We Match Segments, Not Matches

Posted on by

A Segment-ology TIDBIT

This is an important point: The DNA Matching algorithms are based on finding a Shared DNA Segment. My DNA Segment matches your DNA Segment. We say: I match you; but, technically what we mean is my DNA matches your DNA – our DNA segments overlap and match – they match enough to satisfy the algorithm.

Above 15cM, the matching algorithms are designed to determine an Identical By Descent (IBD) segment. My DNA is identical to your DNA (all the SNPs are the same) over enough DNA that it can only be that way because we both got that segment of DNA from the same Ancestor. This is the foundational concept of autosomal DNA testing.

If a person only shares one IBD segment with us, we call that person a Match. We match the Match. We are basically equating the Match to the Shared DNA Segment. And most of our “Matches” share only one IBD segment with us. But not all…

Some of our “Matches” share two or more DNA segments with us. In these cases, we need to be careful how we speak. Each of these Shared DNA Segments is an independent event. Most of the time, multiple Shared DNA Segments will be from the same Common Ancestor, but that’s not a requirement of the biology. From my spreadsheet of over 20,000 shared segments, I can attest that there are many instances of multiple segments from one Match coming from different Ancestors (as well as many instances of Matches who are genealogically related to me in multiple ways).

I raise this point because I found myself assigning my 3C Match with 6 Shared DNA Segments to our Common 2xG grandparents. That is, in my spreadsheet, I assigned all 6 segments to the same 2xG grandparents. Analyzing each of these resulting Triangulated Groups, I found one that was “off”, “strange”, “out of kilter”… The other Matches in that TG were related to me on a different line. A little investigation into my 3C’s Ancestry revealed we were also 7C on a different line.  

I also raise this point to illustrate the importance of segments in genetic genealogy.

The point here is that we don’t really “match” a Match, we only match a part of their DNA.

Notwithstanding… like most of us, I will continue to say that “I have a Match” and that “I match person A”. It’s just important to remember that part of our DNA matches part of our Match’s DNA.

[22BF] [ Segment-ology: We Match Segments, Not Matches TIDBIT by Jim Bartlett 20230813

10 thoughts on “We Match Segments, Not Matches”

  1. Jim, I actually have 3 cousins, descended from 4 daughters of one set of my 4G Grandparents, who are 4th and 5th cousins to each other, and all of whom match on a single segment… of X DNA. (One is descended from 2 daughters, through a 1st cousin marriage.)

    Like

    Reply ↓

  2. Exactly so Jim. Gets back to the importance of ‘segment’ triangulation. All too often those ‘extra’ smaller segments are either false positives or from a different more distant ancestor. It can even happen with close cousins. Muddies the waters, particularly with clusters where you don’t have the segment data!

    Liked by 1 person

    Reply ↓

  3. I wish there was a way to know clearly and conveniently the “qualitative” differences to bear in mind.. as well as the why. (I have to revisit some of these situations, on and off, over the years, and I keep forgetting what I know.) For example, I understand that these is an apparent tendency of the X chromosome as well as the segment near the (“left”) end of ch. 15 to “resist” break down from one generation to the next. — This blog’s topic addresses one of these things to bear in mind. — I have German ancestry primarily, almost exclusively really. So I am slightly perplexed when I see a match in Sweden or Poland in my top ten matches at MyHeritage. But yes… it comes down to an analysis of segments. First the total breaks down to several segments, 5 or 6 usually, and, second, all these segments are small (>15 cM, so perhaps false?). – It does help if your match is only a half match and/or most of the other, arguably irrelevant ethnicities are quite contrasting. For example, your promising match is half Vietnamese and you are not all related to any other Vietnamese person. Sadly, I am now wondering about the 5 segments I share with a known 4th cousin (longest segment is 17 cM). Our common branch is from East Frisia. Those folk did have a tendency to marry others from a nearby community, who could be as close as a second cousin… before the early 20th century.

    Like

    Reply ↓

Leave a comment

This site uses Akismet to reduce spam. Learn how your comment data is processed.