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| Modern Genetic Analysis Chapter 18. Quantitative Genetics Norm of Reaction and Phenotypic Distribution The phenotypic distribution of a trait, as we have seen, is a function of the average differences between genotypes and of the variation between genotypically identical individuals. But both are in turn functions of the sequence of environments in which the organisms develop and live. For a given genotype, each environment will result in a given phenotype (for the moment, ignoring developmental noise). Then a distribution of environments will be reflected biologically as a distribution of phenotypes. The way in which the environmental distribution is transformed into the phenotypic distribution is determined by the norm of reaction, as shown in Figure 18-5. The horizontal axis is environment (say, temperature) and the vertical axis is phenotype (say, plant height). The norm of reaction curve for the genotype shows how each particular temperature results in a particular plant height. So, the dashed lines from the 18°C point on the temperature axis is reflected off the norm of reaction curve to a corresponding plant height on the vertical phenotype axis, and so forth for each temperature. If a large number of individuals develop at, say, 20°C, then a large number of individuals will have the phenotype that corresponds to 20°C, as shown by the dashed line; and, if only small numbers develop at 18°C, few plants will have the corresponding plant height. Then the frequency distribution of developmental environments will be reflected as a frequency distribution of phenotypes as determined by the shape of the norm of reaction curve. It is as if an observer, standing at the vertical phenotype axis, were seeing the environmental distribution, not directly, but reflected in the curved mirror of the norm of reaction. The shape of the curvature will determine how the environmental distribution is distorted on the phenotype axis. So, the norm of reaction in Figure 18-5 falls very rapidly at lower temperatures (the phenotype changes rapidly with small changes in temperature) but flattens out at higher temperatures, so the plant height is much less sensitive to temperature differences at the higher temperatures. The result is that the symmetric environmental distribution is converted into an asymmetric phenotype distri-bution with a long tail at the larger plant heights, corresponding to the lower temperatures.

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