& The Norm of Reaction [ II ]
"Is the difference between groups genetic?"
The Norm of
is a curve
that relates, for a given genotype,
environmental variation to observed phenotypic variation.
The norm of reaction curve can be thought of as a mirror that
the environmental-effect curve into phenotypic space. But, given
genotype, does the norm of reaction explain any observed
differences between groups?
Consider two breeds of cattle, corresponding to genotypes A & B.
[Left] The norm of
reaction for each genotype is flat
the two lines are
cattle raised in either a nutrient-poor free-range environment
curve on the y-axis) or a nutrient-rich feed-lot environment ("Good"
those with genotype A always produce more milk
(narrow red curve on
than do those with genotype B (narrow
blue curve). The range of
breed is quite small and largely independent of the environment:
quantitative genetic study of milkfat
production will conclude that genes have
a strong influence on milkfat production (high
heritability). This corresponds to a pattern in which the difference between breeds is "genetic" in the sense that it is
determined by genotype, independent of environment.
[Middle] The norm
reaction for each genotypes is sloped
the two lines are offset with constant
of the environment for "Poor' to "Good"
a significant increase in
milk quality for both breeds. In any given environment, the
of genotype A
always exceeds that of genotype B. Note
however that the performance of genotype B in the "Good"
environment exceeds that
in the "Poor"
strong environmental component,
production of either breed can be modified by a change in
norm of reaction for each genotype is a complex curve, and the
different shapes. The
two curves are essentially flat and parallel in the "Poor"
environment, and heritability is high as in the first example.
the same two
breeds are moved to the "Good"
environment, both show a marked improvement
in average milkfat production, as in the second example.
cattle with genotype A now
typically produce richer milk
than those with genotype A (mean
blue curve slightly higher), the reverse of the previous
of reaction becomes steeper, both breeds also show a wider
range of milk fat production (broad blue
curves), depending on the exact environmental conditions. A
heritability in this environment will conclude that
genes have relatively low influence on milk fat production (low
heritability), which is mostly a consequence of environmental
Thus the relative
importance of "genetics"
and "environment" cannot be expressed as a unitary heritability
coefficient, which may vary greatly
depending on the environment in which the genes are expressed.
Heritability measured for a single
in a single environment cannot
accurately estimate the Norm of Reaction between groups and environments,
and often may not predict
phenotypic response in a different environment.