Fig. 1.

A hierarchy of genetic modules in a three-locus haploid model. The bottom nodes represent genes 1, 2, and 3 and are the first-level modules. The center nodes represent the interactions of genes 1 and 2, 1 and 3, and 2 and 3 (denoted 12, 13, and 23, respectively) and are the second-level modules. The top node represents the interaction of all three genes and is the third-level module. Notice that a correspondence can be drawn between the s terms in Eq. 1 and the modules in Fig. 1 based on the former’s superscripts and the latter’s labels, and that in such a hierarchy of modules, lower-level modules are multiply used in higher-level modules.