Parents of children with autosomal recessive diseases are not always carriers of the respective mutant alleles.

Classically, each parent of a child with an autosomal recessive disease has been considered to carry at least one copy of the abnormal allele. However, with the increasing ability to characterise the molecular basis of genetic diseases, several exceptions have been reported. The most frequent situation is that only one parent is a carrier of the mutation that is present in the patient in two copies either because of uniparental disomy or because of a de-novo mutation on the gene transmitted by the non-carrier parent. In order to give accurate genetic counselling, in particular when prenatal diagnosis is envisaged, molecular analysis of each of the parents of a child affected with an autosomal recessive disease must be routinely performed.