The so-called endogenous psychoses, i. e. the manic-depressive pychosis and the schizophrenias have still to be treated like functional characters, as their probably existing somatic bases are still unknown. It may be granted that these two groups are genetically different, though there are — particularly in isolates — hereditary types of psychoses mixed with both manic-depressive and schizophrenic symptoms. According to the data of twin research, schizophrenias and cyclic psychoses have no common genetic background.
The very high concordance of monozygotic twins for schizophrenia is liable to prove the high importance of genetic factors, while it does not exclude accessorial environmental influences in its etiology.
A simple recessive inheritance, which seemed to be outruled by most authors, appears now to be possible according to Garrone (1962). Other authors (Slater, Böök) pleaded for irregular dominance of major genes with or without single accessory genes, and others even for an unspecific polygenic origin.
The term “ schizoid ” is vague and does not correspond to a satisfactorily clear conception either clinically or genetically. It may be the incomplete expression of a disposition to schizophrenia and more often that of a recessive gene in heterozygotic state or an incompletely dominant gene. Schizoid features which have been observed twice in children with trisomy-22 are not likely to have anything to do with schizophrenic psychoses.
Most probably the schizophrenias are heterogenic, but not with regard to clinical variations such as hebephrenia, catatonia, dementia simplex or paranoides. At any rate, we should not rely too much on the empirical hereditary prognosis.
The study of the occurrence of schizophrenia in isolates has the advantage that we have to do mostly with one and the same biotype and that we are able to trace the presumable genes throughout 8 and more generations. On the other hand the number of character-bearers is numerically limited.
In an isolate of eastern Switzerland with about 400 individuals of a highly inbred and fertile population, the frequency of schizophrenia is at least 1.75%. There are accumulations of the trait in some families and collateral lines. The oldest cases of presumable schizophrenics go back a hundred years; the more recent ones were or are interned in the two Cantonal asylums. The diagnoses in their records were controlled by prominent Swiss psychiatrists. Among the parents and sibs of those schizophrenics only a few can be judged as “ schizoid ” at a psychopathic degree.
The parental consanguinity of our propositi is about three times higher than the average of this population, which also contains 3.5% of undifferentiated oligophrenia being low-grade in 1%. The frequency of both schizophrenia and oligophrenia does not account at all for the relatively high number of 5 so-called “ Pfropf-schizophrenias ” (i. e., combinations of schizophrenias with inborn mental deficiency); however, Larsson and Sjögren found the same in a Swedish isolate.
While the 14 cases of sure or most probable schizophrenia in our isolate do not clearly show a monomeric recessive inheritance (since Weinberg's sibmethod gives a =10.4% and Bernstein's a priori-method a P = 3.0%), the undifferentiated oligophrenia of all degrees most probably depends on one specific simple recessive gene. It is not unlikely after all that the schizophrenias be inherited in the same way in this population, provided there is a considerably reduced manifestation of a mutated autosomal gene.