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Triangulation in Action at Family Tree DNA

Roberta Estes

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 Family Tree DNA, 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.

What is Triangulation?

Think of triangulation as a three-legged stool – a triangle. Triangulation requires three things:

  1. At least three (not closely related) people must match
  2. On the same reasonably sized segment of DNA and
  3. 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.

Boundaries

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. As always, I’m the background grey and these matches are all being compared against me.

I triangulate with them 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.

I’ve overlayed additional triangulation groups below, so you can compare the groups.

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, extending to the left. 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 – At right, we see 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 far 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. The start location varies as well. 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 spot 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 in this group, of course.

Slight Variations

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 such as Phased Family Matching which we’ll talk about in a minute, but shared matches don’t necessarily mean triangulation has occurred, 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.

Triangulation Solutions

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 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.

You can find step-by-step transfer instructions to and from each vendor, here.

I wrote more specifically about triangulation here and here.

Let’s start by looking at triangulation at Family Tree DNA.

Triangulation at Family Tree DNA

Family Tree DNA has two different tools that can be used separately in different circumstances to determine whether or not your segments triangulate.

Phased Family Matching can be used for triangulation.

The Matrix tool can be utilized for people who aren’t designated through Phased Family Matching as maternal or paternal matches to suggest or eliminate triangulation.

First, go to the Family Finder section of your personal page.

We’ll be working with Matches, the Chromosome Browser, and the Matrix.

Phased Family Matching

At Family Tree DNA, I’ve tested my cousins:

I’ve linked the test results of those cousins to my tree in their proper location, which allows Family Tree DNA to do something called Phased Family Matching.

If you don’t have a tree and don’t link your DNA results and those of your family members, Family Tree DNA can’t perform Phased Family Matching.

I explained phasing in the introductory article.

Testing your parents is wonderful if that’s possible, but parents aren’t always available to test. At Family Tree DNA, you don’t need to have tested your parents in order to have phased matches.

In essence, Family Tree DNA uses the DNA of known cousins, third cousins or closer, to assign matches to maternal or paternal tabs, or sides, also sometimes referred to as buckets. I wrote about Phased Family Matching here and here.

You can see that of my 4806 matches, 1101 are assigned to my paternal side, 884 to my maternal side and 4 are assigned to both.

My cousin Charlene is assigned to my paternal side, as shown by the blue icon, because I linked her to the correct position in my tree, as is my cousin, David, below.

Conversely, my cousin Cheryl is assigned maternally because I linked her as well.

These specific people are assigned maternally and paternally because I linked them to their proper place in my tree. These matches will allows Family Tree DNA to link other testers to the proper side of my tree too, because they match me and my cousin on the same segments – in essence phasing a large number of my matches for me which facilitates triangulation.

Linking Matches on Your Tree

In order to cause Phased Family Matching, aka, “bucketing” to occur, I linked my own test and that of my known 3rd cousins or closer to their proper places in my tree at Family Tree DNA.

If you don’t create a tree or upload a GEDCOM file and link yourself and your known matches, your matches can’t be assigned to maternal and paternal sides.

By utilizing the matching DNA between you and known close relatives on your maternal and paternal sides, Family Tree DNA assigns other people who match both of you on those same segments to the same side of your tree.

If you select matches from the same side of your tree and they match on the same segments, they triangulate.

Of course, that’s assuming the person doesn’t match you on both sides of your tree.

You can also download your matching segments in a file and sort to see who matches on the same locations, but the parental side designation (bucketing) is not reflected in the segment download file. Bucketing is reflected in the match download file which is a different file.

There are two separate download files, but they can be merged.

Two Download Files

The first file, your match download file, provides information about your matches such as their haplogroups, surnames and contact information, including bucketing assignment, but not the actual matching segment data.

The match file tells you a great deal and is both sortable and searchable. You can search for any surname, for example, or you can sort for everyone in the Paternal or Maternal matching bucket. You can creatively combine parts of this file with the matching segments file in order to quickly flag the people on your paternal side. Knowledge about how to work with spreadsheets is a plus.

