A Segment-ology TIDBIT
23andMe has a great feature. It starts out as a standard In Common With (ICW) list for each Match you are sharing with (old Shared Genomes Matches and new Open Sharing Matches). This ICW list is near the bottom of your Match’s page. But the difference with other ICW lists is the Shared DNA column. The Matches marked with a “Yes” have overlapping segments – and over 99% of the time they form a Triangulated Group (TG).
So go to your DNA Relatives page and scroll to the bottom and click on the “Download aggregate data” link. You’ll get a spreadsheet of all your matches and most of the 23andMe data. Sort the spreadsheet, and delete the ones with no segment data. Then sort on Chromosome Number and Chromosome Start to put them in a particular order. Add a column called “TG ID”. Now you’re all set to begin Triangulating.
Start with the first Match in the spreadsheet (let’s call it A). Click on the hyperlink* that takes you to A’s page, scroll down to the ICW list and note in your spreadsheet Match A and each Match with a “Yes”. Since you are starting on Chr 01, call this TG: 01A, and put 01A in the TG ID column for A and each “Yes” Match. This pretty much identifies all the other 23andMe Matches that are in a TG with (A). The whole TG 01A (of 23andMe Matches) is created through one Match! There may be a few that don’t overlap A enough to form a shared segment at 23andMe, but all you have to do is go down your spreadsheet list of 23andMe Matches and select the next Match (B) that is not already in a TG; click on B’s hyperlink and look at their ICW list for “Yes” Matches with (B) – some will either overlap with (A) (call all of them 01A, too); or they all form a new TG (say 01B) – one or the other. Then continue with the next Match not already in a TG.
One could probably go through their entire 23andMe list of shared Matches in a few hours, creating TGs for all of them. There may be some with no ICW “Yes” Matches – give them their own TG; and move on. Be careful with Matches with more than one shared segment – make sure to treat each segment individually – this may take a little extra analysis.
Remember, TGs represent segments (from an ancestor) on one of your chromosomes. They are equivalent to phased data. I consider all shared segments at 23andMe which Triangulate to be IBD. All of them should be in a TG on one side (parent), or the other.
If you have known relatives in any of the TGs you can assign those TGs as Paternal or Maternal. This often allows other, overlapping TGs to also be assigned to a side, using logic.
*Note1: clicking on the 23andMe hyperlink (Link to Compare View) is a little tricky – I usually just copy (Cntr-C) the spreadsheet URL, and paste (Cntr-V) it into the URL bar of any open 23andMe page – hit Enter. It goes pretty fast.
Note2: feel free to use any TG ID numbering system you want. I think it’s wise to start with the Chromosome number. But you can name your TGs Bill, or Bob, or Sue if you want. You are creating groups that will tie to ancestral lines.
ARE YOU READING THIS FTDNA? ALL YOU NEED TO ADD IS A YES!! And AncestryDNA could add a similar feature, and hell might freeze over, too.
Enjoy easy Triangulation at 23andMe…
[22M] Segment-ology: Triangulation at 23andMe TIDBIT; by Jim Bartlett 20170720