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From ISOGG Wiki

Endogamy is the practice of marrying within the same ethnic, cultural, social, religious or tribal group. In endogamous populations everyone will descend from the same small gene pool. People will be related to each other in a recent genealogical timeframe on multiple ancestral pathways and the same ancestors will, therefore, appear in many different places on their pedigree chart. Endogamy can be the result of a conscious decision or cultural pressure to marry within the selected group but also occurs as a result of geographical isolation (for example, in island communities).

Examples of endogamous groups include Jews, Polynesians, Low German Mennonites, the Amish, Acadians or Cajuns (French settlers in what is now Nova Scotia, Canada), French Canadians, people from many Arab countries, people from Newfoundland and people from many islands. Endogamy is also a problem in early Colonial American populations.

The interpretation of DNA results from endogamous populations can be particularly challenging because such people will typically have large numbers of matches in the DNA databases.

The interpretation of autosomal DNA matches can be particularly difficult, especially in the case of endogamous populations where the pedigrees cannot be traced back beyond the 1800s. The relationships will often be more distant than predicted.

The Family Tree DNA Family Finder matching algorithms were modified with effect from 21st April 2011 to downweight matches between Jews in order to provide more accurate relationship predictions.[1] 23andMe make similar adjustments for customers with Ashkenazi ancestry.[2]

For people with Jewish ancestry it has been suggested that you need to have at least one long half-identical region of 23 cMs or more to have some hope of finding the relationship in genealogical time.[3]



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