The occurrence of Parkinson's disease and of essential tremor was examined in the parents and siblings of 52 Parkinson's disease patients with onset before the age of 45 years. The expected numbers of cases with Parkinson's disease or essential tremor were calculated according to the age and sex specific incidence rates of Parkinson's disease and essential tremor in the general population. Among the parents, there was one case of Parkinson's disease (expected 1.2), and 10 cases of essential tremor (expected 5.4); among the siblings there were two cases of Parkinson's disease (expected 0.7), and three cases of essential tremor (expected 5.3). The observed and expected incidence of Parkinson's disease or essential tremor were not significantly different. This does not support the inheritance of early-onset Parkinson's disease, or the association of Parkinson's disease with essential tremor.
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