Sunday, March 26, 2017

Jewish DNA and Endogamy ~ One Example

I have written about endogamy before:
DNA ~ Chromosome Browser and Endogamy
One Jewish Family's DNA Ethnicity Results (at FamilyTreeDNA)
and I usually mention it in my blog posts about DNA.

Due to endogamy, my husband's match list at FamilyTreeDNA has 9,902 matches. Yes, that's a lot. (I have 2,403 matches, and some of them appear as closer relatives than they really are due to endogamy, because of my many colonial New England ancestors.)

Recently I noticed a fellow blogger's name in the match list for my husband's DNA test results, at both FamilyTreeDNA and at GEDmatch. She shares more DNA with my husband than she does with either of his parents! This match is Emily Garber of (going) the Extra Yad, a blog to check out if you have Jewish ancestry.

Matching results to Emily at FamilyTreeDNA show the following:
Person (Total) Shared cM Longest Block (cM) Predicted relationship
My Husband 123 cM 13 cM 3rd-5th cousin
His Dad 122 cM 11 cM 4th-distant cousin
His Mom 98 cM 12 cM 4th-distant cousin

If you just look at the predicted relationship and think logically, you can see that something is unusual here: how can my husband be a 3rd to 5th cousin with Emily and his parents be a more distant relationship: 4th cousin to distant cousin to her? Endogamy: my husband happened to receive DNA from his parents in such a way that caused FamilyTreeDNA's algorithm to calculate his relationship to be closer than it really is.

I should note that Emily and I have compared family trees and cannot confirm a relationship between her and my husband or his parents.


When I explore the chromosome browser, I can see where it is that FamilyTreeDNA shows a larger longest block matching Emily to my husband than either of his parents.

The longest block that my husband matches Emily is on chromosome 8 from position 127,226,471 to 136,074,788 for a total of 13.09 cM.

Emily matches my father-in-law on chromosome 8 from position 127,226,471 to 133,420,302 for a total of 8.7 cM. (His longest segment with Emily is on chromosome 7.)

Emily matches my mother-in-law on chromosome 8 from position 130,793,738 to 136,074,788 for a total of 7.73 cM. (Her longest segment with Emily is on chromosome 6.)

Emily kindly shared a screen shot of these results with me so you can see this graphically. Her matching segments with my husband are in orange; matching segments with my father-in-law are in blue; and matching segments with my mother-in-law are in green. Look at the right side of chromosome 8 and you'll hopefully see what I've explained above.


Note that my parents-in-law are not known to be related, though they do share a little bit of DNA. The following table is from GEDmatch and shows where they share DNA and how little they share.

IMPORTANT: As has been noted by many genetic genealogists, especially those who research endogamous populations, 17.3 cM total shared DNA and a largest segment of 8.7 cM is very small, making it virtually impossible to determine a common ancestor and confirm your relationship with that match. See this recent post by Lara Diamond about sorting your DNA matches by largest segment to guide you in which of your matches to contact.


  1. Elizabeth, this is very helpful as I begin the journey of doing what you're doing using FTDNA and Gedmatch. So far, it seems matches are maybe 6 generations back, far too distant for us to know names/relationships. On the other hand, some known 2d cousins are there and I can use those DNA details as context for analyzing other matches. I can see it's going to be a long journey.

    1. My advice is to read everything you can and listen to any and all speakers about DNA and you'll eventually understand the concepts. Of course, getting as many known 2nd, 3rd, and 4th cousins to test is helpful - I'm still working on that!
      Thanks for reading and commenting!

  2. Great post. I've pretty much given up on DNA analysis except as a way of confirming what I've learned in my research. Even when I have kits that share over 150 cM and a shared segment over 20 cM, I've been unable to find a connection even going back 4-5 generations. And before that there weren't many Jews with surnames, so it's pretty much impossible to figure out a connection.

    1. I don't spend too much time on the DNA analysis, at least for my husband, because of the reason you state. But I'm ready in case a closer relative tests!

      Thanks for the comment.