Recently, I published the article, Hitting a Genealogy Home Run Using Your Double-Sided Two-Faced Chromosomes While Avoiding Imposters. The “Home Run” article explains why you want to use a chromosome browser, what you’re seeing and what it means to you.
This article, and the rest in the “Triangulation in Action” series introduces triangulation at FamilyTreeDNA, MyHeritage, 23andMe, GedMatch and DNAPainter, explaining how to use triangulation to confirm descent from a common ancestor. You may want to read the introductory article first.
This first section, “What is Triangulation” is a generic tutorial. If you don’t need the tutorial, skip to the “Triangulation at 23andMe” section.
What is Triangulation?
Think of triangulation as a three-legged stool – a triangle. Triangulation requires three things:
- At least three (not closely related) people must match
- On the same reasonably sized segment of DNA and
- Descend from a common ancestor
Triangulation is the foundation of confirming descent from a common ancestor, and thereby assigning a specific segment to that ancestor. Without triangulation, you might just have a match to someone else by chance. You can confirm mathematical triangulation, numbers 1 and 2, above, without knowing the identity of the common ancestor.
Reasonably sized segments are generally considered to be 7cM or above on chromosomes 1-22 and 15cM or above for the X chromosome.
Triangulation means that all three, or more, people much match on a common segment. However, what you’re likely to see is that some people don’t match on the entire segment, meaning more or less than others as demonstrated in the following examples.
You can see that I match 5 different cousins who I know descend from my father’s side on chromosome 15 above. “I” am the grey background against which everyone else is being compared.
I triangulate with these matches in different ways, forming multiple triangulation groups that I’ve discussed individually, below.
Triangulation Group 1
Group 1 – On the left group of matches, above, I triangulate with the blue, red and orange person on the amount of DNA that is common between all of them, shown in the black box. This is triangulation group 1.
Triangulation Group 2
Group 2 – However, if you look just at the blue and orange triangulated matches bracketed in green, I triangulate on slightly more. This group excludes the red person because their beginning point is not the same, or even close. This is triangulation group 2.
Triangulation Group 3 and 4
Group 3 – In the right group of matches, there are two large triangulation groups. Triangulation group 3 includes the common portions of blue, red, teal and orange matches.
Group 4 – Triangulation group 4 is the skinny group at right and includes the common portion of the blue, teal and dark blue matches.
Triangulation Groups 5 and 6
Group 5 – There are also two more triangulation groups. The larger green bracketed group includes only the blue and teal people because their end locations are to the right of the end locations of the red and orange matches. This is triangulation group 5.
Group 6 – The smaller green bracketed group includes only the blue and teal person because their start locations are before the dark blue person. This is triangulation group 6.
There’s actually one more triangulation group. Can you see it?
Triangulation Group 7
Group 7 – The tan group includes the red, teal and orange matches but only the areas where they all overlap. This excludes the top blue match because their start location is different. Triangulation group 7 only extends to the end of the red and orange matches, because those are the same locations, while the teal match extends further to the right. That extension is excluded, of course.
Matches with only slight start and end differences are probably descended from the same ancestor, but we can’t say that for sure (at this point) so we only include actual mathematically matching segments in a triangulation group.
You can see that triangulation groups often overlap because group members share more or less DNA with each other. Normally we don’t bother to number the groups – we just look at the alignment. I numbered them for illustration purposes.
Shared or In-Common-With Matching
Triangulation is not the same thing as a 3-way shared “in-common-with” match. You may share DNA with those two people, but on entirely different segments from entirely different ancestors. If those other two people match each other, it can be on a segment where you don’t match either of them, and thanks to an ancestor that they share who isn’t in your line at all. Shared matches are a great hint, especially in addition to other information, but shared matches don’t necessarily mean triangulation although it’s a great place to start looking.
I have shared matches where I match one person on my maternal side, one on my paternal side, and they match each other through a completely different ancestor on an entirely different segment. However, we don’t triangulate because we don’t all match each other on the SAME segment of DNA. Yes, it can be confusing.
Just remember, each of your segments, and matches, has its own individual history.
Imputation Can Affect Matching
Over the years the chips on which our DNA is processed at the vendors have changed. Each new generation of chips tests a different number of markers, and sometimes different markers – with the overlaps between the entire suite of chips being less than optimal.
I can verify that most vendors use imputation to level the playing field, and even though two vendors have never verified that fact, I’m relatively certain that they all do. That’s the only way they could match to their own prior “only somewhat compatible” chip versions.
The net-net of this is that you may see some differences in matching segments at different vendors, even when you’re comparing the same people. Imputation generally “fills in the blanks,” but doesn’t create large swatches of non-existent DNA. I wrote about the concept of imputation here.
What I’d like for you to take away from this discussion is to be focused on the big picture – if and how people triangulate which is the function important to genealogy. Not if the start and end segments are exactly the same.
Each of the major vendors, except Ancestry who does not have a chromosome browser, offers some type of triangulation solution, so let’s look at what each vendor offers. If you and your Ancestry matches have uploaded to GedMatch, Family Tree DNA or MyHeritage, you can triangulate with them there. Otherwise, you can’t triangulate Ancestry results, so encourage your Ancestry matches to transfer.
I wrote more specifically about triangulation here and here.
Let’s look at triangulation at 23andMe.
Triangulation at 23andMe
At 23andMe, click on “DNA Relatives” in the Ancestry dropdown at the top of your page.
You will then see your list of matches.
