A Triangulation Overview

A Segment-ology TIDBIT

Triangulation is a tool. It’s a process that can help us with our genealogy. It is not the only tool in our kit bag – there are many other tools that also utilize DNA, including InCommonWith Lists, Matching Segment Lists, Matrix displays, Shared Matches, Clustering, Circles, etc, etc. This blogpost is an overview of Triangulation.

With atDNA we have been using Triangulation to mean two different things:

Segment Triangulation of shared segments (a focus of this blog), and

Ancestry Triangulation (having at least 3 Matches in a Triangulated Group (TG) all match on the same ancestral line; sharing a Common Ancestor (CA) on that line.

In the atDNA community we often conflate these two concepts, and they are very much intertwined. I tend to think first of forming a TG and then looking at the genealogy to determine the side (maternal or paternal) and then finding various MRCAs. But some start with the genealogy and look for Triangulation to add evidence that a CA is correct. Both ways will work, they are intertwined in genetic genealogy, so in this overview I will also conflate them. Here are some overview points about Triangulation:

We look for at least 3 Matches, Much of our work as genealogists involves one-on-one – finding a Common Ancestor with a Match – that’s OK, but it’s not Triangulation.

We look at overlapping DNA segments. ICW and other tools don’t require overlapping segments – that’s OK, but they are not Triangulation.

We look for 3 “segment” legs. This means the 3 people (usually you and two Matches) that form a Triangulated Group are not closely related. But once a TG of 3 cousins is formed, other close relatives can be added to the TG. It’s the TG forming that needs 3 strong legs. So 3 siblings and their parent do not form a TG, but they can be in one.

The shared segments that form a TG must be IBD. From experience we’ve found that:

  • “all” shared segments over 15cM are IBD;
  • shared segments under about 7cM are false most of the time; and
  • the process of comparing overlapping shared segments in a TG will cull out many in the 7 to 15cM range which do not match – I consider these to be false segments.

Blaine Bettinger is working to define Triangulation – not to preclude the use of other tools – but to help us better understand Triangulation as a tool. I use Triangulation as a tool to primarily sort and group all of my IBD segments. I’ve formed about 400 separate TGs over my 45 chromosomes. New Matches always fall into one of these TGs (close Matches may span two or more TGs – it’s OK). This is segment Triangulation. With close relatives, I’ve been able to determine the side for these 400 TGs. This is a huge benefit because new Matches almost always Triangulate with other Matches already in a TG; and I then know which side our Common Ancestor must be on. This is an excellent use of the Triangulation tool.

Ancestry Triangulation does not preclude me from also using the information of Circles, or ICW lists, or ethnic makeup, of even genealogy records or Trees or discussions with Matches to determine CAs.

Within a TG we may find a CA with a Match. As we have pointed out many times: shared DNA plus a CA does NOT mean that the shared DNA came from that CA, or that the CA is somehow “proved” because there is also a shared segment – maybe, but also maybe not. But, by finding 3 Matches in a TG who all share the same CA (Ancestry Triangulation), we increase our confidence (not “prove”) that the CA is linked to the shared segment; and with more Ancestry Triangulation (and/or walking the ancestor back), we increase our confidence even more.

If Triangulation leads to a conclusion that a CA is not linked by the shared DNA, we can still be cousins on that CA, and we can still use ICW, Circles, etc. to pursue a genealogy goal. But we should not say that DNA supports that cousinship conclusion.

IMO, a TG has characteristics that help us in our genealogy goals. Triangulation is a strong tool that takes advantage of our shared DNA with Matches.

I applaud Blaine’s effort to try to define Triangulation and provide some standards for its use.

The above is adapted from my recent post to the Genetic Genealogy Tips and Techniques Facebook Group.


[22N] Segment-ology: A Triangulation Overview TIDBIT; by Jim Bartlett 20170728

Triangulation at 23andMe

A Segment-ology TIDBIT

23andMe has a great feature. It starts out as a standard In Common With (ICW) list for each Match you are sharing with (old Shared Genomes Matches and new Open Sharing Matches). This ICW list is near the bottom of your Match’s page. But the difference with other ICW lists is the Shared DNA column. The Matches marked with a “Yes” have overlapping segments – and over 99% of the time they form a Triangulated Group (TG).

So go to your DNA Relatives page and scroll to the bottom and click on the “Download aggregate data” link. You’ll get a spreadsheet of all your matches and most of the 23andMe data. Sort the spreadsheet, and delete the ones with no segment data. Then sort on Chromosome Number and Chromosome Start to put them in a particular order. Add a column called “TG ID”. Now you’re all set to begin Triangulating.

