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.
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.
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.
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