Tag Archive | Triangulation

DNA Triangulation 2017 – My Rootstech Talk

Rootstech is almost here and I can’t wait. It is not just the great talks that I love but also all the wonderful displays, shows, and tools in the exhibit hall and, of course, seeing old friends. Naturally I am going in a day early to spend time in that wonderful Family History Library, practically next door to the conference!

My talk is about triangulation, a technique used with DNA test results to prove descent from a common ancestor. I hope to see you there, Thursday at 11:00 in the morning in Ballroom J. As always my slides will be posted at slides.com/kittycooper after the presentation.

Wold line cousins Kitty (me), Ed, Marlys

If you have been to previous versions of this presentation, there will be much that is new this time around. It is amazing how much the DNA technology for triangulation has progressed in just a year. Most American 23andme kits are now on the new experience which includes an automated triangulation feature. Plus GEDmatch has added Triangulation Groups (TGs) to its Tier 1 tools.

Furthermore one of the semi-finalists in the RootsTech 2017 Innovator Showdown is a new Double Match Tool from Louis Kessler that provides triangulation for different ftDNA kits that you have the Chromosome Browser Results (CBR) for. Click here for his blog and description of DMT. Genetic genealogy has come a long way!

There is also news for those of you who remember the story of how I used triangulation to confirm a thin paper trail and prove that Kristine is my Wold side cousin (click here for that blog post and here for the follow up). The DNA test for Marlys, one of Charlie’s 26 grandchildren, came in and provides further proof. Not only that but Marlys had known the story all along about that first child of Charlie’s!

A Triangulation Feature on the New 23andme

An exciting triangulation feature has just been released at 23andme. It is only for profiles that have transitioned to the new site and have selected open sharing (for now). The feature shows you the relatives you have in common with one other profile and whether or not they triangulate with the two of you.

To use this tool you go to Tools > DNA relatives > People and pick a DNA relative to look at. At the top of the comparison page there is a menu item “Relatives.” Click on it to skip down the page where you will see a display like this (names removed for privacy):


When there is a “Yes” in the right hand column, there is a DNA segment in common between the two people you are comparing and this third person thus you triangulate. A “No” means that although you each share DNA with this relative, there is no segment of DNA where all three of you match. Click on the Yes (or a No) to see the three of you compared in the chromosome browser.

Once in the chromosome browser, you can use the “Update View – Edit” to add more people to the comparison with the first person. This is the same chromosome browser that you get to from Tools > DNA relatives > DNA

Jason Lee has done a nice write up with more details on his DNA blog at Tumblr:

Sadly you cannot use this tool with relatives still on the old site, since they have not yet been able to opt in to “Open Sharing” yet.

More on the SCGS Jamboree: Free Live Streaming

I am very grateful that Ancestry.com has sponsored live streaming of the Jamboree since my foot problems are keeping me home now. There is one selected presentation available in each time slot.

TriangulationSlidesSee the schedule here: http://genealogyjamboree.com/2015/schedule-streaming.html and how to get set up for it here http://genealogyjamboree.com/2015/streaming-overview.html

I really enjoyed my day and a half at the Jamboree. I discovered that most of the San Diego folk had come by train! There is a stop next door at the Burbank Airport and the hotel has a van that will fetch you. Next year …

As always I loved listening to Cece Moore. I learned a few things from her presentation about some of her adoptee success stories. Many more genetic genealogy stars were there: Blaine Bettinger, Tim Janzen, Angie Bush, Judy Russell, and Kathy Johnston. Sadly I had to miss most of their presentations since I gave two of my own.

I was pleasantly surprised by the full house at my triangulation lecture. It was exciting to be live streamed. A number of folk told me afterwards that they felt like they finally understood this concept. What I did was present a number of real life cases from my own research where I used DNA triangulation to figure out relationships. That seemed to work well.

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Triangulation: Proving a Common Ancestor

The same question seems to come up over and over again among those new to autosomal DNA testing. If I match A and B on the same segment why is that not enough to prove they match each other and we have a common ancestor?

The reason the ancestor is not proven is that you have two strands of DNA on each chromosome (remember there are 23 pairs of chromosomes) and the testing mechanism cannot differentiate between the two of them. So A could match the piece from your mother and B could match the piece from your father or one of them could even be a false match to a mix of alleles from both parents (see my post on IBC for more on that concept)

The way to prove the common ancestor is to see if A and B match each other in the same place that they match you. This is what we call triangulation.

Triangulation example

Kristine’s shared DNA with other Wold descendants, relationships are to her (to me in parenthesis)

About a year ago I blogged about how, after many years, a change in spelling on the paper trail had led fellow genealogist Dennis to think his wife Kristine was perhaps descended from my great-grandmother’s brother Carl. To prove this I suggested he test her autosomal DNA.
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When is a DNA segment match a real match? IBD or IBS or IBC?

The current technology for personal genome testing cannot tell you which of the two chromosomes, maternal or paternal, in a pair that an allele comes from. It can tell you that there is an AG at a specific position and a CT at the next position but not whether the A came from your mother or your father. This leads to much confusion about DNA segment matching.


Kitty and Shipley; siblings sharing 47% of their DNA

The matches that these testing companies find are for stretches of DNA that are half identical regions (HIRs). This is due to the fact that a relative who shares a DNA segment from a common ancestor with you will match you along the chromosome you got from the parent who is descended from that ancestor. Thus your new relative will match you for half the alleles in those positions. Only a sibling will share fully identical regions of DNA. Click here for a page that has a picture of the DNA I share with my brother Shipley.

For example, if my Dad gave me AAAAAAAAAAA and my Mom gave me CCCCCCCCCCC then I would seem to match absolutely everyone on that segment because every position has both an A and a C. So an ACACCAACCAC or a CCAACCCACA looks like a match, but only those with an  AAAAAAAAAAA or a  CCCCCCCCCC would be real matches. This is simplistic and the segment runs used for matching are much longer than this to try to avoid that sort of false matching. Also note than when your testing company shows an AC it is really an AT and a CG but just one of the known pairing is shown for brevity.

The term for a real match is IBD, which is an abbreviation for Identical By Descent. The term IBS means Identical by State which would apply to any false match. So in our example, the CCAACCCACAA match would be considered IBS.

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