CA and MRCA

 

Shared IBD segments come from a Common Ancestor (CA). Matching & overlapping IBD segments form Triangulated Groups (TGs). Every Match in a TG with significantly overlapping shared segments will have the same CA! And closer Matches (cousins), will also have a closer CA. So how can we have a close CA and a distant CA when they are in the same TG?  When they are all in the same ancestral line!

Let’s start with a distant cousin (Match) and look at the Common Ancestor.

05C Figure 1

Some notes about Figure 1

– With atDNA the path from the CA can go through males (boxes) and/or females (circles) in any order – it does not matter.

– The CA is one of the two parents above – the DNA that passed down from the CA to you and your 7th cousin (7C) came from one person. In this example, I’ve assumed the mother just to illustrate that it is just one parent. In most cases we don’t know which parent the DNA is from.

– The CA has at least two children: one is an ancestor of your 7C Match (M); and one is the ancestor of you (U)

– In this case the CA is also an MRCA (Most Recent Common Ancestor) – you and your Match don’t relate any closer on this line. However, in genetic genealogy, we tend to call this the CA, rather than the MRCA.

– You and your Match (M), will also share all of the Ancestors of the CA.

– This Figure 1 assumes the CA shown is the correct CA – the one who passed down the shared DNA segment to you and your Match.  We don’t really know if this CA is correct, until we find corroborating evidence – read on.

So how do we confirm that this CA line (either the mother or the father) is the one who passed down the segment you and the Match (M) share (as opposed to some other ancestral line)? One way is by Triangulation. When several people share the same segment, and all have paper trails to the same CA, we assume this CA must be correct. Another method is by “walking the ancestry back”. That is through closer cousins who also share this segment (in a Triangulated Group). We are generally pretty comfortable (when a close cousin shares a lot of DNA with us) that the closer CA is correct. In other words, when a 2nd cousin (2C) shares 220cM with us, and has large individual shared segments with us, we assume the CA is the known Great grandparent. And with large segments this is almost always true. Then if a known 4C shares a good sized segment with us, we also assume the known 3G grandparent is the CA. If all of these occur in the same TG, we need to call the intermediate CAs, MRCAs (Most Recent Common Ancestors) to distinguish them from the CA of the TG. Let’s see how this looks in Figure 2.

05C Figure 2

Some notes on Figure 2:

– The Tree for your Match (7C) and you is the same as in figure 1.

– A matching 2C on the same segment will have an MRCA with you on the G grandparent.

– Matches with 4C and 6C are also shown with MRCAs on the ancestral line from you to the CA.

– Everyone in Figure 2 descends from the CA.

This scenario, with intermediate MRCAs, adds a lot of confidence to the CA being the Ancestor who passed down the DNA that all of you (Match M, you, 2C, 4C and 6C) share.

Note that the intermediate MRCAs could have just as easily been on the Matches line. And/or you and the Match may both have intermediate MRCAs. The key point is that the MRCAs are in the ancestral line to the CA.

This concept applies equally to TGs. Each TG really represents a segment from an Ancestor to you. A “tight” TG – one with significantly overlapping segments among all the Matches in the TG – will have a CA just like a shared segment does. And all the Matches in the TG will share that CA. A “wide” TG – with “cascading” segments such that one at the beginning of the TG doesn’t overlap one at the end of the TG – may well turn out to be two TGs, with two CAs… more on that in a different post.

So there is always an Ancestor who is the most distant MRCA of your TG, for a given threshold. That means that any Ancestor who is more distant would not show up as a Match (using the given threshold, say 7cM), because the segment from the more distant Ancestor down to you would be too small at that distance to match anyone. For example, you may have gotten only 6cM from a 7G grandparent. In that case you would never get any Matches who were cousins on that 7G grandparent, using a 7cM match threshold. Others may get large enough segments from that same 7G grandparent, and maybe get some Matches, but you would not.

It appears this most distant MRCA of a TG may be fairly deep in our Trees in many cases. As a result we are having a hard time finding them. Our best tactic then is testing close cousins, and finding intermediate cousins among all of our Matches. This means testing at all companies and uploading to GEDmatch to get the most Matches you can. We never know when the key intermediate Match will show up – they won’t always have significantly larger segments. And lowering the threshold at GEDmatch, in general, will only result in even more distant cousins and more distant CAs.

Summary

A shared segment is from a Common Ancestor (CA) with a Match (cousin).

Closer cousins would have MRCAs with you who are descendants of the CA. Your Matches may also have closer cousins with MRCAs who also descend from the CA.

These “intermediate” MRCAs increase the probability that the CA passed down the shared segments.

