Just because two (or more) people match you on the same segment does NOT mean they are related to each other.
This is a fundamental concept of DNA matching and of using a chromosome browser.
I want to make this concept crystal clear.
This past week, I’ve had two people contact me with the same question that’s based up on a critical misunderstanding, or maybe just lack of understanding.
It’s not intuitive – in fact, it’s counter-intuitive. I understand why they don’t understand.
It seems logical that if two or more people show up as a match to you on the chromosome browser, on the same segment, you’ve hit a home run and all you need to do is to identify their common ancestor who will also be your common ancestor, or at least related. Right?
NOT SO FAST!
Let’s walk through this, step-by-step. Once you “get it,” you’ll never forget it, and you can use this to help other people understand too. Please notice there are lots of links here to other articles I’ve written if you need refreshers or help with terms.
Yay! – I’ve Got Matches
OK, so you’ve just discovered that you have a close match with three people, on the same segment. You’re thrilled! Maybe you’re trying to identify your grandparent, so first or second cousin matches are VERY exciting for you.
They are also close enough matches with large enough segments that you don’t need to worry about false positive matches, meaning identical by chance.
Let’s take a look. I’m using FamilyTreeDNA because that’s where the majority of my family has tested, plus they have a nice chromosome browser and their unique matrix tool.
We have three nice-sized matches to people estimated to be my first or second cousins. I’ve selected all three and compared them in the chromosome browser. The large red match is 87 cM and the blue and teal matches are 39 cM each, and completely within the 87 cM segment, so completely overlapping.
I’ve hit the mother-lode, right?
All I need to do is identify THEIR common ancestor and I’ll surely find mine.
Right???
Nope
Just because they all three match ME on this same segment does NOT mean they all match each other and are from the same side of my family. All three people DO NOT NECESSARILY have the same ancestor. From this information alone, we cannot tell.
I know this seems counterintuitive, especially since you’re seeing them all on MY chromosomes – which are the background pallet.
However, remember that I have two chromosomes. One from my father and one from my mother.
These matches are ALWAYS FROM THE PERSPECTIVE OF THE TESTER.
So, I’m going to see matches in exactly the same location – matches on my mother’s chromosome and matches on my father’s chromosomes – painted on the same segment locations of my chromosome.
Let’s prove that in the simplest of ways.
Mom and Dad
This is my kit, compared with my Dad and Mom.
I only took a screen shot of my first several chromosomes, but you can see that I match both of my parents on the full length of each chromosome – on the same exact segments.
I am the background – the pallet upon which my matches are painted.
First, my father is painted, then my mother – their match to me displayed on my chromosomes.
I assure you, my father and mother are NOT related to each other. I’ll prove it.
I could simply select one parent, then look for the other parent on the shared matches list.
Or, I could use the Matrix tool, especially if I wanted to see if a group of people are related to me and also to each other.
The Matrix
The Matrix tool is available under “See More,” in the Autosomal DNA Results & Tools section.
The Matrix allows you to select 10 or fewer matches to see if they are matches to each other. We already know they are matches to you.
I added my parents into the matrix.
My parents do not match each other, meaning they are not genetically related, because their intersecting cell is not blue.
Next, let’s select those three other people I match and see if they match each other.
Yes indeed, we can see that Cheryl and Donald match each other, but Amos matches NEITHER Cheryl nor Don. Yet, the segments of Cheryl and Donald, who had the 39 cM blue and teal segments on the chromosome browser fall entirely within Amos’s 87 cM segment.
Therefore, if Cheryl and Donald do not match Amos, that means that Cheryl and Donald are from one side of my family, and Amos is from the other. This is absolutely true in this instance because we are comparing the exact same segment on my DNA, so everyone has to match me maternally or paternally, or by chance (IBC.) The segment size alone removes the possibility of IBC.
Each parent gave me one copy of chromosome 4, so everyone who matches me on chromosome 4 must match one or the other parent on that chromosome segment.
I’ve added my parents back into the comparison, at the bottom, with the three matches on chromosome 4. Now you can see that same segment again, and everyone matches me, parents included, of course.
