Background: Schizophrenia spectrum psychosis is a chronic mental disorder, based on a range of genes whose expression can be altered by different environmental factors.
Subjects and methods: In this archival and demographic study of genetic isolate we show an incidence of schizophrenia spectrum psychosis in a minority population and transmission of schizophrenia spectrum psychosis through several generations of two families in an isolated rural area of Dalmatia from the second half of 19th and the first half of 20th century.
Results: The incidence of schizophrenia spectrum psychosis was between 0.58 and 2.58 per 1000 inhabitants. The genogram of Family 1 shows that out of 182 descendants in six generations, there were 27 people with schizophrenia spectrum psychosis. Schizophrenia spectrum psychosis occurred in two men who were born from a relationship of a mother with her relative. The genogram of Family 2 shows that 19 out of 86 descendants in five generations suffered from schizophreniform psychosis.
Conclusion: The high occurrence of schizophrenia spectrum psychosis in an isolated village in central Dalmatia is in accordance with the results of earlier studies in that region. The fact that psychosis occurs in varying frequency, without obvious regularity, can be explained by the irregular expression of risk genes as well as by exogenous factors which may affect the activity of the key chromosome areas. Occurrence of psychosis in people born from incestuous relationships indicates the importance of genetic factors in the development of the disorder.