The prevalence of myotonic dystrophy (MyD) in the Saguenay-Lac-Saint-Jean (SLSJ) region (Quebec, Canada) is 30 to 60 times the world's prevalence. We identified 746 patients (673 still alive) distributed in 88 families. Using a population-based register of the SLSJ area and several marriage repositories from northeastern Quebec, we could trace back all patients to a couple who settled in "Nouvelle-France" in 1657. The MyD gene was then passed on over 10 to 14 generations. This genealogical reconstruction is a strong argument in favor of the genetic homogeneity of MyD in the SLSJ region.
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Mathieu et al. (1990) studied this question.