FAMILIAL PSYCHOSIS AND LINEAGE

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 identif ies two separate diseases with differing symptom patterns and outcomes . However, Kraepelin recognised that intermediate states, as exemplifi ed by Kasanin's concept of << schizoaffective >> illness are common. I n contrast to Kraepelin's separation of manic-depressive illness from schizophrenia, the concept of a continuum of psychosis implies that th ere are gradations of illness between unipolar depressive, through bip olar affective and schizoaffective illness, to schizophrenia. This con cept is strongly supported by the majority of family studies. Moreover , no simple clinical demarcations of affective from schizophrenic illn esses can be made. The family findings are compatible with the hypothe sis that the same gene or genes contribute to susceptibility to both s chizophrenia and affective disorder : a single locus that may be varia ble between generations, and that is represented by homologous loci on the X and Y chromosomes.