Properties of the major gene index and related functions

Two recent methods for detecting major genes under continuous variation are investigated, by analysis of both simulated and real data. The major gene index of Karlin is shown to be sensitive to the distribution of gene effects in such a way that it may not detect major genes in certain cases. The intrafamily correlations of Matthysse et al. are shown to yield values difficult to reconcile with established patterns of inheritance for certain traits.