Marrying within the group can fool a DNA test into seeing closer cousins
Consumer DNA tests estimate kinship by how much genetic material two people share. In communities where people have married within the group for generations, family trees loop back on themselves, and matches can look far closer than they really are. The Aari of highland Ethiopia have practised this for about 4500 years.
Endogamy means marrying only inside one's own social group, religion, caste or ethnicity and treating outsiders as unsuitable partners; exogamy is the opposite. Some traditions permit a way around it through conversion, so the incoming spouse joins the partner's faith. Minorities have long used it to stay distinct within larger societies, though it can also harden sectarian divides and resistance to integration.
Genetics is where it gets complicated. Repeated marriage inside a closed group raises the frequency of unions between first, second and third cousins. When a cousin marriage appears in someone's known ancestry, genealogists call it pedigree collapse: the same ancestors turn up along several branches, so autosomal DNA matches come out stronger than the nearest link would predict. Double cousins, by contrast, do not collapse a pedigree. Over long periods such patterns can add noise that makes people from one region seem more closely related than they are. Health effects are usually modest unless inbreeding is close or the population becomes very small.
Isolation can force the pattern. On Tristan da Cunha, a remote South Atlantic island, the small community is almost entirely endogamous, and University of Toronto research has linked cases of glaucoma and asthma there to it. The uncontacted Sentinelese of North Sentinel Island live the same way by circumstance. In the Northern Cyprus village of Galinoporni, it is said, everyone is related.
Elsewhere it is a matter of choice or doctrine. The Urapmin of Papua New Guinea practise strict endogamy, yet their inherited kinship classes overlap so much that most people belong to all of them. Among the Serer of Senegambia, it is mandated by traditional spirituality. Pamiris in Tajikistan prefer it as an Ismaili, Pamiri-speaking minority, and the Habsburg and Borbón royal houses of Spain kept marriages within their own circle.
Source: Endogamy