Start with the first Match in the spreadsheet (let’s call it A). Click on the hyperlink* that takes you to A’s page, scroll down to the ICW list and note in your spreadsheet Match A and each Match with a “Yes”. Since you are starting on Chr 01, call this TG: 01A, and put 01A in the TG ID column for A and each “Yes” Match. This pretty much identifies all the other 23andMe Matches that are in a TG with (A). The whole TG 01A (of 23andMe Matches) is created through one Match! There may be a few that don’t overlap A enough to form a shared segment at 23andMe, but all you have to do is go down your spreadsheet list of 23andMe Matches and select the next Match (B) that is not already in a TG; click on B’s hyperlink and look at their ICW list for “Yes” Matches with (B) – some will either overlap with (A) (call all of them 01A, too); or they all form a new TG (say 01B) – one or the other. Then continue with the next Match not already in a TG.

One could probably go through their entire 23andMe list of shared Matches in a few hours, creating TGs for all of them. There may be some with no ICW “Yes” Matches – give them their own TG; and move on. Be careful with Matches with more than one shared segment – make sure to treat each segment individually – this may take a little extra analysis.

Remember, TGs represent segments (from an ancestor) on one of your chromosomes. They are equivalent to phased data. I consider all shared segments at 23andMe which Triangulate to be IBD. All of them should be in a TG on one side (parent), or the other.

If you have known relatives in any of the TGs you can assign those TGs as Paternal or Maternal. This often allows other, overlapping TGs to also be assigned to a side, using logic.

*Note1: clicking on the 23andMe hyperlink (Link to Compare View) is a little tricky – I usually just copy (Cntr-C) the spreadsheet URL, and paste (Cntr-V) it into the URL bar of any open 23andMe page – hit Enter. It goes pretty fast.

Note2: feel free to use any TG ID numbering system you want. I think it’s wise to start with the Chromosome number. But you can name your TGs Bill, or Bob, or Sue if you want. You are creating groups that will tie to ancestral lines.

ARE YOU READING THIS FTDNA? ALL YOU NEED TO ADD IS A YES!! And AncestryDNA could add a similar feature, and hell might freeze over, too.

Enjoy easy Triangulation at 23andMe…


[22M] Segment-ology: Triangulation at 23andMe TIDBIT; by Jim Bartlett 20170720

Using a Child to Determine the Side

A Segment-ology TIDBIT

Normally your child’s atDNA cannot indicate which side of your DNA a Match is on. But, a child only gets one side of your DNA at a time. In other words: you have DNA from both of your parents that covers each of your chromosomes from beginning to end; but you can only pass on one side to a child. Your parents are your child’s grandparents – every bit of your DNA passed to a child is either one of the child’s grandparents (your parents) or the other. Please review Bottom-Up and Crossovers to see the approximate 34 crossover points that divide all of your child’s DNA into about 57 segments from your parents. This is a very important concept in atDNA, and well worth the time to study it until you understand it.

So if you have a segment that you cannot figure out which side it’s on (your maternal or paternal side), having a child also match on that segment doesn’t give you any insight. Because: if you don’t know which side it’s on for you, you don’t know which grandparent it’s on for your child.

However, if you have tested at multiple companies, a child may help. Here is an example.

At 23andMe I have 3 Matches who all match me and each other on the same segment – a Triangulated Group (TG). There are no other 23andMe Matches, or any known cousins, who overlap this segment at 23andMe. However at GEDmatch I have a number of Matches with overlapping segments, and I’ve formed two TGs. And with my Dad’s kit at GEDmatch, I know which TG is paternal and which is maternal. I also have a child at GEDmatch, and he matches the maternal TG, but not the paternal TG. So my son got my maternal DNA for this segment. This son was also tested at 23andMe, so checking the ICW list with the 3 Matches above, they all include my son with a “Yes” – meaning an overlapping segment. So I can conclude that the 3 Matches at 23andMe form a maternal TG for me, because my son only got my mother’s DNA on this segment.

This now lets me communicate this information to each of the 3 Matches: our Common Ancestor will be on my mother’s side (and I include a list of Patriarchs of those lines in my message). I’ve made up two standard messages for 23andMe Matches – one for each side – which I can quickly copy and paste into the 23andMe message box. I initially get about 10-20% response – usually with a thank you for providing a concise, and easy to review list of potential Common Ancestors. Over time, additional responses trickle in. It’s a quick and easy way to find those Matches you can work with. And sometimes these “seeds” bear fruit months and even years later.


[22L] Segment-ology: Using a Child to Determine the Side TIDBIT; by Jim Bartlett 20170712