Cousin Marriages From a Genetic Perspective

A blog about gene expression at Discover Magazine took a look at “the individual & social risks of cousin marriage,” and it even had maps.

In the United States there’s a stereotype of cousin marriage being the practice of backward hillbillies or royalty. For typical middle class folk it’s relatively taboo, with different legal regimes by state. The history of cousin marriage in the West has been one of ups & downs. Marriage between close relatives was not unknown in antiquity. The pagan emperor Claudius married his niece Agrippina the Younger, while the Christian emperor Heraclius married his niece Martina. Marriage between cousins were presumably more common.

How did things change?

With the rise in the West of the Roman Catholic Church marriages between cousins were officially more constrained. Adam Bellow argues in In Praise of Nepotism: A Natural History that there’s a material explanation for this: the Roman church used its power over the sacrament of marriage to control the aristocracy.

That’s interesting.

More precisely the coefficient of kinship between two first cousins is 1/8. That means that at any given locus there’s a 1 out of 8 chance that the two individuals will have alleles which are identical by descent, which means that the genetic variant comes down from the same person in the family line.

If the allele is “good,” that is, totally normal/wild type, not associated with any pathology, then we’re in the clear. That’s why most first cousin marriages don’t produce children who are monsters. What a first cousin marriage does is change the odds. How you present these odds matters a great deal in how scary they sound. If I told you than the chance of first cousins having children with a birth defect is 4-7%, vs. 3-4% for a non-consanguineous couple, it might not sound that bad. But if I told you that the odds of having a birth defect is ~50% greater, then it sounds worse.

He got more technical and moved on to discuss Muslims in Britain.

"item"'>

A blog about gene expression at Discover Magazine took a look at “the individual & social risks of cousin marriage,” and it even had maps.

In the United States there’s a stereotype of cousin marriage being the practice of backward hillbillies or royalty. For typical middle class folk it’s relatively taboo, with different legal regimes by state. The history of cousin marriage in the West has been one of ups & downs. Marriage between close relatives was not unknown in antiquity. The pagan emperor Claudius married his niece Agrippina the Younger, while the Christian emperor Heraclius married his niece Martina. Marriage between cousins were presumably more common.

How did things change?

With the rise in the West of the Roman Catholic Church marriages between cousins were officially more constrained. Adam Bellow argues in In Praise of Nepotism: A Natural History that there’s a material explanation for this: the Roman church used its power over the sacrament of marriage to control the aristocracy.

That’s interesting.

More precisely the coefficient of kinship between two first cousins is 1/8. That means that at any given locus there’s a 1 out of 8 chance that the two individuals will have alleles which are identical by descent, which means that the genetic variant comes down from the same person in the family line.

If the allele is “good,” that is, totally normal/wild type, not associated with any pathology, then we’re in the clear. That’s why most first cousin marriages don’t produce children who are monsters. What a first cousin marriage does is change the odds. How you present these odds matters a great deal in how scary they sound. If I told you than the chance of first cousins having children with a birth defect is 4-7%, vs. 3-4% for a non-consanguineous couple, it might not sound that bad. But if I told you that the odds of having a birth defect is ~50% greater, then it sounds worse.

He got more technical and moved on to discuss Muslims in Britain.

Posted in: consanguineous,cousins,Islam,religion,science,UK