Moderate and severe learning disabilities affect about 1% of newborns. Chromosome abnormalities and single gene disorders must be excluded in children with severe learning disabilities (intelligence quotient <50), but many have no specific demonstrable cause. The empiric figures give a recurrence risk to siblings of about 3%, rising to 25% after two affected children. The risk to the children of a sister with an affected brother may be difficult to assess and X-linked recessive inheritance must be considered.
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