Each Triangulated Group represents a segment of DNA that come from a specific ancestor in your ancestry. It comes from that ancestor, down one specific path, to you. Even if you have the same ancestor in your ancestry more than once, a specific TG segment comes from only one instance of that ancestor, down one specific path to you.
Review Ancestral Segments
Let’s review ancestral segments. Please also refer to the posts and figures for Segments: Bottom-Up and Segments: Top-Down. At each generation of ancestors, each one passes down specific segments to you. In other words, your entire DNA is made up of segments from your ancestors of each generation. All 4 of your grandparents passed down various specific segments to you that make up all of the DNA on all of your chromosomes – all of your DNA came, in various segments, from those 4 ancestors. The same statement can be made of all 8 of your Great grandparents – you have different segments from each of your 8 G grandparents, which, in total, provide all of your DNA on every chromosome. Some Great grandparents may not contribute to each and every chromosome, but all of your chromosomes will be a patch-work-looking quilt of segments from your Great grandparents. The same is true for every generation: your DNA is made up from segments from your ancestors in that generation. As you go back 6 or 7 or 8 generations, some of the ancestors of that generation will drop out. That is you don’t get DNA from every one of them. They are still your ancestors, you just didn’t inherit DNA from them. Review The Porcupine Chart.
Another way to put it is that segments from distant ancestors are combined to make larger segments in closer ancestors. The smaller segments are still there, it’s just that the closer ancestor passed down larger segments which are made up of the smaller segments from their ancestors. Your parent passes complete chromosomes (very large segments) to you, which are made up of smaller segments from his/her ancestors. See the graphics in Top-Down.
Mapping with Triangulated Groups
Now the hard part of chromosome mapping with Triangulated Groups (TGs) is that the TG segments don’t generally create a map for any specific generation. The TG segments could come from ancestors of different generations. It all depends on how your TGs are laid out, and on the shared segments you get with Matches. You will only “see” TG segments that are made up of shared segments from Matches. So some TG segments may be from closer generations than others. But, in general, your TG segments will be adjacent to each other from the start to the end of each chromosome.
Cousinships with Matches in a TG
A TG is made up of Matches with shared segments. Review: The Anatomy of a TG. The shared segments (with Matches) in a TG will generally fall into 3 categories:
- Shared segments with Matches who are cousins on the Common Ancestor who created your TG segment. For a TG segment created by a 6G grandparent, this would be 7th cousins. Note: it would be very unlikely for you to have more than three exact 7th cousins (each one from a different child) from a 6G grandparent on one segment (i.e. in a TG). More on this later.
- Shared segments with Matches who are closer cousins – they share a Common Ancestor with you who is closer than the 6G grandparent who created your TG segment. Note: it would be very unlikely for you to have more than three exact 4th or 5th or 6th cousins (each one from a different child) from the same xG grandparent on one segment (i.e. in a TG). More on this later. These closer cousins would share a Most Recent Common Ancestor (MRCA) who is in the direct path of descent from the 6G grandparent (who created the TG segment) down to you. Because of this they also share the 6G grandparent ancestor with you. But, in this case, you will both descend from the same child of the 6G grandparent, and that does not count against the “limit” of three 7th cousins from a specific ancestor.
- Shared segments with Matches who are more distant cousins – they share a Common Ancestor with you who is ancestral to your 6G grandparent who created your TG segment. This frequently happens when the shared segment is less the full TG segment from the 6G grandparent (for example). So these Matches could be 8th or 9th or 10th cousins or even more distant. They descend from a smaller “sticky” segment which is passed down to them (along a specific path) and down through your 6G grandparent to you, along the same path as categories 1 and 2 above. So you might be 9th cousins on an 8G grandparent, who is a grandparent of your 6G grandparent. In fact, when your shared segment (in a TG) is in the 7 to 10cM range, the odds are roughly 80% that the Common Ancestor will be more than 10 generations (9th cousins) back. That’s the bad news – most of these smaller shared segments will be at the far reaches, or beyond, our genealogies. The good news is that roughly 20% of the Matches (sharing 7 to 10cM segments) will be closer – spread among 1st to 8th cousins. For shared segments in the 10 to 20cM range, the odds are roughly 60% that the Common Ancestor will be more than 10 generations back – which means 40% of those Matches will be closer cousins. In fact, with 100 shared segments in a TG in the 10-20cM range, you’d probably have two or three 3rd cousins, two or three 4th cousins, two or three 5th cousins, two or three 6th cousins, etc. You can see a graph of these probabilities in this article at ISOGG.
