norm of reaction

The norm of reaction is the full set of phenotypes a single genotype would produce across the range of environments it might encounter. Rather than picturing a genotype as fixing one outcome, picture it as a curve: feed in the environment along the bottom and read off the phenotype along the side, tracing how the same genes express themselves under cool versus warm, lean versus rich, calm versus stressful conditions.

Drawn this way, a norm of reaction makes plasticity visible. A flat curve means the genotype gives much the same phenotype whatever the environment; a steep curve means it is highly responsive. When the curves for two different genotypes are not parallel — crossing or fanning apart — that non-parallelism is exactly a gene-environment interaction, telling you that no single ranking of genotypes holds across all conditions.

The concept is a useful corrective to thinking of a genotype as encoding a fixed trait value. In the wild and in the clinic alike, what a genome does depends on where it does it. Because we can rarely observe one genotype across many environments in humans, full reaction norms are easiest to study in plants, microbes, and clonal animals, where the same genotype can be replicated and grown in many settings.