Saturday, July 9, 2016

DNA ~ Second Cousin Results at AncestryDNA and GEDmatch

A second cousin (GS) has tested with AncestryDNA which shows that he shares 251 cM (centimorgans - a unit of DNA measurement) of DNA across 16 segments. This indicates a second cousin relationship, which we knew. They have the same great-grandparents: Aaron and Rozalia (Szali) Handler. (See their 1882 marriage record from Ilok, Serbia.)

Screen shot from AncestryDNA results

There are plenty of resources at AncestryDNA for a user to learn about and understand the results, and those testing at Ancestry should definitely upload a family tree, which makes this process much more useful for all. However, AncestryDNA does not have a chromosome browser.

I asked that this cousin upload the results to GEDmatch so I could compare his results with the results I have from FamilyTreeDNA and I can compare the DNA results using the chromosome browser there.

For more information about GEDmatch, visit the FAQ pages or the main page of the GEDmatch Wiki. It is a free resource though donations are accepted. (If you have a membership at Legacy Family Tree Webinars, watch this Legacy Family Tree webinar to learn more about GEDmatch.)

I'm still learning about the tools at GEDmatch, but the first thing that a user will want to do (after uploading the DNA file) is to analyze the data using the 'One-to-many' matches link. (You then confirm the details using the 'One-to-one' compare tool.) The behind-the-scenes analysis is a little different than at the big DNA testing companies, so results can be a little different.

The top two matches for this cousin are my father-in-law (HH) and my husband (SH).

And of course because we're talking Ashkenazi Jewish DNA (an endogamous population), he also matches my mother-in-law (AH), but with much less DNA.

Screen shot of GEDmatch results for cousin G.S.

From the 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).

GEDmatch tells me that:
GS and HH share 524.5 cM with the largest segment of 73.0 cM. (First cousin once removed shares about 6.25% of DNA, or about 425-440 cM.)
GS and SH share 292.8 cM with the largest segment of 43.7 cM. (Second cousins share about 3.125% of DNA, or about 213-246 cM)
GS and AH share 66.4 cM with the largest segment of 13.9 cM. (No known relation.)

Three comments:
The total number of cM is different between AncestryDNA and GEDmatch because AncestryDNA removes many of the smaller segments in calculating its total cM. (I believe their algorithm recognizes certain endogamy.)
In the GEDmatch results, the greater than average matching DNA is due to endogamy.
Also, this makes me disinclined to try to figure out how my husband and his parents are related to their DNA matches unless there is more than, say, 100 shared cM and certainly more than 20 cM in the largest matching DNA segment.

I will share what the GEDmatch chromosome browser looks like in an upcoming post.


  1. That GEDMATCH webinar yesterday was amazing and after following your posts, I am getting closer to understanding it well enough to give FTDNA a try.

    1. Keep reading and watching webinars and you'll get it.

      There are a few good genetic genealogy bloggers that I follow: Blaine Bettinger, Roberta Estes, Cece Moore, Kitty Cooper, as well as Jewish bloggers who write about DNA: Lara Diamond and Israel Pickholtz. I've learned a lot by reading their blogs.

      Thanks for the comment.