Andy Lee

4 min read

What is Triangulation on AncestryDNA?

Ancestry DNA Match lists with title about match triangulation

Many beginners doing genetic genealogy find triangulation of their DNA on Ancestry confusing. Learn what it is and what it can do for your family history research.

Knowing that you match another person can confirm your relationship with each other. When you know that you match more than one person and how they match each other, you can find your common genetic ancestry.

What does DNA Triangulation mean?

DNA triangulation involves three or more persons sharing the same DNA segments.

Three or more persons who are matching share DNA is not enough. You actually have to share DNA all on the same segment on a specific chromosome. Meaning a specific location with similar start and endpoints on that same chromosome.

Think of DNA triangulation as points on a triangle.

Person A has to equal B.
Person B has to equal C.
Person C has to equal A.

Scientifically, this means that the DNA segment that these three people share was passed down through this common genetic ancestor.

Even though these shared DNA segments don't necessarily have to be very long, the overlapping genetic sections offer insights into their shared family tree. With sections ranging from 7-10 centimorgans, you can determine your Most Recent Common Ancestor (MRCA).

Triangulated segments that are 15-20 centimorgans are absolutely amazing! It probably indicates a pretty recent ancestor, such as a second great-grandparent.

In this video, you can view helpful drawings about DNA triangulation with Ancestry.

AncestryDNA: What is DNA Triangulation Using Shared Matches?

Ancestry Doesn't Share Segment Data

After taking an autosomal DNA test with Ancestry DNA, you can learn about your ethnicity, genetic communities and see a list of persons with whom you share genetics. You'll also learn about

How many centimorgans you share with your DNA matches.
How many segments you share with your DNA matches.
What is the largest segment that you share?

While the people on your shared matches are likely cousins to some degree on your family tree, you won't see segment information. This genetic genealogy company does not provide indicate upon which chromosome the match occurs and what are the start and end locations are.

What are you supposed to do with each new DNA match to research your ancestors?

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Ancestry Shared Match Triangulation Basics

Ancestry DNA allows you to do something that I call Match Triangulation. A colleague calls it pedigree triangulation using DNA.

In this case,

Person A matches Person B.
Person A matches Person C.
Person C matches Person B.

BUT, you don't know if the match occurs on the same chromosome in the same segment. As such, you don't have enough data to tell whether or not that each person received that DNA from the same common ancestor.

By contrast, with a Segment Triangulation, you can be fairly certain that that segment has been passed down from a common ancestor.

Without a chromosome browser, Ancestry users have to be very careful when they say their matches triangulate.

Problem with Triangulating with Ancestry Matches

While Person A, B, and C might match one another, without segment data, you do not know how.

You may find that:

Person C is related to Person A through their father.
Person C is related to Person B through their mother.

In terms of segment analysis, it's impossible that these three could share the same segment because the relationship is through a father and a mother. This means the shared DNA occurs on different chromosomes.

How Accurate is Ancestry DNA Triangulation?

Have you wondered, can a triangulated group of matches on Ancestry be wrong? The answer is yes, but the real question should be how common are the inaccurate groupings?

There is a 99% chance you can determine the MRCA with a more closely related set of Segment Triangulated matches.

By contrast, we actually see somewhere between 75% and 90% of the Shared Match Triangulated group can find the common ancestor.

As your triangulated group has matches, which share less than 10 centimorgans, the likelihood of false matches increases.

Ancestry only indicates a Shared Match for individuals who share 20 centimorgans and above. As such, researchers will likely find a common ancestor 75 - 90% of the time.

While triangulation on this platform may return the wrong results, it doesn't mean that Match Triangulation is useless.

Should a beginning genealogist do AncestryDNA triangulation?

For a beginning genetic genealogist, triangulation with the Ancestry Shared Match list is perfectly fine.

Ancestry has the largest database. So, you're going to have more matches from them than anywhere else. And in fact, somebody could probably spend their whole life just researching each one of those matches, trying to find out how they fit into their family tree.

Learn how to do some genealogy research to prove relationships. Compare trees created by other DNA test-takers. Use ThruLines and other techniques to find shared ancestors. As you find how people are related, you can color-code your DNA or link people to your family tree.

Once you exhaust your discoveries on Ancestry, it will be time to test with another company to get some of that segment data so that you can do segment triangulation using chromosome browsers.