The Two Meanings of TG

A Segment-ology TIDBIT

A Triangulated Group (TG) has two meanings – segment and group.

SegmentA TG represents a segment of DNA. A TG is defined as a Start Position and an End Position on one Chromosome (on one side – Paternal or Maternal). It describes one part of your DNA – accurate between two cross-over/recombination points*. Thus, a TG is a long string of SNPs on one chromosome – it is phased data on one side or the other. If we were to compare the raw data from each of the Matches in a TG, we’d see that they all had the same value at each point.

GroupA TG is also a group of your Matches. Each Match shares a phased DNA segment with you** that came from one of your Ancestors***.  All of the Matches should share the same Common Ancestor (CA) with you.***

Hedging a little…

* There is no “sign post” in our DNA to indicate the cross-over points, and the matching algorithms cannot exactly determine the start/end points of a segment – they may start before, and/or end after, the real ancestral segment. But they are pretty close. I say the segment ends are “fuzzy” – but the bulk of the TG segment is definitely from one Ancestor.

** It is possible that some Match(es) may have a false segment that happens, by chance, to match the phased string of SNPs – virtually always a segment under 15cM. If this Match/segment is critical to you, the Match can Triangulate at that location to determine if the segment is true or false.

*** The segments that pretty much cover the TG segment should be from the CA. Smaller segments that appear to be from one end or the other of the full TG segment, may well be from Ancestors beyond the CA. Note this is often the best way to determine which parent (in a couple) the DNA segment came from – the more distant Ancestor would be ancestral to one or the other of the couple.

Usually the TG meaning can be gleaned from the context – is the author talking about segments or Matches. If it’s not clear – call the author (often me) out!

[22BO] Segment-ology: The Two Meanings of TG TIDBIT by Jim Bartlett 20230407

6 thoughts on “The Two Meanings of TG

  1. Jim, Thanks for this tidbit. I’ve been thinking of asking your opinion about a bunch of paternal segments I’ve been researching recently. A few months ago, I received a note from a woman in Australia that she found the she matched my on a 39.5 cM segment on Chr 5 at both Ancestry and 23andMe. I took a look on 23andMe and found that beside hers, there were about 20 others. One that matched me on that segment at 47.0 cM an another on Chr 5 of 53.4 cM. Turns out, this is a 21 year-old 3C2R. We’ve identified about half of these Ancestry/23andMe matches and it turns 6 or 7 are 3rd or 4th cousins descending through my paternaalsol 2nd GGF. 3-4 others are descendants of two other of my 5th Ggrandparents’ sons than the one I come through. However, we still haven’t figured the Aussie connection. She got her father to test and his segment is the same size as hers. Their tree goes back to 1840s-1850s in Australia or NZ. So, we still that to work out, plus we’ve added some new ones from GEDmatch and MyHeritage.

    The 5th Ggrandparents were born in Virgina in 1682 / 1692. Paternal parents and grandparents came from Guernsey to Virginia ca. 1640. Maternal parents and grandparents were born in Virginia and Maryland. So, first question is, does it make sense for to have segments this large triangulating where the CA appears to be at least 10 generations back,

    My other question relates to defining the TGs. If the two or three longest segments are, say 50 cM long, and 6-8 others fall between the start and end points of those longest segments, and they all triangulate, according to MyHeritage’s chromosome browser, do you call that one TG, or do you break apart that group of overlapping segments and make name multiple triangulation groups?
    Thanks again for all your teaching on segment-ology!!


    • Doug, Q1: 10 generations seems pretty distant to me too. A TG represents an Ancestral line – from some Ancestor down only one specific line of descent (your Ancestors) to your parents to you. Within a TG you can have segments with Matches who are 1C, 2C, 3C, etc. *Usually* the farther back you go to a CA, the smaller the cM tends to be – but that is a generalization and not a rule – the DNA is random. The best reference is the Shared cM Project at DNA Painter. A good tactic is Walking The Ancestor Back – finding closer cousins on that TG segment, which then narrows down the possibilities for more distant Ancestors for that TG. Most of my TGs remain a “work in progress” and are subject to revision as new Matches come in.
      Q2: Bottom line is it’s a judgment call. One way to think about it is “layers” – each generation is a different layer. At the parent generation, you got 44 full Chromosomes (pretty large segments). At the grandparent generation, you got roughly 112 fairly large segments spread over the 44 chromosomes. etc. Your DNA has hundreds, even thousands of crossover points from your full array of Ancestors – most are deeply buried, and could only be detected by shared segments with 10C and more distant – usually too distant a cousin and too small a cM segment. So we work with what comes up. The key is to try to *see* clear break points in the data. My solution is to start with the largest clear segments, and be prepared to split them into 2 (sometimes 3) parts as the data indicates. Note that the large segment probably came through a closer Ancestors and when it splits, it’s ususally to two grandparents of the close ancestor (TG segment – on one Chr – cannot split going back to mother and father, who would be providing different Chrs)
      Hope this helps – and thanks for your kind feedback. Jim


  2. Perhaps assumed, but you didn’t mention TG requiring two or more matches from different pathways (ie. Not a child or grandchild). I am often unclear on what, if anything, to do with the single matches with no partner, even when they are 15cM’s and higher. Significant, but not Triangulating?


    • James – You are correct. This was a TIDBIT about the two different meanings of TG – not intended to be a complete rehash of previous posts about the full criteria (which also includes Not a sibling or aunt/uncle/neice/nephew) As noted in this post, a TG represents part of your DNA. You have two of each of the autosomes – so TGs along each of 45 chromsomes (46 for females). Segments over 15cM are generally presumed to be Identical By Descent (IBD), and as such they should fall into a TG on one chromosome or the other – one the Maternal one or the Paternal one. All true segments have to fit into the chromosome jigsaw puzzle somewhere. I have a few cases where a 20cM segment does not match any segment on my father’s side, and I haven’t found any other segment on my mother’s side. This happen because my mother’s mother was from an immigrant line with few Matches. So the conclusion is that these segments which don’t match on my father’s side, must be on my mother’s side – probably on my mother’s mother’s side – to be determined, someday. But there is no conflicting data in this case. If you have conflicting data, recheck your Triangulations – we don’t have 3 chromosomes for one location. Jim


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