Congenital myotonic dystrophy in Britain. II. Genetic basis.

Genetic analysis of 54 sibships containing 70 patients with congenital myotonic dystrophy has shown paternal transmission in only one case, the disorder being maternally transmitted in 51 sibships. No instance of new mutation was found. At least half the sibs were unaffected; 9 sibs were affected without definite congenital involvement. No evidence for genetic heterogeneity was found, most affected mothers having few or no symptoms. There was no disturbance of sex ratio for the affected grandparents, nor in the sibships of the affected parents. The genetic data from this study and from previous published reports support the clinic evidence that the congenital form of myotonic dystrophy results from a maternal intrauterine factor affecting those individuals carrying the myotonic dystrophy gene.