A Segment-ology TIDBIT
I love it. My prime objective with atDNA has been to map my genome to the most distant Most Recent Common Ancestors (MRCAs) that I can. The two essential ingredients for a Chromosome Map are Segments and Common Ancestors. So my basic game plan is to collect and Triangulate as many shared segments over 7cM as I can, and find as many MRCAs as I can. I have basically completed Triangulating the shared segments with all of my Matches (culling out many Identical By Chance (IBC) or false segments along the way), and now have 360 Triangulated Groups (TGs) covering 97% of my 45 chromosomes.
It’s now a full-court-press to find MRCAs with the Matches in these TGs. Of course, not all MRCAs will be correct, but the more I can find in each TG, the more data I have to develop and test possibilities.
Two ways to find MRCAs with segment data:
1. Start with MRCAs and get the Matches to test/upload to GEDmatch to determine shared segments [see Shared Ancestry Hints below], and
2. Start with Match segments and review their Trees (including getting them to share private Trees) [alas, so many Matches have no Tree.]
One process that has worked pretty well for me, focuses on AncestryDNA Hints. I have 830 Shared Ancestor Hints (SAHs), and I’ve sent a message to every one of them. It’s a standard message saying I agree with the Hint, but note that we might have other Common Ancestors, too. For that reason, and because I’m mapping my DNA segments to specific ancestral lines, I’d like for them to upload to GEDmatch so we can see the shared segment. It’s easy, and I will do the DNA analysis and give them a report back. About 5-10% of these SAH Matches upload.
Today, in response to my request 7 months ago, I got a message with a GEDmatch kit# for a SAH who is a 5th cousins (5C). At GEDmatch I found our shared segment, typed the info into my Master spreadsheet, and Triangulated with other overlapping segments. The new segment was in one of my few remaining TGs with no known MRCA. So, from the Hint, I now had an MRCA! And it “fit” at the grandparent level with adjacent TGs. I then checked our Shared Matches – there were only 3 – one Private, one No Tree, and one with 57 people. Well, the 57 people Tree had just the barest of a clue – a maiden name without dates or locations. But I knew where to look, and quickly determined it was the same line as my new GEDmatch kit. Wow! Identify an MRCA in a TG, and get another cousin with the same MRCA line in that TG at the same time.
I have over 600 Matches with MRCAs that “fit” at the grandparent level. And it’s becoming easier every day to find and rule potential MRCAs in or out of a TG.
Communicating with Matches to find MRCAs is the key. Sometimes it literally takes years… You’ll only get a small percentage of responses, but the more emails and messages you send out, the more you’ll get back.
[edited to identify abbreviations]
[22R] Segment-ology: When a Plan Comes Together TIDBIT; by Jim Bartlett 20180316