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