Fig. 1.

a Reaction norm allows describing the relationship between the expected value of a phenotypic trait (E(Y)) and an environmental parameter (X) for a specific genotype. The observed values of the phenotypic state (indicated by the Gaussian bell curves) will vary due test variation, measurement error, and due to biological variation induced by variation in other environmental parameters. b The reaction is a genotype specific property: different genotypes ($g_1$, $g_2$) can have different reaction norms, with the effect that for the same environmental parameter value, $x_3$, $g_1$ and $g_2$ produce different expected trait values, k and m. For some x, both genotypes can have the same expected values for y (e.g. $E(y|g_1,x_2)=E(y|g_2,x_2)=l$) and different genotypes can have the same expected trait value under different environmental conditions (e.g. $E(y|g_2,x_1)=E(y|g_1,x_3)=k$). If the reaction norm is flat, we expect the same trait value even under different environmental conditions (e.g. $E(y|g_2,x_3)=E(y|g_2,x_4)=m$)