A Segment-ology TIDBIT
Triangulated Groups (TGs) are one way to group your Matches – grouping Matches who share overlapping DNA segments with each other. The DNA segment represented by a TG, is passed down to you from one of your parents, and from more distant Ancestors on that side. As we find Most Recent Common Ancestors (MRCAs) with Matches in a TG, we begin to learn which ancestral line passed down the TG segment – a Common Ancestor.
Clustering is another way to group your Matches – grouping Matches who share other Shared Matches with each other. The Matches in a Cluster also tend toward a Common Ancestor.
The companies where Triangulation is possible generally do not have many robust Trees. And so the TGs do not have many MRCAs. AncestryDNA has many robust Trees and a ThruLines tool that determines many Common Ancestors (CAs) , but does not provide the DNA data needed for Triangulation.
Is there a way to combine the best of both worlds? I think there is. TGs and Clusters should be grouping on the same thing – an ancestral line – a Common Ancestor. When a TG and a Cluster clearly have some of the same Matches, I think the deeper MRCAs in the Cluster, can be imputed to the TG.
This is another method of Walking The Ancestor Back (WTAB) in a TG. You already have a TG with some MRCAs along the same line. The Most Distant Common Ancestor (MDCA) in a TG usually represents a couple, one of whom passed down the TG segment. The next generation back has only two possibilities: the paternal or maternal side of the MDCA.
The MRCAs in a Cluster aligned with the TG provide a strong clue in reinforcing and extending the CA for a TG.
[22AP] Segment-ology: Extending the MRCA of a TG through Clusters TIDBIT by Jim Bartlett 20191118