Friday, August 2, 2019

The Trials and Tribulations of Triangulation

There are millions of consumers spitting and swabbing their way to family history. An entire industry is on the rise, eager to be the spittoon to the masses and their gateway to genetic genealogy.1

Ethnicity estimates, of course, are a huge draw for the average Joe (“Kiss me, I’m 21% or 39% or 53.9% or 21.9% Irish!).2 My head’s already spinning, and I haven’t even had a pint of Guinness, yet.

Test-takers (particularly Americans where market penetration is greatest) are racking up cousin matches by the thousands. This overload abundance of unidentified relationships makes for Mission Impossible when you just want to pinpoint your link to William the Conqueror and make a better case for Liz’s crown.

Serious family historians fight the good fight, trudging through hordes of matches hoping for public trees that align with their own. If they’re lucky they’ll find a paired match with Average Joe who has a well-documented pedigree making it easy to pinpoint a shared ancestor.

While offering important clues to possible family relationships, these one-to-one matches are not entirely conclusive on their own.

Try Triangulation

What if you’re looking for more substantial evidence to untangle a tightly knotted family mystery? Can you do something a little more sophisticated with those DNA results?


One approach is called triangulation. Genetic genealogist Blaine Bettinger defines it as, "a process or method by which three or more people all sharing an overlapping segment of DNA in common compare their family trees in order to identify a common ancestor or ancestral couple shared by all."3

Triangulation is often touted as the gold standard for verifying a common ancestor among genetic cousins.

I have my own case study where triangulation may provide an answer for a theory of mine. 

Theory: My fifth great-grandfather Thomas Kirk (1778-1846) of Licking County, Ohio was the younger brother to Mary (Kirk) Geiger (1774-1832) also of Licking County. 

I really think they were brother and sister. I've found more than 40 one-to-one DNA matches between descendants of Thomas and Mary. The amounts of shared DNA between those paired matches aligns with the projected relationship-level (usually 5th cousins or 5th cousins once removed) that I would expect to see for the matches if Thomas and Mary were siblings and their parents were the common ancestors for all testers.4

Could triangulation provide more substantive, respected evidence so I could confidently conclude that they were siblings?

All I have to do is find living descendants of Thomas and Mary, test their DNA, and compare for overlapping segments. And those first two steps are mostly done. There's just step three to tackle.

Sounds worthwhile and easy enough, right?

Why is it so difficult to triangulate?

I quickly learned that triangulating autosomal DNA was easier said than done. In fact, I think Euclid had an easier go of it.

The first obstacle I encountered was getting access to the information needed to properly triangulate matches.

Nearly all of those 40+ paired matches were discovered on one monolithic genealogy site.

Guess which site does not provide information on whether matches triangulate nor does it give access to the segment data necessary to triangulate? The big monolithic one, of course.

To triangulate, I would need to persuade my genetic cousins to download their data from the monolith and upload to another entity that offers triangulation capabilities. Not impossible, of course, but frustrating that the approach can't be undertaken at the monolith.

The second obstacle I encountered was where to persuade the matches to transfer their results.

A couple years ago, this would have been a straightforward answer. There's one third party website that was the go-to site for testers to upload their data from most of the consumer DNA websites, especially the monolith.

However, following the apprehending of the Golden State Killer and other violent criminals thanks in part to a lead from a consumer genealogy test uploaded at this third party site, sensational headlines have raised privacy concerns.5 The subsequent spotlight on the use of the site by investigative authorities and a robust dialogue within the genetic genealogy community has been withering.

Was law enforcement surreptitiously mining users' data without their knowledge and consent? While this conversation is ongoing, concerns around ethics and privacy have created an unease that makes testers more cautious with their data.

Seeking a site that largely escaped the clickbait headlines, I settled on a popular genealogy platform based out of Israel that has proven an innovative player in the consumer DNA game and - most importantly to my case study - has a tool that identifies triangulated matches. Opportunely, they offer FREE uploads of DNA data from other companies (although there's a nominal fee to gain access to tools like the triangulation feature), and their Terms and Conditions of service attempt to put at ease concerns about law enforcement access:

"...using the DNA Services for law enforcement purposes, forensic examinations, criminal investigations, "cold case" investigations, identification of unknown deceased people, location of relatives of deceased people using cadaver DNA, and/or all similar purposes, is strictly prohibited, unless a court order is obtained. It is our policy to resist law enforcement inquiries to protect the privacy of our customers."6

The third obstacle I am now encountering has more to do with the science of how DNA is inherited and fades with each subsequent generation, drastically reducing the amount of genetic overlap needed for triangulation. The eldest living generation of Kirk descendants who have tested are 4th and 5th great-grandchildren of Thomas and Mary Kirk's parents.