There’s no way to tell the difference whether the blue, red and teal match is on my mother’s or father’s side without additional information.
Again, let’s prove it.
Everybody, Let’s Dance
I added my Mom and Dad back into the matrix.
You can see that Mom and Cheryl and Donald all match each other, plus me of course, by inference because these are my matches.
You can see that Amos and my Dad match each other, and me of course, but not the other people.
Settled
So, we’ve settled that, right.
In my case, I could provide this great example, because I do in fact have parental tests to use for comparison.
You can see when I remove my Dad and Amos that Cheryl and Donald and my Mom all match each other. If I were to remove my Mom, Cheryl and Donald would match each other.
If I remove Mom, Donald and Cheryl, Dad and Amos match each other.
Of course, you may not have either of your parents’ DNA to use as an anchor for matching. You may, in fact, be searching for a parent or close relative.
If you do have “anchor people,” by all means, use them. In fact, upload or create a tree, link your anchor people and as many others as possible to their profiles in your tree at FamilyTreeDNA so your matches will be automatically bucketed, meaning assigned maternally or paternally. FamilyTreeDNA is the only company that offers linking and triangulated bucketing.
But, if you’re searching for your parents or know nothing about your family, you won’t have an anchor point, so what’s next?
What’s Next?
Using a combination of matching, shared matches and the matrix, you can create your own grouping of matches.
My suggestion is to start with your 10 closest matches.
Pull all 10 into the matrix.
Remember, you will match these people across your chromosomes. The only question the matrix answers is “do my matches match each other,” and a “yes” doesn’t’ necessarily mean they match each other on the same line you match either or both of them on.
I’ve noted how each person is related to me.
You can see that there’s a large block of matches on my paternal side. Some are labeled “Father- both.” These people are related both maternally and paternally to my father, because either the families intermarried, or they are descendants of my paternal grandparents.
Three, Donald, Dennis and Cheryl are related on my mother’s side, but it’s worth noting that Dennis doesn’t match Cheryl or Donald. That doesn’t mean he’s not on my mother’s side, it simply means he descends through her maternal line, not the paternal line like Donald and Cheryl. Remember, we’re not comparing people who match on the same chromosome this time – we’re comparing my closest matches across all chromosomes, so it makes sense that my mother’s maternal matches won’t match her paternal matches, but they would both match Mom if she were in the matrix. Clearly they all match me or they would not be in my match list in the first place.
You could also run a Genetic Affairs AutoCluster or AutoTree to cluster your matches for you into groups, although you can’t select specifically which individuals to include, except by upper and lower thresholds.
Regardless of the method you select, you still need to do the homework to figure out the common ancestors, but it’s a lot easier knowing who also match each other.
Circling Back to the Beginning
Now, when you see those two or three or more people all matching you on the same segment on the chromosome browser, you KNOW that you can’t immediately assume they match you and therefore are all related to each other. It’s possible, and even probable that some of them will match you because they match your mother’s chromosome and some will match your father’s chromosome – so they are from different sides of your family.
The Matrix tool shows you, for groups of 10 or less, who also matches each other.
What you are doing by determining if multiple people share common segments and match each other is triangulation. I wrote about triangulation at each company in the articles below:
Unfortunately, Ancestry does not provide a chromosome browser, so triangulation is not possible, but Ancestry does provide shared matching with some caveats. However, some Ancestry customers do upload their DNA file to FamilyTreeDNA, MyHeritage or GEDmatch. You can find step-by-step download/upload instructions for all vendors, here.
Additional Resources
You’ve probably noticed there are lots of links in this article to other articles that I’ve written. You might want to go back and take a look at those if you’re in the process of educating yourself or need help wrapping your head around the “same segment address – two parents – your matches are not created equal” phenomenon.
Here are a couple of additional articles that will help you understand matching on both parents’ sides, and how to get the most out of matching, segments, triangulation and chromosome browsers.
I prepared a triangulation resource summary article, here:
Enjoy!!
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