If you are looking for family using DNA results you are probably using GEDMatch or one of the other website to help compare your DNA kit results. Triangulation is comparing A to B, then A to C, then B to C, etc. You might match B and B might match C, but that doesn’t mean you match C. Here is some information and advise for those of you using GEDMatch and its Triangulation feature. You have to be a Tier 1 user to get this feature, but Triangulation on GEDMatch is well worth the $10 a month donation.
If you have a match of interest, use yourself or relative and the match of interest and on GEDmatch use the “People who match one or both of two kits.” Enter both kit numbers (yourself or your relative first so that everyone is being compared to you in the later options), and submit. In the top part you get all the people who match the two of you. These people that show up in the top section matching both of you can be in three categories:
1. People who match both of you on the same chromosome and segment. (you have a common ancestral couple)
2. People who match both of you, but on different chromosomes and segments (you probably have a common ancestral couple)
3. People who match both of you, but through different common ancestral couples.
Check all the boxes of all the people who match both of you and click “submit.”
You will then get the GEDmatch visualization options. What’s nice about this screen is no matter which you chose, you can use your back button in the browser to get back to it to use another tool in the visualization options.
If you click on the “2-D Chromosome Browser” you will see all the segments where everyone is matching you. As you scroll down the page, look for the segments that are the colors for the segments greater than 10 CM (ignore pink for example). You will find that most of the time that what you are looking for includes a lot of green segments. So once you’ve found where there are at least three of you are overlapping on the same segment, pursue that as a triangulated segment. Remember that you are not showing on this as a separate bar. It’s showing how people are matching you. Either save this page, or do a screen capture of the segment with the segment beginning and ending points in the table above the chromosome image.
Go back to the Visualization options and use “Segment csv file.” This gives you all the matching chromosomes and segments of all the people who matched both of you. This not only gives the segments where they are matching you and the other kit you used (in the People who match one or both of two kits.) but also shows how they match each other. Even if you do follow that one location where you had two or more people matching you on the same chromosome, you should keep track of other segments because other matches may eventually show up that will match on those, too.
One thing that is difficult with the “Segment csv File” is that if there are close relatives involved (say your aunt and yourself and your sister), it’s going to show a lot of segments where we match each other. Those aren’t going to help you identify the unknown ancestral couple shared with other matches. So this file is created sorted by names of matches. So you can go through the resulting spreadsheet (from opening the csv file) and first delete all rows of close relatives. You can also do this when you can tell that the matches are close relatives. Then delete the rows showing matching segments to each other.
Triangulation on GEDMatch – Why Do It This Way?
Working with GEDmatch this way, you verify triangulated segments by being able to get the Segment csv file. You have to use triagulation because even if two people who match each other are matching you on a segment does not necessarily mean that they are matching each other. It is likely they will be, .but it is possible that they match each other through a different ancestral couple and just happen to match you on the same segment. One may be matching on your maternal chromosome and one the paternal chromosome. Triangulation on GEDMatch helps to prove that they are indeed matching each other on the same segment and the “Segment csv File” does this for you.
To read more about DNA triangulation, there are several articles listed at ISOGG.
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