Nature of the genetic contribution to psychotic illness--a continuum viewpoint.

The recurrent psychoses, rather than, as Kraepelin supposed, constituting 2 major entities, manic depressive illness and schizophrenia, as separate diseases, may be distributed along a continuum that extends from unipolar depressive illness through bipolar and schizoaffective psychosis to schizophrenia with increasing severities of defect state. It is proposed that this continuum rests on a genetic base, variations in the form of the gene accounting for variations in form of psychosis. The simplest interpretation of the continuum is that such variation relates to changes at a single genetic locus. Evidence from a postmortem study of brain structure in schizophrenia suggests that this is the gene that determines the development of asymmetries in the human brain, i.e., the cerebral dominance gene or right shift factor of Annett; a possible genomic location is in the pseudoautosomal region of the sex chromosomes.