Biometrical modeling of twin and family data in Stata

Data on twins or on other types of family structures (for example, nuclear families, siblings, cousins) can be used to estimate the proportion of variability in observed traits (or phenotypes) that is due to genes. The models are essentially multivariate regression models with residual covariance structures dictated by Mendelian genetics. Usually, specialized software for structural equation modeling is used. However, the required covariance structures can also be produced using mixed models and by specifying an appropriate design matrix for the random part of the model. Stata’s xtmixed command can then be used to estimate the models. For binary phenotypes, such as diabetes, the appropriate probit models can be estimated using gllamm.