Segregation analysis of 1885 DMD families: significant departure from the expected proportion of sporadic cases.

The proportion of sporadic cases of Duchenne muscular dystrophy has been estimated by classical segregation analysis in a pooled sample of 1885 sibships from 7 different countries. A significant departure from the theoretical expectations based on mutation-selection equilibrium is observed (segregation frequency = 0.439 +/- 0.017; frequency of sporadic cases = 0.229 +/- 0.026, at the maximum likelihood). The occurrence of germinal mosaicism in some of the mothers of Duchenne cases may account for this peculiar finding, although a possible role of inequality of mutation rates in the two sexes cannot be ruled out.