A Segment-ology TIDBIT
My first post with this title (here) listed 4 ways to get Segment data for AncestryDNA Matches; and then added another way using GEDmatch.
Here is yet another way.
In Walking The Clusters Back (see WTCB 2022 and WTCB SITREP) I’ve now completed the analysis of all Matches over 20cM – almost all fit into one of several hundred Clusters. I’m now integrating the below 20cM Matches who have segment data (a TG ID), usually from GEDmatch – over 800 of them. Most of them have Shared Matches which usually provides a consensus on the Cluster. In checking my Master segment spreadsheet (with all of my Match Shared Segments), I noticed a number of AncestryDNA kits which didn’t yet have a link to an Ancestry profile. It turns out that usually all the Matches with the same TG ID will be in one Cluster. It is a relatively easy task to find that Cluster (particularly with WTCB) with some of the Matches – and then review the few other TG Matches with the other Matches in that Cluster. I usually send a message to the Ancestry Match to confirm they indeed uploaded to GEDmatch (and promise to help them with the DNA if they are).
Result: more AncestryDNA Matches linked to specific DNA segments (TGs).
[22BJ] Segment-ology: Segment Data for Ancestry Matches 2 TIDBIT by Jim Bartlett 20220223
**darn autocorrect, Extensive, not expensive (although the second may also be true).
Jim, thanks for continuing posts with tidbits and tips. I have also found this investigation of Ancestry segment details in Gedmatch kits to be quite fruitful. I was wondering if you know of any good statistics that correlate the segment size with the distance back to MRCA’s? I have heard the 20/20/20 guideline (20% of 20cM size matches extend back 20 generations to the common Ancestor). This seems overly pessimistic to me, I guess I’m just wondering if something like 50% of 20cM segments have a common ancestor within 7 generations might be true. Data from your own very expensive mapping?
James, The DNA is pretty random. As noted by the Shared cM Project, and DNA Painter, there is a lot of overlap of relationships with a given cM shared DNA segment. They show a wide range of possibilities and probabilities for a 20cM shared segment. Someday I will try to do a correlation between Cluster size (and a the probable Ancestor) with the family size. Another insight I think about is the very personal nature of the results – how I got more Matches on some lines than my brother, and vice versa… Right now my focus is on WTCB to see how deep an Ancestor I can find for each TG. Jim