Gene–environment interaction (or
genotype–environment interaction or
G×E) is when two different
genotypes respond to environmental variation in different ways. A
norm of reaction is a graph that shows the relationship between
genes and
environmental factors when phenotypic differences are continuous. They can help illustrate GxE interactions. When the norm of reaction is not parallel, as shown in the figure below, there is a gene by environment interaction. This indicates that each genotype responds to environmental variation in a different way.