Simultaneous genetic analysis of longitudinal means and covariance structure in the simplex model using twin data

D. Soerbom's (1974, 1976) simplex model approach to simultaneous analysis of means and covariance structure was applied to analysis of means observed in a single group. The present approach to the simultaneous biometric analysis of covariance and mean structure is based on the testable assumption that the mean trend can be decomposed into (1) a part that can be ascribed to those genetic and environmental factors that contribute to the individual differences and (2) a part that cannot be so ascribed. Illustrations are given using simulated data and repeatedly measured weight obtained in a sample of 82 female twin pairs on 6 occasions.