Sex-limitation and G × E Interaction

Abstract

As described in Chapter 8, the basic univariate ACE model allows us to estimate genetic and environmental components of phenotypic variance from like-sex MZ and DZ twin data. When data are available from both male and female twin pairs, an investigator may be interested in asking whether the variance profile of a trait is similar across the sexes or whether the magnitude of genetic and environmental influences are sex-dependent. To address this issue, the ACE model may be fitted independently to data from male and female twins, and the parameter estimates compared by inspection. This approach, however, has three severe limitations: (1) it does not test whether the heterogeneity observed across the sexes is significant; (2) it does not attempt to explain the sex differences by fitting a particular sex-limitation model; and (3) it discards potentially useful information by excluding dizygotic opposite-sex twin pairs from the analysis. In the first part of this chapter (section 11.2), we outline three models for exploring sex differences in genetic and environmental effects (i.e., models for sex-limitation) and provide an example of each by analyzing twin data on body mass index (BMI).