Longitudinal data from twin pairs may be used to determine how the genetic effects influencing a quantitative trait change with age. Here a model for mixed longitudinal data of Huggins and Loesch [1998] on unrelated individuals is extended to twin studies. The model is fitted using robust statistical methods and a bootstrap procedure is proposed to estimate the percentiles. The method is applied to longitudinal twin data on body mass index in male and female twin pairs aged 5-18 years.
Copyright 2000 Wiley-Liss, Inc.