Segment Triangulation: Proving an Ancestral Line
Have you heard the term “segment triangulation” associated with DNA and wondered what that meant? Perhaps you wondered if this could help you confirm an ancestor that you’ve researched? Which DNA testing and third party companies offer this tool? In this article, I’ll define the term and show how the process can help you in your genealogy journey of discovering your genetic family tree.
I previously wrote about pedigree triangulation and used the following visual from the ISOGG Wiki page to explain the concept. You and your DNA match both received DNA from your Most Common Recent Ancestor (MRCA). In pedigree triangulation, you compare your family trees to find the path back to the MRCA.
Now let’s talk about segment triangulation. Although this also involves a triangle, in this case we’re talking about three DNA matches. If two or more people match with a third person on the same segment of DNA AND they all match with each other on that segment of DNA then they each have inherited that shared segment of DNA from a common ancestor. The following image illustrates a triangulated match between me and two DNA cousins who all share DNA on my maternal copy of chromosome 7.
Segment triangulation is ideal, but not always possible. The amount of DNA you inherit from an ancestor decreases with each generation. Because the DNA randomly recombines with each generation, you will not inherit the same DNA segments from your grandmother as your first cousin. You’ll likely inherit some common segments, but not all.
When autosomal DNA began being used by genealogists, segment triangulation was the gold standard and no testing company had a tool. It required asking DNA matches to upload their raw DNA to GEDMatch for comparison and using spreadsheets to track the segment data. Two testing companies now offer a chromosome browser that show triangulated segments, My Heritage and 23andMe. Additionally, DNA from all the testing companies can be uploaded to GEDMatch and the tools there used to find triangulated segments.
Nicole illustrated how to use chromosome browsers from each DNA test company in her article, “The Chromosome Browser: A Tool for Visualizing Segment Data.” She detailed segment triangulation on 23andMe, My Heritage, and GEDMatch. In this article, I’ll show how I discovered the triangulated segment illustrated above using the chromosome browser on MyHeritage.
Segment Triangulation on MyHeritage
When I viewed my list of DNA matches on MyHeritage, my top matches were my children and my mother, as expected. My next closest match was my mother’s first cousin, Kathleen, recently deceased. I know exactly how we are connected through our shared ancestral couple Charles Cannon Creer and Mary Margaret Peterson. Although the estimated relationship on the screen shot below gives an estimated relationship as great-aunt or 1st-2nd cousin, I know that Kathleen is my 1st cousin once removed. With 22 shared segments, there is a good chance that I’ll have another DNA match who also shares one of those segments.
When I clicked on “Review DNA Match” I was taken to a page that showed DNA matches between both Kathleen and me. In the image below, notice how our shared match “W” is my estimated 3rd – 5th cousin but Kathleen’s estimated 2nd cousin – 2nd cousin once removed. Kathleen shares over three times more DNA with “W” than me. At this point, I didn’t know who this cousin was, but MyHeritage had added an icon indicating a triangulated segment between Kathleen, “W” and me (highlighted by the red box).
Clicking on the icon indicating a triangulated segment, I was taken to the “Chromosome Browser – One-to-many” tool shown below. It highlighted a triangulated segment on chromosome 7. The segments that Kathleen and I share are in red and the segments that W and I share in yellow. Clicking on the rectangle highlighting the triangulated segment, I saw that the shared segment size is 15.5 cM. I share much more DNA with Kathleen than W as evidenced by the many red segments. Not shown are two additional yellow segments I share only with W. If none of our segments had matched up, I wouldn’t have had any triangulated segments with Kathleen and W. This would not have precluded a genetic relationship; it just wouldn’t have been segment triangulation.
How did I use this information in my genealogy? I noticed that Kathleen and W were likely the same generation based on MyHeritage reporting their ages as “80s” and deduced they were probably 2nd cousins. MyHeritage showed a likely relationship between them as 2nd cousins – 2 cousin once removed. I hypothesized that W is most likely a descendant of the ancestral couple a generation back, William Creer and Sarah Jane Miller, my 2nd great grandparents.
Viewing the descendancy tree on FamilySearch for William Creer, I discovered a likely candidate for W: William Edward Creer, deceased in 2004. The name on the DNA match I have shortened to W was listed as William Creer and no other person of that name appeared in the descendancy tree for the right generation. Checking the amount of centimorgans that each match shared with me, the relationships made sense. I shared more with Kathleen because of our closer relationship.
To check my hypothesis, I can contact the administrator for the account of William Creer on MyHeritage and verify the tree, but this triangulated match is strong evidence of my relationship to this ancestral couple. The triangulated segment that we share on chromosome 7 almost certainly came from either William Creer or Sarah Jane Miller.
Genealogy research traced this relationship through family histories and documents and now DNA can be added as another piece of evidence that this is my correct family line. The screenshot below illustrates my descendancy from William Creer and Sarah Jane Miller and it’s nice to know that DNA evidence confirms this relationship.
If you’re new to working with your DNA results, I recommend that you try segment triangulation with an easy project such as confirming a set of 2nd great grandparents as I did in this example. You’ll be more familiar with the names and will probably recognize some of your close DNA cousin matches. Experiment with the segment triangulation tools on My Heritage, 23andMe, and GEDMatch. As you get more comfortable with the concept and tools, you can use segment triangulation to prove additional ancestors in your family tree.
Best of luck in all your genealogical endeavors!