The monozygotic twin half-sib method for analysing maternal effects and sex-linkage in humans.

The influence of maternal effects and sex-linked loci upon the statistics derived from an analysis of the families of monozygotic twins (the so called MZ half-sib design of Nance and Corey, 1976) is examined. If, as in the published analyses, the progeny sexes are considered jointly, maternal effects and sex-linkage may be confused. In the absence of sex-linkage a comparison of maternal and paternal half-sib covariances will often be a more powerful test for maternal effects than the conventional comparison of maternal and paternal parent-offspring covariances. If the progeny sexes are considered separately the analysis of sex-linkage presents no fundamental problems.