Genes, Environment, & The
Norm of Reaction [ I ]
the variation genetic or environmental?"
The Norm of Reaction for a
given genotype is a curve that relates the contribution of environmental variation to observed phenotypic variation.
The norm of reaction curve can be thought of as a genetic mirror
that reflects the environmental into phenotypic space.
Consider a breed of cattle that with a single gene locus "for"
milk-fat content. If the Norm of Reaction is flat [left], the phenotype
produced over the entire range of environments is constant. This
corresponds to what we usually think of as a "genetic":
a particular genotype always produces exactly the same phenotype
irrespective of environment. Classic single-gene traits fall into
this category: peas are round (RR, Rr) or wrinkled (rr) according to genotype,
irrespective of environment
If the norm of reaction is slightly sloped [middle], the same
genotype produces a narrow range of phenotypes. This corresponds
to what we think of as a highly "heritable" trait, in that the
phenotypic variance is much smaller than the environmental
variance. Put another way, the phenotypic value of the trait is
predictable within strict limits, irrespective of environmental
As the slope of the norm of reaction increases
[right], the range of possible phenotypes equals the environmental
range. This corresponds to an "environmental" trait, in which the
phenotype is not predictable from a knowledge of the genotype, but
requires knowledge of the environment. This is true, even if the
trait is 'genetic' in the sense that it is determined by
Given that phenotypic variation within a group
has a heritable component, we next observe a phenotypic difference
groups. We then ask, "Is the difference 'Genetic"?