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How can consanguineous marriage contribute to the occurrence of genetic disease in a population?

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consanguinity (Left) Absence of inbreeding: horizontal lines connect mates, vertical lines connect parents with their child; (right) an inbreeding loop: horizontal bar at top of loop connects brother and sister, all other lines and bars as above

Consanguineous marriage—a union between individuals who share common ancestors—contributes to the occurrence of genetic disease primarily by increasing the probability of homozygosity for deleterious recessive alleles.12

In human genetics, many rare disorders are caused by recessive mutations. Because these mutant genes are typically rare in the general gene pool, it is statistically unlikely for two unrelated parents to both carry the same harmful gene. However, because consanguineous partners share a common forebear, they are significantly more likely to have both inherited the same rare, recessive mutant gene from that ancestor.34

When both parents are carriers of the same recessive mutation, their offspring face a higher risk of inheriting two copies of the mutant gene, a state known as homozygosity by descent. This results in the manifestation of genetic disease, such as inborn errors of metabolism or conditions like albinism and cystic fibrosis. Consequently, the closer the biological relationship between the parents, the higher the probability that their children will inherit identical alleles from a common source.2435