genotypic groups. This revised model allows for accurate characterization and visualization of any gene-environment interaction effects that may exist in the data. We note that alternatively it is possible to use dummy-coded variables; for a three-category genotype two dummy variables and their cross-products with the environmental variable would be necessary. The use of dummy-coded genotypic variables for modeling interaction is mathematically equivalent to including the G2 term, as well as allele-based coding, within a multiple regression framework. Its omission is parallel to the situation in latent variable interaction models for twin studies (Purcell 2002), where non-additive effects of the genotype can mimic or obscure G × E interaction (Van Hulle et al. 2013).
