In genetic genealogy, you can find cousins by discovering segments of DNA you have in common. How does a cross-match help family historians find new common ancestors to add to their family tree?
What is Genetic cross-matching?
DNA cross-matching is a term used on the website GEDmatch to indicate that four or more genetic matches share an overlapping segment of DNA.
To understate a cross-match, let's first explain the following more familiar research term.
What is DNA triangulation?
Let's go over what DNA match triangulation is first. To understand genetic triangulation, imagine you have three people that are DNA matches.
Person A has to match Person B.
Person A has to match Person C.
Person B has to match Person C.
When drawn, this forms a triangle, hence the name triangulation.
However, to ensure a segment rather than a match triangulation, the shared DNA must happen on the same segment. For instance, the match may occur on chromosome number 13, from 5.6 million to 20 million.
What is DNA cross-matching?
Using autosomal DNA information, when you compare four or more matches that triangulate in multiple ways, you create a cross-match.
Person A matches Person B.
Person A matches Person C.
Person A matches Person D.
Person B matches Person C.
Person B matches Person D.
Person C matches Person D.
One triangle is formed when you connect A to B, B to C, and C to A.
Another is formed when you connect A to D, D to C, and C to A.
When Person D also matches Person B, you create a line across two triangles creating the cross-match.
To see how six matches can intersect, watch the video below.
What happens if a person doesn't share DNA with everyone in a cross-match?
It is possible that everyone might not share DNA in the same spot. For instance, in a six-person match group:
Person F may not match A
Person F may not match Person E.
Person F matches Persons B, C, and D.
Persons B, C, and D match everybody else.
This lack of match with Person E and A could be because of the shared segment's length. Perhaps they share a segment of DNA, but it's too small to meet GEDmatch's threshold to call it a match.
However, if Person F matches fewer persons, then perhaps that person matches the others through a different family line, perhaps the paternal side.
For a better explanation, watch the video mentioned above.
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Where can you find a DNA Cross-Match Tool?
On the GEDmatch website, a member can access the triangulation tool.
This tool will process your top 300 matches to finds which ones match at the same location as you match each of them.
There is an option at the bottom of the form to allow cross-match your results or not.
How Do Cross-Matches Help Genealogy Researchers?
With the help of DNA triangulation and cross-matching, as you evaluate how much DNA the matches share, you can determine the likely common ancestor.
Second cousins may share as much as 40 centimorgans and have a great-grandparent in common.
If a triangulated group has third cousins, you can see shared segments between 20 and 40 centimorgans. Their common ancestor would be the 2nd great-grandparent.
As you discover more cross-matched groups, the amount shared centimorgans may explain which generation the common ancestor appears. Stated a different way, our triangulated group may help identify how far back those common ancestors might be.
You don't have to use the cross-matching function on the GEDmatch triangulation tool all the time.
In fact, you probably want to start with just the triangulation to pick out the different triangulated groups. Then choose groups to specifically research and run a cross-match report for just those segments.
More Tips on Using GEDmatch
Explore the following blog posts to continue your genetic genealogy education.