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123456789 Phenotypic value (units of pigment)

Figure 9.3 The effect of environmental variation on phenotypic variation. The phenotypic distribution on the left is produced by two Mendelian loci with all allele frequencies equal to 1/2 as in Figure 9.2. If the environment causes some variation in the phenotype expressed by each genotype, then the distribution of phenotypes produced by polygenic variation becomes both smoother and wider. In this illustration, environmental variation causes 50% of the individuals of each genotype to randomly increase or decrease one unit in phenotypic value. Although the average effect of environmental variation here is a zero change in phenotypic value, the phenotypic variance increases.

of diet and exercise in human disease, where better conditions tend to lessen the frequency or severity of disease phenotypes. Figure 9.3 shows how environmental differences among individuals contribute to the continuous distribution of phenotypic variation. The left-hand panel of Fig. 9.3 shows the five pheno-types produced by a trait due to two diallelic loci. If each phenotypic class expressed by a genotype is modified by environmental variation, the distribution of phenotypes becomes both wider and smoother as shown in the right-hand panel of Fig. 9.3. The environmental variation experienced by individuals is also likely to be a truly continuous variable, unlike the discrete categories of genotypes produced by multiple loci, that then causes continuous variation in phenotypes.

Components of phenotypic variation

Now that we have seen how discrete Mendelian genetic variation for multilocus genotypes combined with continuous environmental variation produces continuous phenotypic distributions, let's represent the genetic and environmental causes of phenotypic variation in notation. In quantitative genetics it is customary to symbolize expected quantitative trait variation with a V. The V variable always bears a subscript to indicate a specific cause of phenotypic variation. The total variation in phenotype is represented by VP and examples of total phenotypic variance are shown in Figs 9.1 and 9.3. This total phenotypic variation has both genetic and environmental causes. The phenotypic variation caused by variation in genotypes in the population is represented by VG. Independently, the variation in phenotype caused by the environment is represented by VE. The equation vp = vg + VE

is used to represent that the total variation in phenotype in a population is the sum of phenotypic variation caused by genotype differences among

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