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In the figure 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 optimized environments than others.
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Thus: a reaction norm represents performance of genotypes (i.e. animals or populations) in a range of environments. Non-parallel reaction norms of genotypes indicate the presence of genotype by environment interaction |