In the numerous twin studies of affective illness and chronic schizophrenia that have been performed in the last half century, the concordance of monozygotic (identical) twins has consistently been much higher than for dizygotic (fraternal) twins16,21. Furthermore, it is quite unusual to find affective illness in one twin and schizophrenia in the other. For both types of disorder, the incidence of psychiatric illness amongst biological relatives of adopted patients is higher than for adoptive relatives. Furthermore, the diagnoses in the biological relatives are similar to those of the patient. These findings suggest that schizophrenia and affective illness represent distinct classes of disorders which are transmitted genetically, though the possibility remains that non-genetic influences such as infection with a slow virus, acting prior to the age of adoption (i.e. during infancy) could mimic genetic transmission.
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