitative variation.

The smallest set of mutually correlated traits consists of two traits. The selected trait is the trait as observed under the macro-environmental conditions applying to the population subjected to selection, and the other trait is the same trait but then as expressed under different macro-environmental conditions.

This chapters deals with various aspects related to selection for several traits.