On the use of multiple thresholds for the determination of the mode of inheritance of semi-continuous traits.

In this article we reconsider the analysis proposed by Reich et al. (1972) for determining the mode of inheritance for semi-continuous traits from intrafamilial data. We consider both the (general) multifactorial and the single-locus model formulation. We show (numerically), by using a much improved numerical analysis technique for the parameter estimation, that there may be many single-locus models that could have given rise to a particular data set, dependent on its size and nature. Finally we question whether the use of monozygotic twins data, or data obtained from defining further levels of severity of the disease, would improve the discrimination between these two types of models.