Genetics, Environment, &
The Norm of Reaction [ II ]
"Is the difference between groups genetic?"
The Norm of Reaction is a
curve that relates, for a given genotype, the contribution of environmental variation to observed phenotypic variation.
The norm of reaction curve can be thought of as a mirror that
reflects the environmental-effect curve into phenotypic space.
But, given an alternative genotype, does the norm of reaction
explain observed phenotypic differences between groups?
Consider two breeds of cattle, corresponding to genotypes A & B, evaluated for milk-fat production, and three forms of the Norm of Reaction curve.
The norm of reaction for each
genotype is flat line,
and the two lines are offset.
Among cattle raised in either a nutrient-poor free-range
environment ("Poor" curve on the y-axis) or a nutrient-rich
feed-lot environment ("Good" curve), those with genotype
A always produce higher milk-fat (narrow red curve on
x-axis) than do those with genotype B (narrow blue
curve). The range of phenotypes for each breed is quite
small and largely independent of the environment: A quantitative
genetic study of milk-fat production will conclude that genes
have a strong influence on milk-fat 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.
The norm of reaction for each genotypes is sloped line, and the two
lines are offset with constant slope. Improvement of the
environment for "Poor' to "Good" results in a significant
increase in milk quality for both breeds. In any given
environment, the performance of genotype A always exceeds that of genotype B. Note however that the performance of
genotype B in the "Good" environment exceeds that of genotype A in the "Poor" environment. Milk-fat
production has high
heritability, but shows a strong environmental component, meaning that milk-fat production of
either breed can be modified by a change in environment.
The norm of reaction for each genotype is a complex curve, and the
curves have different shapes. The two curves are essentially
flat and parallel in the "Poor" environment, and heritability
is high as in the first example. When the same two breeds are
raised in the "Good" environment, both show a marked
improvement in average milk-fat production, as in the second
example. However, cattle with genotype A
now typically produce richer milk than those with genotype
A (mean of blue curve
slightly higher), the reverse of the previous situation.
As the norm of reaction becomes steeper, both breeds also show a
wider range of milk fat production (broad blue
and red curves), depending on the
exact environmental conditions. A study of heritability in the "good"
environment will conclude that genes have relatively low
influence on milk-fat production (low heritability), which is mostly a consequence
of environmental variation.
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 population within a
single environment cannot accurately
estimate the Norm of Reaction between
groups and environments, and may not predict phenotypic
response in a different environment.