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To get insight in the size of GxE you can plot the performance of different genotypes across multiple environments in a graph. On the x-axe of such a graph is the environmental gradient, for example temperature, or protein content in the diet, or some other component in the environment of the animals. On the y-axe is the performance of the animals in the environments. The resulting plot is called a reaction norm. The slope of the reaction norm will tell you how sensitive the genotype is. A horizontal slope indicates absence of sensitivity to the environments that are considered. A slope indicates that performance in one environment is better than in the other. Parallel reaction norms indicate that the two genotypes are equally sensitive to changes in the environment across the environmental range. However, if one reaction norm is more steep than the other, this is an indication that one genotype is more sensitive to a change in the environment than the other and only then we call that a genotype by environment interaction. In the extreme situation the reaction norms even cross each other. That means that in the one environment the one population would be best, whereas in the other environment the other population would be best.

In the figure 4 below is an example of both type of reaction norms to indicate GxE. In the top figure the reaction norms are non-parallel, but not crossing, indicating that one population remains superior to the other in both environments. In the bottom figure the reaction norms are crossing,  indicating that the genetic superiority changes with the environment change. The reaction norms in these figures are straight lines because only two environments are considered. They can become non-linear when more environments are included, and thus multiple performances are compared. Non-horizontal reaction norms indicate that some breeds are more capable of dealing with less optimised optimized environments than others.

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