Consanguinity

Consanguinity increases the risk of a recessive disorder because
both parents are more likely to carry the same defective gene,
that has been inherited from a common ancestor. The rarer the
condition the more likely it is to occur when the parents were
related before marriage. Overall, the increased risk of having a
child with severe abnormalities, including recessive disorders, is
about 3% above the risk in the general population.