How to define the Most Recent Common Ancestor (MRCA)

There are, generally, two discussions about this.

1: One is the MRCA is really a couple – and both your and your DNA Matches descend from this MRCA (couple). You descend from one child of the couple, and your Matches descend from other children of the couple. We may not know which individual in the couple passed down the DNA we share, but we do agree it’s from one or the other.

2: The other discussion is that the child of the couple that you descend from had to have the DNA segment and is then *the* MRCA (singular). But, at least in my mind, this MRCA (singular) is not the MRCA of the Matches, and things get a little complicated trying to have a discussion among the Match-cousins. The various MRCA (singular)s are not “common” to the various Matches.

In Segment-ology, I use MRCA to mean MRCA (couple). This couple is common to Matches who descend from them.

[22BR] Segment-ology: How to Define the Most Recent Common Ancestor by Jim Bartlett 20230513

14 thoughts on “How to define the Most Recent Common Ancestor (MRCA)

    • That’s a good way to be specific. I usually use the Ahnentafel Number of the husband and list the two surnames. Ex: 48M: BAKER/BROWN – I add the M to indicate the Maternal side – easily derived from 48 > 24 > 12 > 6 > 3 (my mother); but much more labor for more distant Ancestors. If I’m entering this info in something that can be sorted I use #A0048M – the # is a holdover from an old search algorithm, that I still find useful. If I’ve imputed this relationship (from a consenus of Shared Matches or a Triangulated Group), I annotate it as #L0048M to show the Match is likely to be this MRCA – and to be focused on that line while reviewing their Tree. Jim


  1. Me too, Jim. I always mark the couple as the MRCA, it also makes you not forget the segment(s) could come from either side! I like to look at it (if we have triangulation) that the ancestor who gave me the segment is DNA confirmed, but the couple is tentative. Once I have identified more MRCA’s further back on either side would then I would mark one or both of the individuals as also DNA confirmed. Sometimes you are left with one half of the couple still tentative, but it then highlights that you may have a problem there if it continues!


  2. Thanks, Jim. It’s good to be aware of the possibility of different definitions…How do you refer to the most recent common ancestor, as opposed to the Most Recent Common Ancestral Couple? That is, the person who is the common ancestor (singular) in half-cousin relationships.

    Liked by 1 person

    • Kathryn, I use MRCA for both – whether it’s one person or a couple, I’m back to that generation. And I’m always looking to walk the MRCA back another generation which would make it moot anyway. And I’m working hard at entering all of the thousands of paths from the MRCA down toward the Match, into my main Tree at Ancestry. Jim


  3. And the case where I am descended from great great grandfather and wife #1; and my match is descended from great great grandfather and his wife #2. In this case I sometimes have to drop the threshold to 4 cMs or so to pick up my match.


      • My thought was that in my example, my match and I are descended from only the gg grandfather and we are not descended from the wifes, In Common. I have no relationship to his gg grandmother and he has no relationship to my gg grandmother in terms of DNA. We only share the gg grandfather. Therefore, we would share only half as many cMs? This is a real life scenerio. I had tree matches with this gg grandfather, but showed shared cMs only in the 4-5 cMs range. Am I off base? Sometimes lowering the threshold to under 7 cMs will pick up population matches? Many thanks.

        Liked by 1 person

      • Caith; In general, we only get one DNA segment from one of two people in an MRCA – so, in general, we don’t get half from each of them. It is technically possible to get a different segment from the husband and from the wife – but very unlikely (and it would be two different segments). This does happen in close relationships, rarely in distant relationships.
        When I use GEDmatch, I can usually find a 3cM or 4cM “segment” with almost any person there – there is a very high probability that the “segments” are false. Your and my DNA is over 99% the same (we all need the same “building block” DNA to create a human, only a small part of our DNA is different. The concept behind “matching DNA” is that the segment must be long enough to pretty much guarantee that it is unique, and could only have come from a Common Ancestor – it’s Identical By Descent (IBD) – it is only identical because it descends from a CA. This can happen with a 4cM Match, but science predicts well over half of the 4cM shared segments are false. I always try to remember to say “shared segments are false” rather than “segments are false”. This is because all of your own DNA is true, even segments below 1cM. The problem comes when the match algorithm compares our DNA to someone else’s DNA to get a “shared DNA segment” – the algorithm gets parts from our mother and our father and creates a false “shared segment” – one that doesn’t come from one Ancestor.
        I don’t use “population DNA” – it’s like saying we get it from the drinking water. Our own DNA is unique. There are some segments that are found in many people. These are usually short segments, and they can create pile-up areas. These segments are usually from very distant Ancestors – well beyond our normal genealogical timeframe.
        Hope this helps, Jim


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