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Objective:  Familial occurrence of tardive dyskinesia (TD) and schizophrenia has been hypothesized to confer risk to the development of TD. We investigated these hypotheses in a large patient sample applying standardized methods for phenotype characterization. Method:  Two hundred and twenty‐two patients with a diagnosis of schizophrenia or schizoaffective disorder were assessed for TD and for family history of schizophrenia or schizoaffective disorder. Thirty‐nine patients had 40 affected first‐degree family members, one patient having two first‐degree relatives. Of these, 17 pairs and one triplet were personally examined. Results:  1) There was a tendency for TD in the affected relatives to be associated with the TD status of the index‐patient; this finding was unrelated to age and doses of neuroleptic medication. 2) No association between a family history of schizophrenia or schizoaffective disorder and TD was found. Conclusion:  A family history of TD might represent a risk factor for TD, whereas a family history of schizophrenia does not.

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Although numerous genetic and anthropological markers are available for determining zygosity of twins, there is still a need for a more practical and informative method in zygosity diagnosis. Dinucleotide repeats or other short repeats (microsatellites) are highly variable between individuals and offer a simple, fast, cheap, and exact approach for zygosity determination. The feasibility of a set of microsatellites to be used for this purpose is demonstrated.

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