We have examined data on six closely linked microsatellite loci on chromosome 9q34 from 59 Ashkenazi Jewish families with idiopathic torsion dystonia (ITD). Our data show that the vast majority (>90%) of early–onset ITD cases in the Ashkenazi population are due to a single founder mutation, which we estimate first appeared approximately 350 years ago. We also show that carriers preferentially originate from the northern part of the historic Jewish Pale of settlement (Lithuania and Byelorussia). The recent origin of this dominant mutation and its current high frequency (between 1/6, 000 and 1/2, 000) suggest that the Ashkenazi population descends from a limited group of founders, and emphasize the importance of genetic drift in determining disease allele frequencies in this population.
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