Selection with Regard to a Trait with Qualitative Variation

Plant breeding aims at the genetic improvement of plant material. Thus among candidates for selection (clones, (pure) lines, hybrids, families or individual plants) those resembling most closely the ideal of the breeder are selected. The genetic improvement due to selection often deviates from the ultimate goal. One of the causes is that natural selection interferes with the artifi- cial selection. Thus the phenotype(s) favoured by the breeder (under artifi- cial selection) may differ from the phenotype(s) best prepared for ‘the struggle for life’ (under natural selection). Another cause for a disappointing result from artificial selection is the fact that the phenotype of a candidate is a poor indicator of the quality of its genotype. The phenotype may give a misleading impression of the genotype because of dominance, of epistasis or because of the growing conditions. This chapter considers impacts of artificial selection on the genotypic composition with regard to traits with qualitative variation. Some attention is given to effects of natural selection. Selection with regard to traits with quantitative variation is considered in later chapters.