The Fundamental Building Blocks of Genetic Genealogy

A Segment-ology TIDBIT

In genetic genealogy, there are two fundamental building blocks: Ancestors and DNA Segments.

As genetic genealogists, virtually everything we do revolves around these two key elements. The Ancestors are really Common Ancestors (CAs) with a Match; and the DNA Segments can be grouped into Triangulated Groups (TGs). See How To Triangulate here. Each of your TGs is really a DNA segment (on one of your Chromosomes) that came from an Ancestor.

In Segmentology, the Two Fundamental Building Blocks are:

  1. Common Ancestors (CAs) – see my Shorthand ID for a CA here.
  2. Triangulated Groups (TGs) – see my Shorthand ID for a TG here.

These two fundamental building blocks, and their shorthand IDs, are valuable tools. Here are some examples:

Reasonable. Suppose I have a cousin/Match: 36P/4C on 01S24, with a 38.7cM shared segment. This looks reasonable. CA 36P has an ancestral line down to me as: 36-18-9-4-2-1, so it agrees with the 2-4 in 01S24 and both are on the P-side. And 38.7cM is in the range for a 4C.

Unreasonable. Suppose I have a Match on TG 01S24, with a 38.7cM shared segment, and then find a Common Ancestor 856M – I quickly know there are issues. 856M is a Maternal Ancestor and 01S24 is a Paternal TG. Also if the Match shares 38.7cM, the CA is not likely to be as far out as CA 856 [8th cousin range].

Impossible. Similarly, suppose I have determined a 256P CA with a Match, [paternal side]. The Match subsequently uploads to GEDmatch, and I find that our shared DNA segment is in TG 08B36 [maternal side]. We may still be a genealogy cousin on our CA 256P, but we have another CA on my maternal side who is linked to 08B36. Side note: this actually happened to me when I started with autosomal DNA. I worked hard to find CAs with 100 Matches before I really understood how to use the DNA. Later I determined that 25 of these CAs were impossible for the DNA segments which the Matches and I shared. 25% of the CAs were not linked to the DNA. Every time I find a CA without segment information, I think about this 25% error rate…

Very helpful. These two fundamental building blocks, and their shorthand IDs, are very valuable in analyzing various CAs we may find in a TG; or in reviewing a list of InCommonWith or Shared Matches, or Match Clusters. It takes some work to type them into the Notes boxes (at AncestryDNA, MyHeritage, FTDNA), but they sure are handy and helpful with analysis of groups. I’ll blog more about how to use these building blocks.


  1. Finding CAs is genealogy work! We have to do this work – by reviewing a Match’s posted information or by contacting them (and sometimes by building their Trees).
  2. Forming TGs is a mechanical process – also work! I recommend trying to get as many shared DNA segments as you can into the appropriate TGs. Grouping your segments into TGs will save you time in the long run. See The Benefits of Triangulation here.
  3. Your TGs and CAs have certain specific links. Each TG will be linked to a specific ancestral line – often including several CAs at different generations with different cousin/Matches (aka Walking the Ancestor Back). Each CA will be linked to only certain TGs. Distant CAs may have only one TG; Intermediate CAs may have a few TGs and Close CAs will be linked to several TGs. See Figure 3 in this blogpost for an idea of how many segments (TGs) ancestors at different generations are likely to have. The point is that each of your Ancestors will link to only certain TGs, or none.


[22Y] Segment-ology: The Fundamental Building Blocks of Genetic Genealogy by Jim Bartlett 20190203

4 thoughts on “The Fundamental Building Blocks of Genetic Genealogy

    • Thanks, I have many Triangulated Groups with Matches who have Common Ancestors with me on different lines – but, our DNA comes to us from only one ancestral line. Only one line can be correct. Usually we’ll see a consensus in a TG. Jim


  1. Ancestry’s new search feature as of 2018, is the greatest search tool they have, for me personally. This is the ability to SEARCH in your MATCH’S TREE. I can take the tree of a match with only one person who has a name, but no dates or locations. Sometimes I can search that upstream to find a tree CA. This is Ancestry’s best kept secret! And, of course, this is a tree match, and not a TG.


    • caith,
      You are correct that finding a CA is one one part. We still need the TG, with other Matches in the TG having CAs on the same line. We can come close by looking at our Notes in Shared Matches and/or a Cluster Matrix. Jim


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