Risk counselling in autosomal dominant disorders with undetermined penetrance

A method is presented for estimating the probability of an affected child being born to a clinically unaffected subject who is at risk for having inherited a rare gene for an autosomal dominant disorder of unknown penetrance. The maximal risk is 8.6% for children of persons at 50% risk for having inherited the mutant gene regardless of the true penetrance of the disorder in question. Applications of this maximal risk figure, which should be of benefit in various counselling situations, are summarised.