I've done videos about triangulation before. We had one viewer ask about
crossmatching. And whether or not I could explain crossmatching so it's more understandable?. Well,
today I'm going to try to do that for you. Howdy, 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. Crossmatching 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 crossmatch your results or not. Now during
my video on triangulation, I went over really briefly about what crossmatching means and why you
should do it. There's some options there as far as how much crossmatching is allowed. But basically,
let's go over what triangulation is first, and then we can go over crossmatching. 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 going to 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 a little triangle there. It all connects.
But with this fourth match, what if we 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 crossmatch 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. TO 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 got to add this next one E. Well, E to D.
E to C. E to B. E to A. And then we got to add this last one. F to E. F to A. F to B. F to C.
And F to D. So we have every single one of these matches all matching each other
on a certain chromosome. Let's say chromosome number 5 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 crossmatching 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, and we won't worry about where they're matching. But it is possible that,
hey, we might not have everything in the same spot. So, 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 going to 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 crossmatched group.
But F would not be. 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 maybe,
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, 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 web page.
If you have instead of six, 200 of these. Then that's actually going to 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 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 them. And I've put along the bottom where A, B, C, D, E, and F are.
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
crossmatching 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 going to
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 you know. Maybe it is as much as 40 centimorgans or so,
possibly more. But you know, a very sizable chunk. Once we add in the next group, the
D and E, they're going to 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 backward 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 crossmatching function all the time when you're using the
triangulated groups. In fact, you probably want to 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 crossmatch just those ones on those segments. Now,
if you'd like to learn more about how to use a triangulation tool on GEDmatch,
you can watch this video up here. But if you want to learn something else about DNA,
then why don't you take a look at this video down below. Our free guide. Start again.