[Spreading of the gene for myotonic dystrophy in Saguenay (Quebec)]

The prevalence of the myotonic dystrophy (Steinert disease) is about 1/475 in the Saguenay region, located in the North-east of the province of Québec (Canada). About 600 cases are currently known in a population of 285,000 inhabitants. This disease is an autosomal dominant disorder which causes a general muscular degeneration. Usually, it is also associated with a lower fertility, if not sterility, among the affected families. Another element of the phenotype is a higher infant mortality rate among cases. In the light of those traits, the very high prevalence of the disease in the Saguenay population becomes rather puzzling. It is the subject of this research. Using a computerized population register, we have been able to analyze the genealogies of the patients and their family history. In the overall, two basic factors account for the wide transmission of the gene: a) an immigration stream from a neighbouring region (Charlevoix) may have brought between 57 and 77 patients into the Saguenay since 1840; b) the cases proved to be by and large as fertile as controls (an average of 9.2 births among 85 affected families). The paper also presents a genealogical inference program used to ascertain cases among ancestors, along with data on the history of the gene, nuptiality, geographical and occupational mobility. In the overall, we have not been able to bring out an important selective disadvantage against the patients.