At this level, there's very little DNA inherited from 4th and 5th great-grandparents: 1.56% and 0.78% respectively.7 The odds that the tested matches have inherited any DNA from Thomas and Mary's parents are slim, and the chance that they have inherited the same, overlapping bits of DNA required to triangulate a match are even slimmer!

And yet, despite all of these obstacles, I have identified my first triangulated match.

Two 4th great-grandchildren of Thomas Kirk - each descended from a different line - triangulate with a single descendant of Mary (Kirk) Geiger. It's a small 10.3 cM triangulated match, but, right now, it's my beloved inspiration and motivation to persevere through the trials and tribulations of triangulation.

While I wait for more Kirk and Geiger descendants to transfer their data and pray for overlapping segments, I find myself asking, "Why is triangulation so difficult if the rewards are potentially so great?" Surely it doesn't have to be this hard (shaking fist at that monolith).



  1. I feel the exact same way. It is so frustrating. The ancestors I'm interested in are my 3rd great-grandparents so the segment(s) would be a bit larger than the nice looking 10 cMs you have triangulated. I have large groups of matches who triangulate but cannot get the known cousins to transfer from the monolith. I keep using the tools available and hope one day it will be easier.

    1. I'm working with kits that are a generation or two closer in time. My kit doesn't even track with most of these Geiger matches. I suppose I'm too young ;)

      As far as encouraging cousins to transfer from the monolith, I've now taken to monthly updates on the case study's progress. Perhaps if I keep them updated they'll eventually see a role for them in the research and want to play a part.

    2. I could do something similar to your monthly updates on the case study's progress. But first I need to write the preliminary post about the DNA question which needs to be solved. It would simplify the email correspondence to have the post to refer. Thanks for the idea, Michael.

    3. It is very helpful to have DNA posts to reference that provide a more in-depth explanation. I often link to blog posts - especially on DNA - when chatting with genetic cousins. Good luck!

  2. Imagine adding to your experience the phenomenon of endogamy where, as in my case, almost all Ashkenazi Jews share at least some tiny portion of DNA. We all could triangulate, and yet we'd not find a common ancestor because that theoretical common ancestor is from one of about 350 families from 500 years ago when all Jews were marrying cousins and other relatives. I have thousands of matches, including 100s who are second or third cousins (but not really since I've identified all my second and third cousins through traditional research).

    So for me---unless I already have someone on my tree and the DNA confirms the connection, DNA has been almost entirely useless.

    1. I don't even want to imagine! This process is complicated enough and onerous enough, I don't need any other obstacles. But I have real sympathy for those confronted with endogamy. Just like a widely-available triangulation tool (particularly at the monolith), I'm sure you're hoping for technology to devise a way to parse through the tight relationships a few hundred years ago and reveal meaningful match data. Fingers crossed for you, too!

    2. Thanks, Michael---I like the way you dream!

    3. I just saw an advertisement for an eConference on DNA by Family Fanatics, which will include a session on endogamy by Leah Larkin. Maybe this is of interest:

    4. Leah's a friend---she's helped me with my DNA work (unfortunately with no real success). I will check this out. Thanks, Michael!

  3. You "settled on a popular genealogy platform based out of Israel that has proven an innovative player in the consumer DNA game and - most importantly to my case study - has a tool that identifies triangulated matches. they offer FREE uploads of DNA data from other companies (nominal fee to gain access to tools like the triangulation feature), and their Terms and Conditions of service attempt to put at ease concerns about law enforcement access. Can you list the name of the company? m

  4. My DNA has been uploaded at My Heritage for over a year. My husband's and daughter's DNA was uploaded there a few months ago. They have German ancestors. I had read that MH had more continental Europeans than Ancestry. But because the site is in German I refer to it as a German company. So I am surprised to learn you were writing about a place I am already connected to. Thanks, Mary

    1. In the sense of using My Heritage I did not beat you to anything beyond trees. I was first there about 10 years ago because three cousins only had their trees there. I was invited and totally lost at the site. Was just there looking at triangulation with my daughter and 4 close cousins. I like the color chart. The Excel charts not so much because I cannot fully open them. I can only save the color chart by doing screen shots. Do you agree that that making screen shots is the only way to save a the color triangulation chart? Need 4 screen shots for the 22 chromosomes. Thanks, Mary

    2. As far as I know, the only way to save the visual of the triangulated segment is to make a screenshot. That's what I've been doing in addition to downloading the data in Excel.

  5. Thanks for this post. I too wish we could somehow convince the monolith to provide what we have all been begging for. It cannot be that difficult. Pretty please.