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 “Transfers” or “Triangulation at DNAPainter” 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.
All vendors except Ancestry offer some type of triangulation.
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.
Transfer your results in order to obtain the maximum number of matches possible. Every vendor has people in their data base that haven’t tested elsewhere.
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
Triangulation at DNAPainter
Once you identify your ancestral segments with matches, or using triangulation, you can paint them on your maternal or paternal chromosomes utilizing DNAPainter.
The great aspect of DNAPainter is that you don’t have to triangulate in order to use DNAPainter. Just identifying matches as maternal or paternal allows you to visually see where on your maternal or paternal chromosomes your matches fall, in essence triangulating groups for you.
DNAPainter assigns colors to each ancestor and shows your match names, which I’ve disabled in this example for privacy. I’ve also optionally painted my ethnicity segments from 23andMe, which I discussed in this article.
Above, on chromosome 22, I’ve painted matches that I know descend from either my mother’s (pink) or father’s (blue) side. At DNAPainter, I DO have both a maternal and paternal chromosome, but they are only useful AFTER I figure out which side of my family a match comes from, or if I paint my Family Matching bucketed maternal and paternal matches in an upload file from Family Tree DNA. I wrote instructions for how to do that, here. The combination of Family Matching and DNAPainter is awesome!
Looking at the graphic above, I know that three separate people who match me descend from the bright pink ancestor on my maternal chromosome; Curtis Lore and his wife. I’ve assigned Curtis the bright pink color, and now every match that I paint assigned to Curtis and his wife is colored pink.
One person descends from Curtis’s parents, Anthony Lore and his wife Rachel Hill who I’ve assigned as green.
Until someone else matches me and descends either from Anthony Lore’s parents or Rachel Hill’s parents on this green segment, I won’t know which of those two ancestors, or both, provided (pieces of) that segment to me.
Anthony Lore and Rachel Hill are my great-great-grandparents and Curtis Lore is their son. Even if I only have 2 matches on this segment, one pink and one green, I would know that the green portion of my maternal chromosome 22 is attributed to Anthony and Rachel which means I inherited that green segment from my pink ancestor, Curtis Lore.
In order to determine the source of the two pink triangulated matches at far right, I’ll need to wait until someone from either Curtis’s line or his wife Nora Kirsch’s line match me on that same segment.
We build these groups of triangulated segments slowly, creating in essence a timeline on our chromosomes. It seems like it’s taking forever, but four generations distance with 2 separate triangulated segments really isn’t bad at all!
At DNAPainter, triangulation is as simple as painting your identified matches, either individually, one by one, or using the group import features. I would only recommend utilizing that feature at Family Tree DNA where their Family Matching software divides your matches into maternal and paternal, allowing DNAPainter to paint them on the correct chromosome. Otherwise, the segments are painted, but you can’t tell which side, maternal or paternal, they come from, so I don’t find painting all matches useful without some way to differentiate between maternal and paternal. After all, the point and power of a chromosome browser is to determine how each person is related, from which side, and from which ancestor.
In the article, DNAPainter Instructions and Resources, I compiled my various articles about the many ways to use DNAPainter, including an introduction.
Be sure to test at or transfer to each vendor who provides segment information. Unfortunately, Ancestry does not, but you can transfer your ancestry results to Family Tree DNA, MyHeritage and GedMatch, each of which has unique features that the others don’t have. Transferring and matching is free at each vendor.
I wrote transfer instructions for each vendor, here.
Then, paint and triangulate all in one step at DNAPainter.
I receive a small contribution when you click on some of the links to vendors in my articles. This does NOT increase the price you pay but helps me to keep the lights on and this informational blog free for everyone. Please click on the links in the articles or to the vendors below if you are purchasing products or DNA testing.
Thank you so much.
DNA Purchases and Free Transfers
- MyHeritage DNA only
- MyHeritage DNA plus Health
- MyHeritage FREE DNA file upload
- 23andMe Ancestry
- 23andMe Ancestry Plus Health
- Legacy Tree Genealogistsfor genealogy research