Who Ya Gonna Call? (hint: NOT Ghostbusters!)


A Segment-ology TIDBIT

The Visual Phasing process looks at full chromosomes of three siblings to determine grandparent crossover points. The Leeds Method uses Matches over 90cM to group by grandparents. Great grandparents rely on 2nd cousins (2C) Matches which average 229cM. Even out to 4xG grandparents, we rely on 5Cs at an average of 25cM. What if your genealogy question or interest is more distant?  

I recently broke through a 48 year brick wall. My known ancestor was Wilson BROWN c1751-1793 who died without a Will or any other document listing his wife or children. I’ve always known his name, because the marriage license of Keziah BROWN to Elliott BAKER in 1801 listed her father as Wilson BROWN, decd – but little else, except a probable brother Isham BROWN. Finally, the 1776 Will of James BROWN came to light – it listed 16 children including Wilson and Isham. With literally no Trees with Wilson BROWN, we have to find Match cousins from James BROWN – who would be my 6Cs.

Who Ya Gonna Call?


The average for a 6C relationship is 18cM – and over 70% of the segments are under 20cM. We have to find and use and group these under-20cM Matches in order to build a case for a 6C relationship. Boy, did I pick the wrong surname to test this out – BROWN. So I’m searching my maternal Matches with a BROWN surname below 20cM – there are many.  I’m now down to 11cM, and the “hits” in VA, NC, SC, TN, KY area are showing up. Some appear to be single “hits” in otherwise large BROWN families (not helpful);  but some are starting to group on particular lines (promising). I think by the time I get down to 8cM Matches, I will have a number of strong candidate BROWN families, with a number of potential cousins on each line. I’m letting these small segment Matches tell me lines I’m related to.

Now, I recognize that some of these small segments may be false. At the 7cM level, we expect about half to be false. But the flip side is  half will be true (Identical By Descent). When I see what appears to be a single line of descent from a BROWN ancestor in the 1700s, I can well accept that it may be a false segment. On the other hand, if a number of Matches all descend from several children of the same BROWN patriarch, I’m more inclined to think that consensus indicates true, matching, segments. Even if we insist that half of these shared segments are false, we still have a lot of them which are true and all pointing to the same family.

In my case I’m sure my BROWN line is BROWN Y-DNA Group 40 – so a link to known Group 40 lines is another reinforcing piece of evidence. Also, from my Walk The Clusters Back process, I’ve identified almost all of my greater-than-20cM Matches to a Cluster and many of those to a Triangulated Group (TG) segment. Many of the under-20cM Matches have over-20cM Shared Matches (SMs). Sometimes there is a clear SM consensus (to a TG), and sometimes the SMs don’t have a clear consensus. When there is a clear SM consensus on a suspected “BROWN” TG, more often than not, I can build a Match’s BROWN ancestor back to the patriarch of a consensus group. This further reinforces these family groups.

BOTTOM LINE – If you are looking for cousins at the 6C or 7C or 8C level, you have to rely on Small Segments! And, IMO, when you factor in that they form a solid consensus group in one family, a high percentage of them will be true segments.

[22BP] Segment-ology: Who Ya Gonna Call? TIDBIT by Jim Bartlett 20230428

The Two Meanings of TG


A Segment-ology TIDBIT

A Triangulated Group (TG) has two meanings – segment and group.

SegmentA TG represents a segment of DNA. A TG is defined as a Start Position and an End Position on one Chromosome (on one side – Paternal or Maternal). It describes one part of your DNA – accurate between two cross-over/recombination points*. Thus, a TG is a long string of SNPs on one chromosome – it is phased data on one side or the other. If we were to compare the raw data from each of the Matches in a TG, we’d see that they all had the same value at each point.

GroupA TG is also a group of your Matches. Each Match shares a phased DNA segment with you** that came from one of your Ancestors***.  All of the Matches should share the same Common Ancestor (CA) with you.***

Hedging a little…

* There is no “sign post” in our DNA to indicate the cross-over points, and the matching algorithms cannot exactly determine the start/end points of a segment – they may start before, and/or end after, the real ancestral segment. But they are pretty close. I say the segment ends are “fuzzy” – but the bulk of the TG segment is definitely from one Ancestor.

** It is possible that some Match(es) may have a false segment that happens, by chance, to match the phased string of SNPs – virtually always a segment under 15cM. If this Match/segment is critical to you, the Match can Triangulate at that location to determine if the segment is true or false.

*** The segments that pretty much cover the TG segment should be from the CA. Smaller segments that appear to be from one end or the other of the full TG segment, may well be from Ancestors beyond the CA. Note this is often the best way to determine which parent (in a couple) the DNA segment came from – the more distant Ancestor would be ancestral to one or the other of the couple.

Usually the TG meaning can be gleaned from the context – is the author talking about segments or Matches. If it’s not clear – call the author (often me) out!

[22BO] Segment-ology: The Two Meanings of TG TIDBIT by Jim Bartlett 20230407