Norm of Reaction

Genes, Environment, & The Norm of Reaction [ I ]
"Is the variation genetic or environmental?"

The Norm of Reaction is a multi-locus genetic 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 locus that influences milk-fat content. If the Norm of Reaction is flat [left], the phenotype produced over the entire range of environments (X-axis) is constant (flat line, Y-axis). This corresponds to the ordinary concept of a "genetic" trait: 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: 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 variation.

    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 alone, but requires knowledge of the environment. The trait remains 'genetic', because it is determined by genes.

    By this analysis, we can show that phenotypic variation within a group has a heritable genetic component. We then analyze phenotypic differences between two groups, and ask, "Is the difference 'Genetic"?