Genes, Environment, & The Norm of Reaction [ I ]
"Is 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 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,  as the phenotype is not at all predictable from a knowledge of the genotype, but requires knowledge of the environment.

    Given that phenotypic variation within a group has a heritable component, we next observe a phenotypic difference between two groups. We then ask, "Is the difference 'Genetic'"?

Figures after Futuyma 1997; text material © 2015 by Steven M. Carr