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Concepts – Why Genetic Genealogy and Triangulation?

Roberta Estes

One of the questions often asked is why triangulation in genetic genealogy is so important.

Before I answer that, let’s take a look at why genealogists use autosomal DNA for genetic genealogy in the first place.

Why Genetic Genealogy?

Aside from ethnicity testing, genetic genealogists utilize autosomal DNA testing to further their genealogical research or confirm the research they have already performed. Genetic genealogy cannot stand alone on DNA evidence, but must include traditional genealogical research. DNA is simply another tool in the genealogist’s tool box – albeit a critical one.

There are three established primary vendors in this field, Family Tree DNA, Ancestry and 23andMe, plus a few newcomers. All three vendors offer autosomal DNA tests utilized by genetic genealogists in various ways. If you want to learn more about the differences between these vendors’ offerings, please read the article, “Which DNA Test is Best?”

In order to achieve genealogical goals, there are four criteria that need to be met. All are required to achieve triangulation which is the only way to confirm a genealogical ancestral match to a specific ancestor.

Family Tree DNA assigns matches to either maternal, paternal or both sides of the tester’s tree based on connecting the DNA of relatives, up through third cousins, who have tested to their appropriate location in the tester’s tree.

In the example above, you can see the individuals linked to my tree include my mother with her Family Finder test, plus her two first cousins, Donald and Cheryl Ferverda who have also tested.

Of the vendors, 23andMe does not have a Gedcom upload or ability for testers to display trees and for the vendor to utilize to match surnames, although they can link to external trees. Ancestry provides “tree matching,” shown above, and Ancestry and Family Tree DNA, shown below, both provide surname matching.

Family Tree DNA’s chromosome browser is shown above.

Each individual tester will have two groups of matches on the same segment, one group from their mother’s side of the tree and one from their father’s side of the tree. Each tester carries DNA inherited from both parents on two different “sides” of each chromosome. You can read more about that in the article, One Chromosome, Two Sides, No Zipper – ICW and the Matrix.

Of the three vendors, Ancestry does not provide segment matching, a chromosome browser, nor any segment information, so testers cannot perform this step at Ancestry.

23andMe does provide this information, but each tester must individually “opt in” to data sharing, and many do not. If testers do not globally “opt in” they must authorize sharing individually for every match, so testers will not be able to see the chromosome segment information for many 23andMe matches. In my case, only about 60% are sharing.

Family Tree DNA provides a chromosome browser, the file download capability with segment information, and everyone authorizes sharing of information when they initially test – so there is no opt-in confusion.

Ancestry and 23andMe raw DNA data files can be transferred to both Family Tree DNA and GedMatch where chromosome browsers and other tools are available. For more information about transferring files, please read Autosomal DNA Transfers – Which Companies Accept Which Tests?

Triangulation – The process used to combine all three of the above steps in order to assign specific segments of the tester’s DNA to specific ancestors, by virtue of:

In order for a group of matches to triangulate, they must match each other on the same segment of DNA and they must all share a common ancestor.

Triangulation is part DNA, meaning the inheritance, part technology, meaning the ability to show that all testers in a match group all match each other and on the same segment, and part genealogy, meaning the ability to identify the common ancestor of the group of individuals.

The following chart shows a portion of my match download file on chromosome 5 from Family Tree DNA.

As you can see, these matches all cover significant portions of the same segment on chromosome 5.

Without further investigation, we know that I match all of these people, but we don’t know what that information is telling us about my genealogy. We don’t know who matches each other, and we can’t tell which people are from my mother’s and father’s sides. We also don’t know who the common ancestor is or common ancestors are.

However, looking at the trees of the individuals involved, or contacting them for further information, and/or recognizing known cousins from a specific line all combine to contribute to the identification of our common ancestors.

Below is the same spreadsheet, now greatly enriched after my genealogy work is applied to the DNA matches in two additional columns.

I’ve colored my triangulated groups pink for my mother’s side and blue for my father’s side.

In this case, I also have access to my cousins’ DNA match results, so I can view their matches as well, looking for common matches on my match list.

One of the reasons genealogists always suggest testing older family members and as many cousins as possible is because triangulation becomes much easier with known cousins from particular lines to point the way to the common ancestor. In this case, one cousin, Joe, is from my mother’s side and one, Lou, is from my father’s side.

By looking at my matches’ genealogy, I’ve now been able to assign this particular segment on chromosome 5, on my mother’s side to ancestors Johann Michael Miller and his wife Susanna Berchtol. The same segment, on my father’s side is inherited from Charles Dodson and his wife, Ann, last name unknown.

In order to achieve triangulation, the common ancestor must be determined for the match group. Once triangulation is achieved, descent from the common ancestor is confirmed.

Unless you are dealing with very close known relatives, like the Ferverda first cousins, there is no other way to prove a genetic connection to a specific ancestor.

At Family Tree DNA, I can utilize the chromosome browser and the ICW and matrix tools to determine which of this group matches each other. At 23andMe, I can utilize their shared DNA matching tool. This information can then be recorded in my DNA spreadsheet, as illustrated above.

Triangulation cannot be achieved at Ancestry or utilizing their tools. Ancestry’s DNA Circles provide extended match groups, indicating who matches whom for a particular ancestor shown in a tester’s tree, but do not indicate that the matches are on the same segment. Circles do not guarantee that Circle members are matching on DNA from that ancestor, only that they do match and show a common ancestor in their tree.  The third triangulation step of segment matching is missing.  Ancestry does not provide segment information in any format, so Ancestry customers who want to triangulate can either retest elsewhere or download their data files to either Family Tree DNA or GedMatch for free.

Summary

Before the advent of genetic genealogy, genealogists had to take it on faith that the paper trail was accurate, and that there was no misattributed parentage – either through formal or informal adoption or hanky-panky.  That’s not the case anymore.

Today, DNA through triangulation can prove ancestry for groups of people to a common ancestor by identifying segments that have descended from that ancestor and are found in multiple descendants today.

Of course, the next step is to break down those remaining brick walls. For example, what is the birth name of Ann, wife of Charles Dodson, whose surname is unknown? Logically, the DNA descended from a couple, meaning Charles and Ann, contains DNA from both individuals. We don’t know if that segment on chromosome 5 is from Ann, Charles, or parts from both, BUT, if we begin to see a further breakdown to another, unknown family line among the Charles and Ann segments, that might be a clue.

One day, in the future, we’ll be able to identify our unknown family lines through DNA matches and other people’s triangulation. That indeed, is the Holy Grail.

Additional Resources

If you’d like to read more specific information about autosomal DNA matching and triangulation, be sure to read the links in the article, above. The following articles may be of interest as well:

If you think you might come up short, because you have only one known cousin who has tested, well, think again.

Here’s wishing you lots of triangulated matches!!!

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