Using a Group Common Ancestor

A Triangulation (and grouping) Concept

We have spent a lot of time and effort to describe *how* to group our Matches: segment Triangulation, DNA Painting, Shared Match Clustering. Each of these processes results in a group of Matches that should have a Common Ancestor (CA). This is an important concept.

But the main thing is to *use* this concept – to use the information found in these groups. If a group is formed around a CA, then all of the Matches in the group should share a CA. Once a CA is found, each Match in the group should also have that group CA, or be a closer cousin with an MRCA that descends from the group CA, or have a more distant MRCA which is ancestral to the group CA. In other words, all the Matches in a group should have the same distant CA.

So… if we find a CA for a group, the other Matches in the group should have the same CA line. This is a powerful focus – let’s *use* it. We should be able to look at other Matches in the group (who have Trees) and find that CA – either directly through a search, or indirectly by building out their Tree.

I illustrated this in Case 3 of Chapter 1 (Lessons Learned from Triangulating a Genome) of “Advanced Genetic Genealogy: Techniques and Case Studies” – here or here. This was all about one of my TGs which I call [04P36]. At Ancestry, I found a few cousins (who had uploaded to GEDmatch) in that TG who  shared my HIGGINBOTHAM ancestry. Armed with that hint, I searched for HIGGINBOTHAMs in other Matches (in that TG) who had trees. I also contacted Matches from FTDNA, 23andMe and MyHeritage – and several replied that they had the same HIGGINBOTHAM Ancestry. In the end I found 14 different Matches ranging from 4C to 8C on this HIGGINBOTHAM line in TG [04P36].

Because TG [04P36] came down a line of descent with the HIGGINBOTHAM surname in 5 generations, this case was an easier example – searching for one distinct surname. If a group represents a CA with a male-female zig-zag line of descent to me, it will be harder – the surname will change often. However, each line of descent (from a given Ancestor) is fixed – and we may find Match cousins with MRCAs of different surnames, but they will all be on the same ancestral line. This is akin to “Genealogy Triangulation” – getting an alignment of multiple cousins on one line.

Finding one Match with a CA in a group is not the end of the story – it’s a clue to the beginning of more research. If we find a CA for a group, but no other Match seems to have that CA, maybe we need to look for a different CA. The “correct” CA for each group should lead to Genealogy Triangulation – agreement by other Matches on the same ancestral line. If you find a CA in a group, *use* it to find more Matches on that same line. Seek CA agreement among Matches in each group.


[08D] Segment-ology: Using a Group Common Ancestor Concept by Jim Bartlett 20200620

5 thoughts on “Using a Group Common Ancestor

  1. Pingback: Friday's Family History Finds | Empty Branches on the Family Tree

  2. Pingback: Let the Matches Tell Us the Cluster Common Ancestor | segment-ology

  3. Ultimately, DNA matching and triangulation are only useful when a corresponding paper trail exists somewhere “out there.” Although a group may have a CA, if that CA lived before a time when records were being kept, there is really no way to establish who the CA may be. One is left only with the knowledge that the members of a TG are “somehow” related, but just how, will most probably be never known.

    This has been my major disappointment with genetic genealogy. My ancestors are Irish on my mother’s side and Ukrainian on my father’s side. The Irish records in the area that they came from go back to the 1840s at the earliest. My genetic matches indicate that our CA most probably was born in the mid to late 1700s. It is the same with my father’s side, as the birth records for his village start in 1801 and the death an marriage records start in 1840. However, for those with families whose documented lineage goes back into the 1600s or even earlier, genetic genealogy can be a real help in determining who the CA may be when working with TGs.

    Sadly, I don’t think that will happen for me unless I find a distant cousin with some form of well-documented family records that goes further back than the civil or church records of the time. My experience, so far, has been been the opposite, in that, I am the one with the most detailed records, having started on my family history back in 1970.


    • Terry, I’ll offer two thoughts: 1) Walk the Ancestor Back – this may require more tests for closer cousins. But once you’ve pinned down a 2xG grandparent, there are only two options for the next generation back in that TG or Cluster. 2) I think we are getting much closer to using genetic genealogy to create “virtual” Trees. I’m not sure how this will play out, but I’ve already found a couple of instances where I’m pretty sure of the “solution” without a paper trail. We’l see how it develops… Jim


      • I would be very interested to hear more about your approach when you deem it ready.

        I have documented back to a complete list of 2X great grandparents and estimates of birthdates from several of my 3X great grandparents from death dates. The situation is complex. I was born in 1943, and so these 2X and 3X ancestors are all born in the mid to late 1700s. My grandfather on my father’s side was born in 1865 and was 27 years older than my grandmother. They had 13 children the youngest of whom was born when my grandfather was 65. The spread in age creates some distance in looking at generational matches. My maternal grandfather was born in 1874. I think these connections give me a somewhat more distant look back in time with respect to DNA matching that would not normally be had, had my grandparents been born.

        I have many distant cousins who match at DNA cM values well above the 7 cM threshold. A complicating factor is that there is some level of endogamy present in my rural Irish and Ukrainian families, both of which came from small geographic areas and married within their faith in their small communities. This has tended to produce cM matching levels that are in excess of what one would expect for 4th to 6th cousins. For example, I have a fourth cousin with a documented relationship who matches me on Ancestry with 113 cM over 6 segments of DNA.

        I am ever hopeful that as the DNA technology and data analysis techniques develop further we will be able to make more positive identifications or at least better educated guesses as to who our CA are.

        Please keep us posted on your always interesting thinking on genetic genealogy. Your blog is much appreciated. Terry


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