Multiple-Threshold Transmission of Affective Disorders

Data on bipolar and unipolar affective disorders were gathered on first-degree relatives of 255 patients with both illness types. As consistent with a model of continuous liability, bipolar probands were found to have more bipolar relatives and more relatives with any affective disorder than unipolar probands. Multiple-threshold models of inheritance were applied to the data using clinical polarity as a threshold determinant. The hypothesis of multifactorial inheritance was ruled out. Autosomal single-major-locus inheritance provided an acceptable fit to the data. It is proposed that separate genetic mechanisms for bipolar and unipolar disorders need not be present. The two illness types are represented in the model at different thresholds on a single continuum of genetic-environmental liability in which bipolar illness is claimed to be more deviant genetically than unipolar illness.