The Norm of Reaction
The
Norm
of Reaction is a conceptualization of phenotypic
variation (X-axis)
as the result of a complex genotype
expressed 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 variation in temperature on the
height of a
genotypically uniform 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 1.0 oC from 19.0 oC
to 20.0 oC produces a sharp 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 "common garden" whose temperature ranges over 18 ~ 22 oC are expected to vary in height from 28 ~ 36 cm with a mode of 30 cm, skewed toward taller plants. In a more tightly controlled environment (an incubator), plant raised at exactly 20oC will have the maximum height of exactly 30 cm, depending on the heritability of height.
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 "common garden" whose temperature ranges over 18 ~ 22 oC are expected to vary in height from 28 ~ 36 cm with a mode of 30 cm, skewed toward taller plants. In a more tightly controlled environment (an incubator), plant raised at exactly 20oC will have the maximum height of exactly 30 cm, depending on the heritability of height.