Components of the Phenotypic Value of Traits with Quantitative Variation

Many of the important traits of horticultural or agricultural crops display quantitative variation. The phenotypic values observed for such a trait tend to depend both on the quality of the growing conditions as well as on the (complex) genotype with regard to loci affecting the trait. The goal of horticulturists and agronomists is the manipulation of the growing conditions in such a way that the performance of the crop better obeys the goals of the growers and consumers. The goal of breeders is improvement, by means of selection, of the (average) genotypic value concerning the trait. For breeders it is, therefore, important to have some understanding of the degree in which the phenotypic expression of traits with quantitative variation is due to the genetic make-up. Breeders should select the candidates with the most attractive genotypic values, not those with the most attractive phenotypic values. The partitioning of the phenotypic values of the candidates into components, including components of the genotypic value, is therefore a topic to be considered seriously.