Grouping genotypes by a cluster method directly related to genotype-environment interaction mean square.

To facilitate the interpretation of data from a genotype by environment (GE) experiment when the GE interaction is large, a cluster method is proposed to group genotypics according to their response to the environments. The dissimilarity index between a pair of genotypes is defined in terms of distance adjusted for the average effects of genotypes, and Sokal and Michener's (1958) unweighted pair-group method is used in the clustering algorithm. The new index, constructed in each cluster cycle for any group, is shown to be equivalent to within group GE interaction mean square under 2-way ANOVA. Thus, if the F-value is used as an empirical stopping criterion for clustering, there will be no significant GE interaction within groups and the genotypes within the groups can be compared by their average effects. The method of analysis is illustrated by an example.