Genetic analysis of manic‐depressive Illness

Abstract

Two threshold models, a single locus model, and two combined models are fitted to data on familial incidence of bipolar affective disorder in 194 nuclear families ascertained through a bipolar proband. The relative fit of alternative tramsmission models is tested by a likelihood ratio chi-square with the degrees of freedom defined by the difference in the number of parameters estimated by each model. All parameters are estimated by the method of maximum likelihood. The simplest threshold model, permitting only a single background familial correlatiron, is found to provide a statistically poorer fit than any of the alternative models, and may be rejected as a model for the etiology of bipolar affective disorder. The four remaining models are statistically indistinguishable. It is suggested, however, that the involvement of a major locus in the etiology of this disorder deserves further scrutiny since any of the models incorporating a major locus, with or without a multifactorial background, are consistently associated with greater likelihoods than the complex threshold model. It is also noted that diagnostic criteria are critical in the analysis. In the present study, relatives of probands are considered affected if a diagnosis of bipolar or unipolar affective disorder is present. When only bipolar relatives are considered affected, none of the transmission models may be rejected. Finally, the results of these analyses are found to be independent of the ascertainment parameter.