Caution in locating the gene(s) for affective disorder.

Among the common psychiatric illnesses a genetic predisposition is most apparent, and evidence for dominant inheritance strongest, in manic-depressive disease. The protean manifestations encourage the speculation that it is a group of conditions with differing aetiology. Bipolar affective illness in which episodes of illness of both depressive and manic character, frequently unrelated to situational difficulties at onset, are separated by periods of normality and unimpaired functional capacity is the most distinct and delimited form of the disorder and therefore the most promising for a linkage study. The familial aggregation of cases has long been held to indicate a major genetic factor in manicdepressive disorders but this information has not been sufficient to be of practical use clinically. Considerable attention has therefore been attracted by the report (Egeland et al. 1987) that linkage analysis has both defined and located a dominant gene responsible for bipolar affective illness in Amish families.