[Familial psychosis and filiation].

Since the last decade of the nineteenth century, psychiatric nosology has been dominated by Kraepelin's binary system: the notion that the distinction between manic-depressive illness and schizophrenia identifies two separate diseases with differing symptom patterns and outcomes. However, Kraepelin recognised that intermediate states, as exemplified by Kasanin's concept of "schizoaffective" illness are common. In contrast to Kraepelin's separation of manic-depressive illness from schizophrenia, the concept of a continuum of psychosis implies that there are gradations of illness between unipolar depressive, through bipolar affective and schizoaffective illness, to schizophrenia. This concept is strongly supported by the majority of family studies. Moreover, no simple clinical demarcations of affective from schizophrenic illnesses can be made. The family findings are compatible with the hypothesis that the same gene or genes contribute to susceptibility to both schizophrenia and affective disorder: a single locus that may be variable between generations, and that is represented by homologous loci on the X and Y chromosomes.