The MRCA Knothole!

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

9 thoughts on “The MRCA Knothole!

  1. Jim, I like this concept. It provides a graphic portrayal of how our quest to identify the common ancestor shared by a DNA match must take into account that the most recent common ancestor is passing along DNA that this ancestor, in turn, received from his or her ancestors. I like the funnel and knothold metaphors!


  2. Jim, I doubt you remember me, but we met at Carol Petranek’s Family History Seminar two years ago. We sat together at the back of the chapel and visited between presentations. My wife and I were missionaries for the LDS Church.

    Could you suggest a good primer for understanding DNA evidence in establishing familial relationships? I have a 3rd great grandfather who is the end of one of my lines that we have searched for for over 20 years. I have DNA information from 3 descendants, and know of a woman who has 3 sets of DNA from other descendants of this individual. I’m not sure how to proceed. -Ken Knight


    • Ken, I would recommend “Genetic Genealogy in Practice” by Blaine T. Bettinger and Debbie Parker Wayne c2016 from the NGS. Chapter 7 “Incorporating DNA Testing in a Family Study” and Chapter 8 “Incorporating DNA Evidence in a Written Conclusion”. This book is unique in that it includes exercises at the end of each Chapter. This and other resources are easily research through: – noodle there and then click on the autosomal DNA link to drill down more. From a Segmentology perspective, I’d upload your kits to GEDmatch and Triangulate as many Matches as I could for each kit. Then, look for TGs that are probably from your 3G grandfather of interest, and collaborate with all of the Matches in those TGs. Good Luck! -Jim


  3. Thanks Jim. When I reach a MRCA “pair” (not sure who the ‘donor’ is, maternal or paternal), for cataloging, I step back towards myself and identify the known ‘donor’ who gave the segment(s) to me. So in the case above, where the 3C matches the pair, I would ID the donor (to me) as the Female below the pair, our “GGM”. We could probably also conclude that is the donor for our 2C too (with paper trail to follow up). I do this simply so I can stop at the last “known”, and cap it psychologically lol That way when I look at it, it isn’t something to be solved every time.


    • Clark, I do roughly the same thing. In my spreadsheet I have 12 narrow columns – one for each generation going back. So for each identified genealogy MRCA (may be genetic MRCA, or not), I’ll note the Ahnentafel numbers out to the MRCA. So the first column is G2, and I put in a 2 or 3 depending on if the MRCA is on my father or mother’s side. If it’s a 3, then the next column has to be a 6 or 7, etc. The last Ahnentafel number is almost always even – for the male in the MRCA pair – and I bold it, to indicate (to me) that it could be the male or female). Your way works. too (I just like to “push” a little). However, your way has the advantage in sorts to summarize all known MRCA children… (remembering that they are probably genealogy MRCA, and only maybe genetic MRCAs). -Jim


  4. Pingback: MRCA Knothole Guildelines | segment-ology

    • Linda, Thanks for your positive feedback. It’s a big help to me too – first to articulate it to myself, and then to try and present the concept in a clear, plain English way. Visuals often take a while to generate, but, hopefully, they are usually worth it. -Jim


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