VG and VE


Figure 9.6 Examples of phenotypic variation due to genetic (VG), environmental (VE), and genotype-by-environment (VGxE) causes shown in norm-of-reaction plots. In all graphs, the phenotypic values of four genotypes within each of two environments (here called A and B) are plotted. Lines connect the phenotypic values of one genotype measured in the two environments. (a) Genotypic variation, where the four genotypes have different phenotypic values but the phenotypic value of each genotype does not change between environments. (b) Environmental variation: all genotypes have identical phenotypes (lines are staggered so that each can be seen) but the phenotype changes between environments. (c) Both genotypic and environmental variation: genotypes differ in phenotype and genotypes have different phenotypes in the different environments. (d) and (e) Genotype-by-environment interaction, with genotypes differing in the phenotypic value expressed in two or more environments. One type of genotype-by-environment interaction is characterized by lines connecting the genotypes that are not parallel (d), leading to changes in the phenotypic variance. In a second type of genotype-by-environment interaction, the rank order of phenotypic values exhibited by genotypes changes across environments and leads to crossing lines in norm-of-reaction plots (e).

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