Small Segments Needed for Distant Ancestors

Segment-ology TIDBIT

To find bio-Parents we usually use Matches in the 90-300cM range. Grouping Shared Matches in this range usually gives us four Clusters – one for each grandparent. For a bio-Grandparent, we need to lower the threshold some to include third cousins (3C), and select the Clusters that would include the bio-Ancestor we want to find (in other words select-out the known Clusters). See my 2022 blog post: Finding Bio-Ancestors here.  That blog post includes a handy Crib Sheet to orient this kind of project. The Crib Sheet indicates the estimated number of Triangulate Groups (TGs) involved, but Shared Match Clusters also work. Since this is largely a genealogy project, using Shared Match groups (Clusters) at AncestryDNA is usually the best place to work on these projects.

For more distant bio-Ancestors, smaller cM Matches are needed. Try various cM thresholds, down to a 20cM threshold, and select the Clusters that point to your Target bio-Ancestor.

For even more distant bio-Ancestors, I subscribe to the concept of ignoring the cMs, and just focus on the genealogy. AncestryDNA only shows Shared Matches down to 20cM, but Clustering can be done at the other companies, down to small cMs. Grouping by segment Triangulation can also be done, and then selecting the TGs that point to the bio-Ancestor.

If you get a hint of a surname, or a specific geography, you can search your Matches at AncestryDNA, to find under-20cM Matches that may have Shared Matches – indicating the Cluster they would be in.

For 4x to 7xGreat grandparents, many of the Match cMs will be under 20cM. Remember this is a genealogy project – the cMs don’t matter.

In all projects looking for (or even just confirming) a bio-Ancestor – let our Matches identify you Ancestor, by determining their Common Ancestor – see the link in the first paragraph.

The main point of this blog post is that Matches with small segments are needed to work on distant Ancestors.

[22BM] Segment-ology: Small Segments Needed for Distant Ancestors TIDBIT by Jim Bartlett 20230306

9 thoughts on “Small Segments Needed for Distant Ancestors

  1. How I long for the day that will allow users that have tested their atDNA with them to download the segment data for their DNA matches. I know it is wishful thinking at this point…


      • Indeed! I have contacted so many matches about transferring their raw DNA data, that I feel like an unpaid spokesperson for the other major commercial genetic testing companies. As you say, a few all-nighters would achieve much with that treasured Ancestry match data. Ancestry’s SideView mode is such a tease!


  2. Debate is underway at and after the just completed Roots Tech presentations. Two significant genetic genealogy experts are suggesting chromosome/ segment data and work is really unnecessary. Comments include things like, autosomal dna is not useful for genealogy research beyond 2X grandparents and for 99% of research, chromosome and segment work is merely icing on the cake; shared matches are all that is needed. I feel these comments are very misguided and chromosome/ segment work is instrumental in a great deal of genealogy research.

    Liked by 1 person

    • Just to clarify, because based on conversation with you in GGTT I’m one of the two experts, I’ve never said “chromosome/segment data and work is really unnecessary,” or that “autosomal DNA is not useful for genealogy research beyond 2X grandparents.” Please don’t ascribe those sentiments to me. What I did say is that for 99% of people, “segment data (i.e., a chromosome browser) is icing on the cake of shared matching.” It’s important to remember that those of us working deeply with segment data are a TINY percentage of overall genetic genealogists, and many genealogists are unable to work with segment data. However, I’ve blogged about, given presentations about, and written books about segment data and triangulation and chromosome mapping, educating 1000s of people about how to do it. I think they’re wonderful tools, and I use them regularly. One significant problem is that many genetic genealogists have said for years that segment data is “better” than other methodologies, that (for example) shared matching is misleading and inferior because it doesn’t include triangulation. Of course, for many, many reasons we know that isn’t true; both methodologies are just fine depending on the analysis involved. But this is a debate that has been going on for many years now, and I don’t see it ending anytime soon unfortunately. I noted in GGTT, one of the wonderful things about genetic genealogy is that we are all free to use whatever tool(s) we prefer!


      • Blaine, Thanks for your feedback – and for your leadership in the GG community. I think we are in total harmony on amost all things factual, as well as our desire to push the envelope. I am indeed in the TINY group you noted. I mentioned no names, but reflected feedback I’ve gotten in my Advanced DNA SIG, and other conversations, usually asking if, IMO, these statements are true. They are certainly good conversation starters, and we do need to listen and reflect on them. I posted two blogs to express my views. Thanks for highlighting them in GGTT. Jim PS: sorry this is late, for some reason it was in the WordPress Spam folder – I fished it out.


    • Leading genetic genealogists are promoting an approach that would never be tolerated with paper trail evidence. It’s an unscientific standard that boils down to this: “If AncestryDNA doesn’t have it, you don’t need it.” It’s an embarrassing shame.


      • I think this is a business model that has grown with the large number of folks who have tested at AncestryDNA. Most of these leading genetic genealogists do recommend testing at multiple companies, but the vast percentage of testers are content with a single test at Ancestry (and many of those are only interested in the ethnicity estimates, if that). Working with DNA segments is a relatively advanced process, and only a small percentage of folks will include segments in their analysis. As an analogy, most folks have a “handyman” tool box with a hammer, two screw drivers and pliers….
        I am thankful for the folks who follow Segment-ology, and help push the envelope on the DNA side of genetic genealogy. Jim


    • James, I agree with your post. They are addressing the 30 million, or so, folks who have taken an autosomal DNA test, rather that the less than .01% of them who subscribe to this blog. Their point – for the masses – is that they can find answers to relatively close relationships with fairly easy techniques. Nevertheless, Diahan’s Shared Match process is not simple – it’s not point-and-click – but it works. I use it, I blogged about it last year. It is a powerful technique, and probably the best one for the millions of AncestryDNA-only users. Those folks are playing to their audience, and most of them are in the business of DNA education… I just wish they were not so strident – and more readily acknowledge that segments are also a good tool, and that atDNA can be used back 9 generations, or more. Segments and Chromosome Mapping is critical, IMO, for ferreting out the distant NPEs. that are in our Trees. Jim


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