A Triangulation Concept
I often get questions along the lines of: “do my Matches have the same TGs?” or “can I form TGs for my 11 kits in one spreadsheet!” The answers are an emphatic: “NO!” and “NO!” Most of us are pretty ego-centric with our DNA analysis – and this is good! And while we are the center of our own universe of DNA segments, each of our Matches is, likewise, the center of their universe of DNA segments. Each of us gets random segments of DNA from our ancestors – random size segment(s) and random placement somewhere on our chromosomes. Once we get past the 2C (2nd cousin) or 3C level, it is quite amazing that we share DNA with more distant cousins at all. But we have many, many cousins, and many of them beat the odds and we share a DNA segment. However, this does NOT mean that we and a Match both got the exact same DNA segment from a Common Ancestor (CA) – that very rarely happens. We get a segment, and our Match gets a segment, and what we “see” in a Chromosome Browser is the portion of our individual segments from the CA that overlaps – the shared part of our segments. When we form a TG with various Match-segments (most matching each other), there are usually fairly well defined start and end locations to the TG*. Of the shared segments in a TG: some will start at the start of the TG, and end before the end of the TG; some will end at the end of the TG; some will “float” within the TG; and some, particularly with closer cousins, will be larger than the TG. These are all normal and expected [see an example in Figure 6 here]. The point of this concept is that each of the Matches in the TG will have their own unique TG – representing the full segment they got from the CA. If your shared segment with a Match, aligns with the start location of a TG, there is a very good chance that the Match’s segment from the CA began before yours did, and the Match’s TG has an earlier start location. Try this experiment: Take two (or more) Matches at GEDmatch with shared segments that start at the start of one of your TGs, and compare them to each other. Often their overlapping segment will start before your TG. In fact, using the Shared Segment search utility at GEDmatch, you can probably find other Matches that match in a Match’s TG, that don’t appear in your TG (and those Matches have segments that don’t overlap enough with your DNA to form a shared segment). The bottom lines for this concept are that Triangulation should be done on one “base” person (usually you) at a time; and there is more to the DNA passed down from an Ancestor than what is shown by any one descendant’s TG. Each of our TGs are pretty unique, and our Matches will not have the same TGs or chromosome map.
[*Sometimes the TG start and end locations are a little fuzzy (see here), but our focus should be on the bulk of the TG.]
08A Segment-ology: Your TGs are pretty unique! Concept by Jim Bartlett 20170907