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The Norm of Reaction


    The Norm of Reaction is a conceptualization of phenotypic variation (X-axis) as the result of interaction of a complex 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 steep: an increase of 1.0oC from 18.0oC to 19.0oC produces a 1.5 cm increase in average height. At higher temperatures, the curve is shallow: a 1.0oC increase from 21.0oC to 22.0oC produces a 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 whose temperature ranges over 20 ± 2oC are expected to vary in height over a mode of 
20 ± 2 cm, skewed toward taller plants. 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 ©2024 by Steven M. Carr