severe learning disability

By contrast, the parents of a child with moderate or severe
learning disability (intelligence quotient50) are usually
of normal intelligence. A specific cause is more likely when
the retardation is severe and may include chromosomal
abnormalities and genetic disorders. The risk of recurrence
depends on the diagnosis but in severe non-specific retardation
is about 3% for siblings. A higher recurrence risk is observed
after the birth of an affected male because some of these cases
represent X linked disorders. Recurrence risks are also higher
(about 15%) if the parents are consanguineous, because of the
increased likelihood of an autosomal recessive aetiology. The
recurrence risk for any couple increases to 25% after the birth
of two affected children.