A Segment-ology TIDBIT
mtDNA is passed from a female Ancestor down the all-female line to each of us. A Triangulated Group (TG) DNA segment is passed down from an Ancestor to us. The concept of DNA being passed down a specific ancestral line – from an Ancestor to us – is the same. Such is also the case for Y-DNA – it is passed from an Ancestor down the all-male line to a man. In the case of mtDNA, the ancestral path is all females; in the case of Y-DNA, the ancestral path is all males; but in the case of atDNA, the ancestral path can zig-zag between male and female Ancestors. Any of our Ancestors could pass an atDNA segment down to us.
The point is the TG segment is found only on one specific ancestral line (like the mt or Y line). However, it is still a genealogy task to figure out which line. As we “walk the segment back” from our own DNA back up our ancestry, there are only two options at each generation. If we know a TG segment is on our maternal side, the next generation back must be one of the maternal grandparents – and so on.
Just as we use mtDNA or Y-DNA, looking for a someone who shares that same DNA with us, to find our Common Ancestor; so, too, we understand that our atDNA Matches in a TG (thus sharing that same atDNA with us) will have a Common Ancestor with us.
This is just another way to think about our DNA segments – they are just as focused as the mt or Y on *one* ancestral line.
[22BX] Segment-ology: Triangulated Group Segments Are Like mtDNA TIDBIT by Jim Bartlett 20230728
segment-ology 
Hi Jim,Love the idea of zigzag paths. The Y and mtDNA are fine but so limited. Slowly working on my spreadsheet. I’ve been adding my personal segments with determined recombination points and which grandparent (if figured out) to the spreadsheet in hopes it will point the way for the groups to go. I have very few matches and they have a few trees. So I’m trying to get any hint I can. This way I know to look on a certain branch for a match. I color code them so they stand out in the spreadsheet.Debbie
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Thanks for your feedback. I use a code for each TG. Example: 01S24 is on Chr01; S is 19th letter, so the TG starts about 190 Mbp; and 24 means on my father’s [2} father’s [4] line (using ahnentafel numbers. In the end, each TG will also be “tagged” with a more distant MRCA – in this case 01S24 goes out to Ancestor 592P [a lot of research to get there] Jim
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Jim, another one of your great posts. I’ve successfully correlated TG’s to MRCA’s for almost 100 of my cousins. You’ve previously indicated that each of us inherits some 300-400 TG’s, each of which is “born” in a specific ancestor. Is it a useful exercise to identify these progenitor ancestors? Furthermore, is it possible to trace lineages even further back than these progenitor ancestors?
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Andy, Thanks for your kind, encouraging, feedback;>j Answers: maybe and yes! IMO, it’s useful to try and determine the progenitor Ancestors who originated the TG segments… 1. It’s fun; 2. It’s interesting; 3. It may push your genetic Tree back; 4. It adds evidence that your Tree is correct (or not); and 5. It helps identify recombination crossover points (if you are into Chromosome Mapping at each generation (which I think is a great Quality Control exercise – there’s an average of 36 crossovers on each side in each generation). Can we trace back further that the progenitors? Absolutely! Some of our Matches in a TG (who have less that the full TG segment) may well share a Common Ancestor more distant than the TG progenitor. It depends somewhat on your goals – I’m pushing it, generally, out to 7xG grandparents (some a little farther, most limited by Brick Walls/dearth of records. IMO, atDNA “works” out to 7xG grandparents – that’s the 8C level – Ancestry used to give us Circles out to 8C… Yes, this involves small segment – but when they clearly Cluster, it think we are on the right track. If I were a Consultant, I’d need to caveat it more; but I’m a hobby genealogist – and this is fun! And everyone gets to chose their own ovjectives… Jim
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Thank you, Jim, for this little gem! A light bulb came on in my brain, reading this. Of course! It seems so obvious, now that you say it. 😀
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Thanks for your feedback – that same lightbulb came on for me – so I blogged it. Each person has a different way of learning and using this stuff. TGs in a spreadsheet are the same data as DNA Pairnter uses – some like the visual, some like the spreadsheet…. Jim
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