Click to enlarge

This download is available at the bottom of the Family Finder match page.

You can download all of your matches, or just those in a filtered view, such as in-common-with or as the result of a surname search.

The second file, your matching segments file, is available on the chromosome browser page.

The matching segments file includes the match name along with the matching chromosome segments and number of matching SNPs.

If you click through to the chromosome browser from your main page, as shown below, with NO MATCHES SELECTED, you will be able to download ALL matching segments.

You’ll see “Download All Segments” in the upper right-hand corner.

From that Chromosome Browser page, you will also have the ability to select matches to show on the browser.

If you select people on the match page before clicking on the chromosome browser or select matches on the chromosome browser page, then clicking on “Download Segments,” will only download the matching segments of the people that you have currently selected to match against in the browser.

Combinations of Tools and Filters

Of course, if both people are assigned to your same parental side bucket, and they both only match you on one large segment – and it’s the same segment, then you must triangulate.

If they aren’t both assigned to a parental bucket, then you can’t make that determination using parental side designations.

Is there a tool that allows you to compare people against each other at the same time to see if your matches also match each other?

Glad you asked.

Yes, there is.

The Matrix

Let’s say that you want to see if a group of people who you match also match each other.

Family Tree DNA provides a Matrix tool that allows you to select 10 (or fewer) matches in order to determine if your matches also match each other.

I’ve entered Cheryl, Charlene and David. You can see that David and Charlene match each other, and Cheryl doesn’t match either Charlene or David.

Of course, we know that’s accurate because:

Even though they match on the same segment on the chromosome browser, the fact that they are bucketed to different parental sides, and that the matrix shows that Cheryl doesn’t match either Charlene and David, confirms that David and Charlene triangulate with me, while Cheryl is not a member of that triangulation group.

This is exactly why triangulation is important. Looking at the image above, the only thing you know is that they all 3 match you – but with the additional information about bucketing and the matrix, we know that only the two bottom people, Charlene and David triangulate with me. Note that I’ve added the maternal and paternal icons for clarity.

However, if I didn’t have this knowledge, or not everyone was bucketed, the Matrix tool would be extremely useful. The matrix tool uses the matching threshold of approximately 7.69 cM.

The matrix doesn’t tell you if these people match each other on the same segment where they match you,

However, there’s a good probability that they do, especially if only one matching segment is involved.

You can check the chromosome browser to see if they both match you on the same segment. It’s possible if they don’t match you on the same segment that they match each other on different segments, and possibly through a different ancestor. You may need to reach out to them to ask if they match each other, and if they have known genealogy if they aren’t bucketed.

By utilizing the Matrix tool, you can isolate people to maternal and paternal sides of your tree.

Other Resources to Identify Common Ancestors

Be sure to check other clues at Family Tree DNA such as:

Shared surnames, shown on your matches page, with common surnames that you share bolded

Trees, indicated by the blue pedigree icon on the match page.

Y and mitochondrial DNA haplogroups and matching. You can view your matches haplogroup and other information by clicking on their profile picture on your matches page.

Advanced Matching can be utilized to see if you match on combined tests, or in common projects.

This article discusses the 9 different autosomal tools available at Family Tree DNA.

What About You?

Do you have a tree at Family Tree DNA?

Have you connected your test and any family members to your tree?

Can you test a family member, third cousins or closer, or have them transfer a kit from another vendor?

Here’s how to transfer:

How many people do you have on your paternal and maternal tabs on your Family Finder matches page?

You can paint every single one of the people who are designated as maternal or paternal at DNAPainter to your grandparents on the respective maternal or paternal side. DNAPainter Instructions and Resources will explain how, and why.

Join me soon for similar articles about how to work with triangulation at MyHeritage, 23andMe, GedMatch and DNAPainter.

Most of all – have fun!

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Disclosure

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.

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