Principles of Plant Breeding

Environment and quantitative variation

• All genes are expressed in an environment (Phenotype =Genotype X Environmental effect). However, quantitativetraits tend to be influenced to a greater degree than qualitative traits.
• In polygenic inheritance, segregation occurs at a large number of loci affecting a trait. ThePhenotypic expression of polygenic traits is dependingon variation in environmental factors to which plants in the population are subjected. Polygenic variation cannot be classified into discrete groups (i.e., variation is continuous). This is because of the large number of segregating loci, each with effects so small that it is not possible to identify of individual gene effects in the segregating population. Instead, biometrics is used to describe the population in termsof means and variances. Continuous variation is caused by environmental variation and genetic variation due tothe simultaneous segregation of many genes affecting the trait.
• In 1910, a Swedish geneticist, Nilsson-Ehle provided a classic demonstration of polygenic inheritance