This question comes up often. The answer is: we cannot tell from just the fact that two shared DNA segments overlap in a chromosome browser. Here is the picture we see:
In this picture, you are normally A and you have two Matches, B and C, which show as overlapping on Chromosome 6. Because they overlap, is this Triangulation? Do A, B and C shared the same Common Ancestor? We cannot tell from this picture.
Assuming the shared DNA segments are Identical By Descent (IBD) – generally true for all such shared segments over 15cM – there are two possibilities:
- They are on different Chromosome 06’s in A. Remember we have two of each Chromosome – one from our mother and one from our father.
In this case, we are (somehow) looking at just A’s two Chromosome 06’s and showing where the shared DNA segments are on A’s DNA. It looks just like the picture we saw in the browser – two overlapping DNA segments. But in this case A & B are sharing on A’s maternal Chromosome 06; and A & C are sharing on A’s paternal Chromosome 06. These two Chromosome 06’s are physically separate (think of two strands of spaghetti). Because A & B have a shared DNA segment, they have a Common Ancestor (CA) who passed that DNA down to them. Because we know in this example that it’s on the maternal Chromosome (the one from A’s mother), we know the CA is on A’s maternal side. Similarly, we know the CA with C is on A’s paternal side. Yes, there is a very unlikely chance that these two CA’s could be the same person, and the DNA segment came down two very different paths to A’s mother and father. I’ll not be sarcastic here – you can decide for yourself if you think that is possible (or what the probability is) in your case.* In general, in genetic genealogy, we conclude that B & C are probably not related to each other – at least not on this segment.
- Alternatively, the two shared segments are on the same Chromosome 06 in A – let’s say, for example, they are both on the maternal side (imagine the two bars below on one Chromosome).
In this case, we are (somehow) looking at just A’s one maternal Chromosome 06, and showing where the shared DNA segments are. Again, it looks just like the picture we saw in the browser – two overlapping DNA segments. But in this case A & B and A & C are sharing on A’s maternal Chromosome 06 (they are both on the same strand of spaghetti). From the beginning of the A & C shared segment to the end of the A & B shared segment, we are looking at the exact same place on A’s Chromosome 06. For there to be a match, all the tested markers (SNPs) are the same. In general, in genetic genealogy, we take this to mean that this DNA came from the same Common Ancestor. It came from that CA down to A and to B and to C. Because both B and C share this same segment of DNA found on one Chromosome 06 in A, both and B and C should themselves show up as a Match to each other. After all they have the same DNA over this area of their own Chromosome 06.
You may have noticed that I stated each explanation of the two possibilities with: “In this case, we are (somehow) looking at…” Well we can’t just look at just one chromosome in a browser and compare it to someone else’s DNA. We don’t have that technology for genealogy DNA testing. But if we could, that is what we would see (probably without the color coding). But we cannot! We can only visualize it. So what can we do?
We use reverse logic. In the first possibility, we noted that B & C wouldn’t match each other; and in the second possibility, we noted that B & C should match each other. That is information we often can determine (at 23andMe, MyHeritage and GEDmatch – and round-aboutly at FTDNA). So, we say that if A matches B, and A matches C on the same/overlapping DNA segment, AND B matches C there too, it indicates the second possibility above – the three of them share the same Common Ancestor. This case of A=B=C=A is called segment Triangulation, and the three Matches are in a Triangulated Group [TG]. There is more about Triangulation here.
In my case, I have close to 20,000 Match/segments – each shared DNA segment is in one of 372 TGs which cover all of my DNA. In other words, these 372 TGs form a segment map of my 45 Chromosomes. The objective now is to determine the Ancestors who passed these TGs down to my parents and then to me.
*If you want to check to see if you have the same segment from your mother and your father, upload your DNA to http://www.GEDmatch.com and use the “Are Your Parents Related” program. It will show you any such segments, which is good information to have in any case.
[11D] Segment-ology: Are Overlapping Segments Triangulated? by Jim Bartlett 20200414