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 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 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 more 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 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 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. Milkfat production has high heritability, but shows a strong environmental component, meaning that milkfat production of either breed can be modified by a change in environment.

    [Right] 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 moved to the "Good" environment, both show a marked improvement in average milkfat 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 this 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 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.