Most of the traits improved through breeding like yield, height, drought resistance, disease resistance in many species, etc. are quantitative. They are also called polygenic, continuous, multifactorial or complex traits. Quantitative traits are the result of cumulative action of many genes and their interactions with the environment. Thus, it can create a range of individuals that vary among themselves with continuous distribution of phenotypes. A quantitative trait is assumed to be controlled by the cumulative effect of numerous genes, known as quantitative trait loci (QTLs), as per multiple-factor hypothesis by Nilsson-Ehle (a Swedish geneticist in 1909) and East (an American in 1916). Hence, a single phenotypic trait is regulated by several QTLs.
Multiple-factor hypothesis (Nilsson-Ehle) Models, Assumptions and predictions Partition of variance components Linearity The infinitesimal model Types of gene action Quantifying gene action Population mean Phenotypic variance Breeding value Heritability Estimating additive variance and heritability Models for combining ability analysis Biparental progenies (BIP) Polycross Topcross North Carolina designs Diallels Multiple regression analysis Stability analysis Regression approaches Genetic architecture of quantitative traits
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