Inbreeding: is it dangerous to marry your cousin?

If the law allows marriage between first cousins, is this practice risky for future children? We know that inbreeding increases the risk of birth defects, genetic disease and infant mortality.

What do Louis XIV, Queen Victoria, Charles Darwin and Albert Einstein have in common? Inbreeding. They all married their first cousins, that is to say, they have the same grandparents. In the 18th century, the practice was still relatively common in the countryside in France (5 to 10% of marriages). Today, this remains relatively rare even if it is authorized by law.

Why is inbreeding dangerous?

But is this dangerous for his offspring? It would seem that yes, according to a study published in 2013 in The Lancet carried out within a Pakistani community (where consanguineous marriages are frequent) in England. It appears that the risk of congenital anomaly is doubled when the parents are cousins, even if it remains fairly low (6% instead of 3%). According to another American study, the risk of having a child suffering from a mental or physical handicap is increased by 50% for couples of first cousins ​​or of first born.

Increased likelihood of carrying two defective genes

But why does the fact of marrying cousins ​​increase the risk of inbreeding? Our chromosomes include two alleles, one from the mother and one from the father. When an allele is defective (for example, a carrier of sickle cell disease), it generally goes unnoticed if it is recessive. But, if the two alleles are defective, the disease will then be expressed.

However, being from genetically close parents increases the probability that the two are carriers of these mutated recessive genes, inherited from a common ancestor. The risk is all the greater as the parents are closely related; for first cousins, this remains limited because we are in the 4th degree (grandparents in common)….

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