The power of the classical twin study

In the absence of genotype-environment interactions, distributional skewness and mean-variance regression in DZ twins are found to be more powerful tests of directional dominance than the standard model fitting procedure and these tests may be worthwhile in future studies.

Abstract

A method based on the non-central chi-square distribution is developed for the calculation of sample sizes required to reject, with given probability, models of variation when they are " wrong ". The method is illustrated with reference to simple alternative models of variation in MZ and DZ twins reared together. Simulation of twin experiments finds the empirical power in good agreement with that predicted by the method. Tables are produced showing the sample sizes required for 95 per cent rejection at the 5 per cent level of inappropriate models of variation. For equivalent cases it is always found easier to reject an inappropriate simple genetical model of variation than an inappropriate simple environmental model. For several frequently encountered cases, more than 600 pairs of twins would be required to reject inappropriate alternative models. The optimum proportion of MZ and DZ twins in a sample will vary with the "true" model of variation but is most likely to be between two-thirds and one-half of DZ twin pairs. The possibility of detecting genetical non-additivity with the classical twin study is investigated by theoretical power calculations and simulation. In the absence of genotype-environment interactions, distributional skewness and mean-variance regression in DZ twins are found to be more powerful tests of directional dominance (or unequal gene frequencies) than the standard model fitting procedure and these tests may be worthwhile in future studies.