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.