23andMe does offer a Mom’s side and Dad’s side option, but only if at least one of your parents has tested AND you and that parent BOTH elect to share with each other. It’s not automatic.
To view your relationship with someone on your match list, click on that person’s name. I selected a known relative on my father’s side, Stacy.
Scroll down to the “Relatives in Common” section where you will see your matches in common with the person you selected. Stacy and I have 284 matches in common.
You can view the relationships of the match to you, and also to the person you’ve selected.
“Yes,” in the shared DNA column indicates that you, the person you selected (Stacy) and this match share DNA on a common segment. In other words, you triangulate.
In this example, Stacy and I share a triangulated segment with my own V4 kit (of course), and with both James and Diana, but not with George or Everett. We both match James and Everett, just not on the same segment, so we don’t triangulate.
Let’s look at James. By clicking on “Yes,” I can view the chromosome browser.
Scrolling down, I see that Stacy (purple), me (background grey) and James (orange) share DNA on only one segment, on chromosome 17.
That segment triangulates between the three of us. I know how I am related to Stacy, but not how I am related to James. I can tell via my matches and triangulation with James that our common segment descends to me through my Vannoy line.
Unfortunately, 23andMe does not support trees in the traditional way, but some people enter surnames and locations, and you can download some Family Search ancestors to 23andMe or place a link to a tree elsewhere. I wrote about that here.
Check your 23andMe matches for surnames, common locations and links to trees.
You can also download your 23andMe segment matches and their information by clicking on Download Aggregate Data at the bottom of your matches page. Segment matches tell you exactly where on each chromosome you match other people.
Segment matches is NOT the same thing as downloading your raw DNA data file to upload to another vendor. See the Transfer section for those instructions.
Other 23andMe Resources to Identify Common Ancestors
23andMe provides additional tools, noted below, with the links to instructional articles I’ve written.
- Relatives in Common shows people you and a match both match
- Ancestral Birthplaces showing the birthplaces of your matches’ ancestors
- Family Surnames
- Link to External tree or FamilySearch
- Shared Ancestry Composition (ethnicity)
- Mom’s Side/Dad’s Side if your parents have tested
- Automated Tree Construction
Have you tested family members, especially everyone in the older generations? You can transfer their kits from Ancestry or 23andMe if they have already tested there to MyHeritage, FamilyTreeDNA or GedMatch.
Here’s how to transfer:
- Ancestry Step by Step Guide: How to Upload-Download DNA Files
- 23andMe Step by Step Guide: How to Upload-Download DNA Files
- MyHeritage Step by Step Guide: How to Upload-Download DNA Files
- Family Tree DNA Step by Step Guide: How to Upload-Download DNA Files
I wrote recently about how to work with triangulation at FamilyTreeDNA. and MyHeritage. Join me soon for similar articles about how to work with triangulation at GedMatch and DNAPainter.
Most of all – have fun!
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Thank you so much.
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- 23andMe Ancestry – autosomal DNA only, no Health
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Very interesting Roberta. We actually provide a visual hint for these overlapping DNA segments using the DNA helix symbol in the 23andme AutoClusters. You can even cluster your matches solely using shared matches that have shared DNA with the tester. Louis Kessler blogged about it some months ago: http://www.beholdgenealogy.com/blog/?p=2952
You are great at making a difficult subject better understood. I was thinking wrongly for some time, because I got the impression that I had to have tested both parents which is impossible. Now, I’ll take a look at this again. Thank you.
Can one use cM segments smaller than 7? Or should I not open that box? 🙂
With appropriate skepticism, yes. I generally don’t simply because there are so many matches with larger segments to work with. If I do use 7 cM or slightly smaller segments, I make sure they are phased and in an appropriate cluster. In order to utilize smaller segments without drawing incorrect conclusions, we really need to understand what we are doing first, with significant experience.
Roberta, thanks for the continuing articles on triangulation. I am still confused about
how one knows if the matches on separate chromosomes are from maternal or paternal dna? Therefore knowing if they are truly triangulated cousins or not. thanks Bill
If they don’t match each other, they are from opposite sides or are matching you by chance. If they match you and each other on that segment, they are from the same side.
The problem I have with all triangulation analysis is that its mostly based on segments that are only half matches. In the examples you show here even the segments that appear to match completely could be glued together differently from what they inherited from each parent. I really don’t know how to handle that. This is very visible in MyHeritage where you can see in a one to one browser matchup that sometimes only a subset of the segments that appear to match completely actually are full matches. That feature unfortunately goes away when you add additional people to the browser display. I hope you will be addressing this when you talk about MyHeritage.
Hi June, I wrote about MyHeritage a couple weeks ago.
Sorry, for some reason I seem to have missed the notice on that one. Can you provided a link?
Look down the right side or type MyHeritage into the search box. I’m not at a computer and actually walking😁
Roberta, I see a 28.91 cM triangulation for my client on 23andMe on the X chromosome. B matches client with 303 cM. C matches client with 137 cM. However, these 3 do not share any other common segments in “lower” 22 chromosomes. Is that triangulation on X significant or not?
Yes. It is. X can persist longer because of the fact it’s passed intact from men to daughters. Remember that an X match of 30 is roughly equivalent to 15-20 on other chromosomes.
Thanks, I admire your ability to actually walk and send a message at the same time 🙂
Well, actually I have to stop😁
Excellent article Roberta …. I’ve been using 23andMe data for some years now ….. creating some larger, more comprehensive spread-sheet files on chromosome data also provided by a couple of the other vendors …. that merged data has been wonderfully revealing !!!
I love those merged files.
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