A Triangulation Concept
An MRCA* in a Triangulated Group (TG) creates a point in your ancestry Tree where the DNA from an Ancestor has to pass through to get to you – a knot-hole of sorts. A DNA segment from your Ancestor passes down a specific line of descent to you. When you and a Match determine a genetic MRCA; that MRCA has to be on that specific line of descent. Draw a mental picture (or see the Figures below) of a funnel, with the MRCA and the line of descent to you represented by the narrow stem; and all of the ancestors of the MRCA represented by the V-shaped funnel itself. Any other genetic MRCA for this TG will be 1) within the funnel area (the ancestors of the MRCA), or 2) somewhere on the stem (descendants down to you). The MRCA Knothole is where the stem meets the funnel. Suppose the genetic MRCA is with a 3C on a 2G grandparent couple (one of 8 such couples in your Tree). The MRCA Knothole is this couple; the stem includes their child/your Great grandparent, your grandparent, your parent and you; the funnel includes only four of your 3G grandparents, eight 4G grandparents, etc. This funnel now eliminates 7/8 of your ancestry from contention for this TG segment! Now suppose you find a 5C with an MRCA in this TG, and the MRCA is in the funnel! BINGO!! If this, too, is a genetic MRCA, you’ve just shifted the MRCA Knothole! The funnel moves back two generations and now excludes 31/32 of your ancestry from contention, and really narrows down the possibilities for more distant MRCAs for this TG. This is the concept of “walking the ancestor back” – see blog post here.
[*MRCA (Most Recent Common Ancestor) here means a genetic MRCA, on in the path of the shared DNA.]
The following Figures show the MRCA Knothole for a 2C and then shifted for a 3C. The funnel for the 3C has only half the Ancestors still in contention for the TG of the Matches.
Figure 1. MRCA Knothole and Funnel for 2C
Figure 2. MRCA Knothole and Funnel for 3C
08B Segment-ology: The MRCA Knothole! Concept by Jim Bartlett 20170909