A Triangulation (and grouping) Concept
We have spent a lot of time and effort to describe *how* to group our Matches: segment Triangulation, DNA Painting, Shared Match Clustering. Each of these processes results in a group of Matches that should have a Common Ancestor (CA). This is an important concept.
But the main thing is to *use* this concept – to use the information found in these groups. If a group is formed around a CA, then all of the Matches in the group should share a CA. Once a CA is found, each Match in the group should also have that group CA, or be a closer cousin with an MRCA that descends from the group CA, or have a more distant MRCA which is ancestral to the group CA. In other words, all the Matches in a group should have the same distant CA.
So… if we find a CA for a group, the other Matches in the group should have the same CA line. This is a powerful focus – let’s *use* it. We should be able to look at other Matches in the group (who have Trees) and find that CA – either directly through a search, or indirectly by building out their Tree.
I illustrated this in Case 3 of Chapter 1 (Lessons Learned from Triangulating a Genome) of “Advanced Genetic Genealogy: Techniques and Case Studies” – here or here. This was all about one of my TGs which I call [04P36]. At Ancestry, I found a few cousins (who had uploaded to GEDmatch) in that TG who shared my HIGGINBOTHAM ancestry. Armed with that hint, I searched for HIGGINBOTHAMs in other Matches (in that TG) who had trees. I also contacted Matches from FTDNA, 23andMe and MyHeritage – and several replied that they had the same HIGGINBOTHAM Ancestry. In the end I found 14 different Matches ranging from 4C to 8C on this HIGGINBOTHAM line in TG [04P36].
Because TG [04P36] came down a line of descent with the HIGGINBOTHAM surname in 5 generations, this case was an easier example – searching for one distinct surname. If a group represents a CA with a male-female zig-zag line of descent to me, it will be harder – the surname will change often. However, each line of descent (from a given Ancestor) is fixed – and we may find Match cousins with MRCAs of different surnames, but they will all be on the same ancestral line. This is akin to “Genealogy Triangulation” – getting an alignment of multiple cousins on one line.
Finding one Match with a CA in a group is not the end of the story – it’s a clue to the beginning of more research. If we find a CA for a group, but no other Match seems to have that CA, maybe we need to look for a different CA. The “correct” CA for each group should lead to Genealogy Triangulation – agreement by other Matches on the same ancestral line. If you find a CA in a group, *use* it to find more Matches on that same line. Seek CA agreement among Matches in each group.
[08D] Segment-ology: Using a Group Common Ancestor Concept by Jim Bartlett 20200620