What is a DNA Triangulation with a Crossmatch on GEDmatch?

Video Transcription

I’ve done videos about triangulation before. We had one viewer ask about cross matching and whether or not I could explain cross matching, so it’s more understandable. Well, today I’m gonna try to do that for you. How do you Welcome to family history fanatics, where we love helping you to climb your family tree and have fun along the way. If you’re new to genetic genealogy, chances are you have tons of questions. Be sure to check out the link in the description below to get our free guide. On the five most common questions we are asked about DNA cross matching is a term that is used on the website, GEDmatch, whenever you are going to use triangulation at the bottom of the form, you have the option of whether you want to cross match your results or not. Now, during my video on triangulation, I went over really briefly about what cross matching means and why you should do it.

There’s some options there as far as how much cross matching is allowed, but basically, let’s go over what triangulation is first, and then we can go over cross matching to understand triangulation. We have three people, three matches here. We have person A and person A has to match person B and person A also has to match person C. So we’ve got two matches that both match A. Now, to triangulate, we have to look at person B and C, and if person B and C match also, you’ll notice that what we have here is we have a triangle, and this is what triangulation is. Now this is all on the same segment. So let’s say maybe this is on chromosome number 13 from 5.6 million to 20 million. So it’s a specific location on a specific chromosome that these three matches triangulate on. Well, if we take that to the next level, what we might look at is we might look at four matches instead of three matches.

Now with four matches, we have some other possibilities. So I’m gonna change colors here, and we can start with our A, B, and C that we just had before. Remember, A to B, A to C and C to B. We have little triangle there, it all connects. But with this fourth match, what if you see that A matches to D and D matches to C and C? Of course, we already know matches to A, we get this other triangle on this other side. The question is really whether or not this last part is going to be true, and that is does D match B? If D matches B, then

What we really have is a triangulated group of four people, which means that they are all crossmatched. So with three, with our triangle, we just had to have those three to create that triangle with four, we’re really creating two triangles plus a cross between that. Maybe that’s where the term cross match comes from. I’m not sure exactly, but this can be done with any number of matches. So let’s take a look at six matches and how that would play out. Start with, and we go around A to B, B to C, A to C. Well, there’s one little triangle, but then we have our D, remember D matches C. D matches A and D matches B. Okay, we’re starting to get some more lines here, which means we gotta add this next one. E. Well, E to D, E to C, E to B, E to A.

And then we gotta add this last one, F to E, F to A, F to B, F to C and F two D. So we have every single one of these matches all matching each other. On a certain chromosome, let’s say chromosome number five, between 152 to 165 million base pairs, they all match in that specific location. Now, some of them may match at a little bit more than that on either the front end or the back end, but all of them share that specific location in common, and that’s what cross matching is. It’s when we have more than just three in a triangulated group that we’ve actually checked to make sure that they all match each other. Now, it is possible, let me erase some here. We won’t worry about where they’re matching, but it is possible that hey, we might not have everything in those same spots.

For for instance, F may not match A and F may not match E, but F matches everything else and everybody else matches everybody else. Well, this could be that because of the length of that segment, it just keeps on getting too small as we add more and more in, and perhaps those ends for F to A and F to E might make it lower than what the threshold limit is for calling it a match. So it may actually be a triangulated group still, but it doesn’t match there. Once we start to see something that is, let’s say F, it

Also  doesn’t match C, I’m gonna just erase this line right here going to C, but it only matches a few of them. It might be that F is getting some of the matches from the paternal side and some of the matches from the maternal side. So in that case, A through E would all be a cross match group, but F would not B. As we look into F more, what we may find here is we may find that F matches C, F matches A and F matches E, which may be once we look at the other relationships, a new triangulated group that is on one of the other sides of the chromosome. That’s possible. Now, the bigger your crossmatch group is, of course, the more complicated it’s going to be, and that’s why there’s some limits on that GEDmatch webpage. If you have instead of six 200 of these, then that’s actually gonna take a lot of computing power to make all of those connections together and see whether or not they’re crossmatched.

So what kind of practicality does this have though for our genealogy research? Well, let’s take a look at a family tree that I’ve just really simply put together. Now, this really just involves one person on each line because they’re just getting the DNA up through those single persons back to a common ancestor for all of ’em. And I’ve put along the bottom where A C, D, E and F R, for instance, if all we had was A, B, and C down here, what we would be able to see is that, hey, they all come back to this most recent common ancestor, and they all happen to be second cousins, which is nice. We found a common ancestor there. But with this family tree, if we happen to add in another person D or even E, we add in D and E. Well then all of these flow back now to one generation earlier, another most recent common ancestor who happens to be the father of that first one and D and E will be third cousins to A, B, and C. Again, if we are able to find a different match that now goes back to another generation, we can see that, hey, with F, all of these go back to one generation before our most recent common ancestor, and

That’s two generations back further than what we found with just the original triangulation group. And F would be fourth cousins with D and E and A, B and C. So the major advantage with cross matching is that as we get more than just three matches together that are triangulating, we are able to show that we have a much bigger group that all triangulates together, which means they all share a common ancestor. Now, when we actually start to look at a family tree or put together a family tree for that, if there are enough branches of that tree, we’re gonna be able to go back more generations than we would with just a single triangulated group coming back to just this family tree. With these three being second cousins, they may share a triangulated segment that maybe is as much as 40 centimorgans or so possibly more, but a very sizable chunk.

Once  we add in the next group, the d and e, they’re gonna be third cousins. Now it’s possible that they could receive a 40 centimorgan segment, but we might actually see a triangulated segment that’s only 20 centimorgans as we go back and include these other two. And likewise, as we continue to go backwards in time with our triangulated group and add in F, we may see that hey, there’s only a 10 centimorgans that is actually shared between all of them. And that’s just because these segments get divided up each generation, and that’s one of the advantages of looking at these larger groups is we can be looking at smaller and smaller segments that are shared between all of the people in our triangulated group to help identify how far back those common ancestors might be. Now, you don’t have to use the cross matching function all the time when you’re using the triangulated groups. In fact, you probably wanna start with just the triangulation to pick out the different triangulated groups. And then for the ones that you want to look at specifically, you might want to go and run a different report to cross match, just those ones on those segments. Now, if you would like to learn more about how to use a triangulation tool on GEDMatch, you can watch this video up here, but if you wanna learn something else about DNA, then why don’t you take a look at this video down below or free.