The Norm of Reaction

    The Norm of Reaction is a conceptualization of the genotype as a complex genetic function that predicts the pattern of phenotypic variation (X-axis) expected over a range of environmental variation (Y-axis). Stated another way, the Norm of Reaction is a genetic mirror that "reflects" environment through the genotype as phenotype. The example shows the effect of varying temperature on the height of a particular variety [genotype] of plant. Note that the curvature of the norm of reaction is not constant. At low temperature, the curve is quite steep, and an increase of 0.5oC from 18.0oC to 18.5oC produces a sharp increase in height. At high temperature, the curve is shallow and the same increase from 21.5oC to 22.0oC produces a much more modest effect.

Figure ©2002 by Griffiths et al.; all text material ©2012 by Steven M. Carr