100 Matches on a TG!
So the next issue is: can we have 100 Matching cousins in a TG? The answer is absolutely! The odds are almost nil that more than 4 children will pass down the same matching segment to cousin descendants. Hence the caution that we almost certainly cannot have more than 4 cousins, from different children of a specific ancestor, passing down matching shared segments to cousins in one TG (i.e. on one segment). Each child inherits a different mix of DNA from the parents – rarely will more than four of them get the same DNA, and much more rarely would their descendants wind up sharing that DNA on the same segment (i.e. in a TG).
However – 100 Matches can easily descend from an ancestor 10 generations back without violating that concept. You and a sibling could share the same segment with two siblings who are 1st cousins from a grandparent. Similarly 4 Matches could be 2nd cousins with no more than two children from each ancestor. And 8 more Matches at 3rd cousin level – up to 512 Matches at 9th cousin level (10 generations back). And you could double that number by adding a once-removed cousin at each level. All without using more than two children at each generation. And this is all within a genealogical timeframe of 10 generations. If you consider that 80% of the Matches sharing 7-10cM will be well beyond the 10 generation level, the potential number of Matches in a TG becomes quite large. Of course this means the Matches are not all exactly at the same cousinship level – they are very likely to be spread over a range.
Large numbers of Mathes in a TG is not a pile-up!
So clearly a large number of Matches in a TG should not sound an alarm. Or arbitrarily define the TG as a pileup area, or require that all the Matches should be discarded. Nonsense! Large TGs, with Matches matching each other on overlapping shared segments, just indicates that many of the Matches will be more distant cousins. Some Matches will be beyond the genealogies of some – it depends on each genealogist and how far back their Tree goes, and the same for their Matches. But, statistically, there will be some closer cousins in the mix of Matches in the TG. We won’t know which are the closer cousins until we share with them.
The ancestor who “created” your TG segment
In the above discussion I note the ancestor who “created” the TG shared segment. This means the most distant ancestor who had that full segment. His or her ancestors did not have that full segment. His or her parent created (at least) that full segment from their parents. And the segment you inherited from that ancestor is all or, probably more likely, a subset of the segment that ancestor passed down, and eventually got to you. We are talking about the unique segment you inherited from that ancestor, and that segment did not exist in any ancestor further back. The TG segment is unique to you and your ancestor.
Your TG/Segment is unique to you
The ancestor who created your TG segment, probably passed down various overlapping segments to your Matches. These segments may be larger or smaller than the segment you got. What you “see” in a shared segment, is the part your Match got that overlaps the segment you got. So from a Match’s perspective, there is almost certainly a different segment from the Common Ancestor than you got – the Match would have a different TG than you from this same ancestor. Or some of your Matches may get segments from a more distant ancestor. These segments would be smaller and, when combined with other DNA, would result in the DNA segment that your ancestor passed down to you. Thus, those other Matches would be more distant cousins, on a smaller segment than what you got. There are many scenarios that would result in a Match (sharing a segment with you) being a more distant cousin – refer back to alternative 3 above.
The name of the atDNA game is sharing and collaboration
This accounts for some Match cousinships which can be pretty distant. I have to emphasize that, although some of the TG Matches will be fairly distant (and often beyond your genealogy), some Matches in each TG are probably within a genealogical timeframe. This brings up two important points when using TGs:
- Contact every Match you can – you never know which ones may have a Common Ancestor you can identify.
- Encourage Matches within a TG to share Trees among the group. Some Matches in a TG may be closer cousins to each other than they are to you, and they may in fact have a Common Ancestor who is ancestral to the ancestor of your TG. Sharing and collaboration are the name of this game.
15A Segment-ology: Understanding an using TGs by Jim Bartlett 20160917