The occurrence in a family of an isolated case of an autosomal dominant disorder with reduced penetrance presents a difficult problem in genetic counselling. It is shown that in such a situation the risk of recurrence in subsequent offspring is given by: (formula; see text) where P is the penetrance (0 less than P less than 1) and f' the relative fitness (0 less than f' less than 1) of affected individuals. In all cases the risks of recurrence will exceed 1 in 20 unless penetrance is high (greater than 0.90) and the relative fitness of affected persons is low (less than 0.6).
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