A Technique for Endogamous DNA Using GedMatch

I've written many times about how much more difficult genetic genealogy is when you're from an endogamous population, such as Ashkenazic Jewish as am I.  Since Ashkenazim are all descended from a relatively small group of people whose descendants all married within that group of descendants, we tend to share a lot of DNA--and the various genetic genealogy programs tend to predict matches are much more closely related than we actually are.  But there are ways to deal with that--and I'll discuss one below using my uncle's DNA.

My uncle's DNA Matches on GedMatch

The above figure is the first screen of my uncle's one-to-many matches on GedMatch.  (If you've tested at any of the big 3 DNA companies--FamilyTreeDNA, Ancestry or 23AndMe--upload your raw data to GedMatch for free.)  GedMatch has an algorithm which calculates the average number of generations apart various kits are from one another.  You'd think that concentrating on the closest predicted kits to find relationships would work--but that doesn't generally work well when you're from an endogamous population.

My uncle has lots of known relatives who have tested (mostly because I asked them to do so, and a few who I've connected with via DNA).  And the closet predicted relatives are some of those relatives.  But there is a clear difference between those known relatives and the predicted relatives--and that is the largest segment shared.  While "Unknown relationship1" (UR1) is predicted to be 3.3 generations from my uncle--the same as his known third cousin who appears two lines below UR1, look at that largest segment.  To me, that says that UR1 is likely much further away than 3.3 generations, and the amount we share is because of endogamy.

There are two people on this first screen who do look interesting--"Unknown Relationship5" (UR5) and "Unknown Relationship9" (UR9).  They have segments in common with me over 20cM and share quite a bit of DNA.  Well, it turns out that UR5's father's family is from the same very small village as my maternal grandmother's family.  I've been able to help UR5 bring his grandfather's family back multiple generations with no interesection to mine.  We're waiting for his grandmother's death certificate to arrive to find out her parents' names--and likely we'll be able to connect our families using that information.

As for UR9, it looks like I should contact him and see what we can figure out.  Maybe there'll be a future blog post on our relationship.

On GedMatch, you can sort by largest segment by clicking on that header.  Doing that with my uncle's DNA yields the following:

My uncle's matches on GedMatch sorted by largest segment

Sorting by largest segment has my uncle's closest known relatives clustered at the top (similar to sorting by estimated relationship), but then there are multiple individuals with at least one large segment in common with him about whom I know of no connection.  That continues onto his next screen of results:

My uncle's matches on GedMatch sorted by largest segment--second screen

You can see that these known cousins all share reasonably large (20cM+) segments with my uncle.  In addition, they also share significantly more DNA in general with my uncle, making them jump out from the rest of the pack.

While it doesn't mean that the others on this list aren't related in a way we can trace, it does demonstrate that in endogamous populations, you need to look at both largest segment and at total DNA shared, and initially investigate those matches with both factors.

This technique can be used with FamilyTreeDNA as well, as it allows for sorting by largest segment and also shows total DNA shared.  Did it work for you?  Let us know in the comments!

Note:  I'm on Twitter.  Follow me (@larasgenealogy).

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