We still do not know if the CA is the mother or the father, but we can be very confident that this is the correct ancestral line, and not some different or alternative ancestral line.

 

 

05C Segment-ology: CA and MRCA by Jim Bartlett 20160101

22 thoughts on “CA and MRCA

  1. Thanks again for another great tutorial, Jim. Your post emphasizes why I need to redouble my efforts to persuade my cousins to test.

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  2. Pingback: Endogamy PART II | segment-ology

  3. You all sent me to google to get definition for “elucidate” and “noodle”. These are not in my everyday vocabulary. Guess my age is showing – have a good day.

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  4. Jim I am confused by your opening paragraph. Two of the sentences do not align with my understanding of DNA. Can you help?

    “Overlapping IBD segments form Triangulated Groups (TGs).”
    I thought overlapping segments did not mean everyone in that group Triangulated. Perhaps only some of my matches who share overlapping segments Triangulate. This would be because some of those in the overlap are from my paternal side and some may be from my maternal when you unzip the chromosome.

    “Every Match with a significantly overlapping shared segment will have the same CA!”
    Base on the last explanation this statement does not make sense to me either. How can all matches who share overlapping segments automatically be considered to share the same CA. Again if the matches are from opposite sides of my family they cannot share the same CA but they may share an overlapping segment.

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    • Karen, you are correct. I got a little sloppy with the intro. I’ve edited the post. This post was not intended to define a Triangulated Group, or to change what I had previously posted on Triangulation. It takes time to redefine a TG every time I use the term, and to include all the caveats and cautions. The main point of this post was to highlight that there can be intermediate MRCAs in a TG.
      I’m not sure what you mean by “unzip the chromosome”. Your paternal and maternal chromosomes are completely separate. You may be thinking about how your paternal chromosome is formed from your father’s two chromosomes…

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  5. Karen – I follow Roberta’s blog, and remember this one. She uses “unzipping” in the sense of separating shared segments into paternal and maternal sides. The two chromosomes are not, somehow, tied together. There is no single chromosome with maternal and paternal parts. And all of the IBD shared segments are separate, they just don’t have a label saying which parent they are from. Roberta’s unzipping analogy is about separating shared segments.

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    • I think we are on the same page now. I was referring to the pictorial representations of a chromosome ex: a Chrom browser shows one line for each chromosome but it actually has data for both our paternal and maternal inherited DNA. Just because two people overlap they may not be related.
      I have an excellent example but have not been able to include here. It shows my husbands two first cousins one from his maternal side and one from his paternal side if I substituted unknown matches for these cousins we would not know which side of his familial line they received their DNA. Using 1st C demonstrates this really well, ergo unzipping the pictorial chromosome.
      Thanks for the nice chat
      Karen

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    • Thank you. It’s critical that we get past sweeping, broad-brush statements about endogamy, and analyze what is actually happening and figure out ways to deal with it. Stay tuned for PART III.

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    • Thanks, Israel – I think this is an important concept for AJ and other endogamous populations. What’s different about A1, A2, etc. is the path they take in passing DNA segments down to me and a Match. I think intermediate cousins, with MRCAs, can help sort it out (to be PART III). These are concepts to use as tools – they are not magic wands to pull rabbits out of hats. There is still hard work to do, but tools can help us whittle away at it.

      Jim – http://www.segmentology.org

      >

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    • Jason, I think I’ve figured out this issue. AncestryDNA claims that more than 3 4C can’t share exactly the same DNA segment (the odd are too great). The way this is worded, I think it is pretty much correct – it says exactly the same segment. If they are talking about an ancestral segment, I’ve not seen any two cousins with exactly the same whole segment from an ancestor. What we “Triangulationists” see are overlapping segments – the part of our ancestral segments which overlap – the shared segment is almost always smaller than the full segment. And in a TG, there are virtually no shared (overlapping) pieces of the full TG that are exactly alike, much less the full ancestral segments from whence the overlapping parts came being exactly the same. So it’s trick wording, or trick interpretations, that are being argued. We are all seeing many Matches and segments in our TGs – the numbers are doubling every 14 months. These overlaps are not exactly alike, and the full ancestral segments they came from are not exactly alike. But there are many such overlaps in our TGs. For some reason, some folks are trying to discredit Triangulation. But I’ve got over 90% of my DNA covered by TGs, and virtually every single new segment “fits” easily within an existing TG. And so the TGs grow with more and more Matches. I’m not wringing my hands in disbelief, I’m shouting hurrah – here are more folks who share the same Common Ancestor – and together some of us might be able to sort it out. As new Matches pour in each week, the odds of finding those intermediate Matches increase. By intermediate I mean the 4C to 7C for whom we have a chance of finding a Common Ancestor.

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