Segment-ology TIDBIT

Let’s analyze a generic Triangulated Group (TG).  There are several facets to this analysis…

Facets related to me:

1. My DNA segment – A Triangulated Group (TG) “segment” is a specific segment of my DNA. It is defined by a Chromosome number, start and end positions, and the total Mbp. The number of SNPs included and the cMs can be obtained through look-up tables on the internet (I have not done that).

2. TG Ancestor – A TG segment of my DNA first came from a specific Ancestor of mine. I’ll call this the TG Ancestor. This TG segment was passed down through descendants of the TG Ancestor, to one of my parents and then to me. All of my Ancestors who descended from the TG Ancestor also had that TG segment in their DNA. NB: the TG Ancestor started with a full Chromosome (a big segment) which he/she passed down – the TG segment was part of that larger segment/Chromosome. This original larger segment was then whittled down through the generations, but each generation had, at least, the full TG segment. NB: A segment may be passed down, one or more generations, intact (i.e. no whittling down), but the TG segment is always intact from the TG Ancestor down to you.

3. TG segment origin – The TG Ancestor received the TG segment (usually a larger segment) through a recombination process. His/her parent recombined DNA from their two parents to create a new chromosome to pass to the TG Ancestor. At this point, in our TG Ancestor, our TG segment is made up of parts from the TG Ancestor’s parent’s two chromosomes – one from each of the TG Ancestor’s grandparents. Thus, this whole TG segment did not exist, on one chromosome, in one person, before this time. The TG Ancestor is the first person to have this particular TG segment.

4. Logic – Matches who share this full TG segment, should also share this Common TG Ancestor – because this TG segment is unique to this TG Ancestor. [It can be argued through logic, that there is a possibility of this exact same segment being created in another person – granted. But the odds are extremely low, and even more distant when you consider this happening in the small subset of your DNA Matches in a TG]

Facets related to Matches and *shared* segments:

5. Cousin segments – In general, our cousins will get somewhat different segments than we do from our Common Ancestors. Apply #2 above to a Match. Our Ancestor passed down a chromosome to their children – some of it identical, some different. The DNA segments passed down through their children and their descendants to our Matches will be randomly different. What we see through a DNA test, is the overlap created by shared DNA segments – the part of our DNA from a Common Ancestor that overlaps. We might get Chr 06: 53-86Mbp and the Match may get Chr 06: 64-97Mpb – the “shared segment” is the overlap: Chr 06: 64-86Mbp.  Our segments from Ancestors are rarely the same as our Matches’ segments from the same Ancestors, but

5. Cousin segments – In general, our cousins will get somewhat different segments than we do from our Common Ancestors. Apply #2 above to a Match. Our Ancestor passed down a chromosome to their children – some of it identical, some different. The DNA segments passed down through their children and their descendants to our Matches will be randomly different. What we see through a DNA test, is the overlap created by shared DNA segments – the part of our DNA from a Common Ancestor that overlaps. We might get Chr 06: 53-86Mbp and the Match may get Chr 06: 64-97Mpb – the “shared segment” is the overlap: Chr 06: 64-86Mbp.  Our segments from Ancestors are rarely the same as our Matches’ segments from the same Ancestors, but the TG segment is always intact from the TG Ancestor down to your Match. Always be mindful of the difference between your own DNA segments, and a “shared segment” with a Match.

6. Cousins on the TG Ancestor – these Matches may share roughly the same amount of DNA as the full TG segment, but some may well share smaller segments. It all depends on the recombinations that occurred in the generations between the TG Ancestor and the Match. Matches in a TG are already analyzed to share at least part of the TG segment with you and with other Matches.

7. Closer Cousins – these Matches also may, or may not share the full segment. Actually close cousins may share somewhat larger segments with us – beyond the scope of the TG segment. This indicates these closer cousins share more than one TG with us and a closer Common Ancestor. However, this closer Common Ancestor would have to be a descendant of the TG Ancestor. Maybe the closer Common Ancestor would be a grandparent or Great grandparent.  

8. Distant Cousins – Other Matches in the TG group may share smaller segments and be related through a more distant Ancestor of the TG Ancestor. Refer to #3 above. We could have a Match cousin related through a parent or grandparent, etc. of the TG Ancestor. In this case the Match would have only received the smaller DNA segment that was part of the full TG segment in the TG Ancestor. It is probable that Matches sharing small segments (in my case down to my 7cM threshold), could be cousins way beyond my genealogy horizon. This is particularly true with pile-up regions within a TG. The whole TG segment may come from a TG Ancestor well within my genealogy horizon, but the pile-up Matches are much more distant (or potentially false segments – a different story).

Summary – the Matches in a TG group can be cousins from many different generations, but all on the same ancestral line. The best estimated guess of relatedness to a TG segment would be a look-up of the cMs and then refer to the Shared cM Project. Generally on Chr X, the relationships may be further back.

Remember a TG segment represents your DNA – only your DNA – your DNA Matches will have a different TG.

[22BL] Segment-ology: Triangulated Group Analysis TIDBIT by Jim Bartlett 20230110