The Norm of Reaction

    The Norm of Reaction is a conceptualization of phenotypic variation (X-axis) as the result of interaction of a complex genetic function (the genotype) over a range of environmental variation (Y-axis). The Norm of Reaction can be thought of as a genetic mirror that "reflects" environment through genotype as phenotype. The example shows the effect of temperature on the height of a particular genotypic variety of plant (e.g., an inbred line). The curvature of the norm of reaction is not constant. At lower temperatures, the curve is quite steep, and an increase of 0.5oC from 18.0oC to 18.5oC produces a sharp increase in average height. At higher temperatures, the curve is shallow and the same half-degree increase from 21.5oC to 22.0oC produces a much more modest effect.

    The Norm of Reaction is a population rather than an individual genetic concept. In the example, a group of plants with the same genotype raised in a greenhouse or "common garden" whose temperature ranges over 20 ± 2oC are expected to vary in height over a range of 4 cm, skewed to taller plants, with a mode of 20 cm. In a more tightly controlled environment (an incubator), plant raised at exactly 20oC will have a height of exactly 20 cm, depending on the heritability of height.

Figure after ©2002 by Griffiths et al.; all text material ©2020 by Steven M. Carr