Pedigree analysis to determine the mode of inheritance in a family with retinitis pigmentosa

A member of a large family with retinitis pigmentosa sought genetic counselling. Recessive inheritance could be discounted as highly unlikely but observation of the pedigree failed to distinguish between autosomal dominant inheritance with reduced penetrance, and X‐linked recessive inheritance with complete penetrance in some female heterozygotes. The difference in predicted recurrence risk under these two models of inheritance is quite striking. A computerized analysis of the likelihood function for the pedigree was undertaken and reveals the X‐linked model to be more than 1W times as likely as the autosomal dominant model.