Percent of Shared Cousins Indicates Relationship


Subtitle: Teamwork in Practice

In my last blogpost about my ancestor Wilson BROWN, I hinted at a large group of Shared Matches to a Thomas BROWN b 1773 (m Nancy LITTON). Over 2,000 people have this Thomas in their Trees at Ancestry. After some collaboration, I was given access to the DNA kit for the person named MATCH in the diagram below [credit to Allen Brown]. I wanted to look at the ThruLines Matches for MATCH. Well… it turns out MATCH has 756 ThruLines Matches from Thomas BROWN b 1773, spread over 7 different children – just WOW! I looked at 276 of them, spread over 5 different children (not including MATCH’s direct ancestor). I clicked on each Match name to see if I was also a match to that person. Drumroll…. 28 of them were also DNA Matches to ME. So, using this information, how is Thomas 1773 related to ME?

Diagram of descent from James 1705 to ME and descent from Thomas 1773 to MATCH:

Remember the rough guidelines that true 2nd cousins (2C) will match 100% of the time; true 3C will match 90% of the time; true 4C 50%; true 5C 10%; and true 6C 2%? My little exercise resulted in ME matching 28 of 278 given cousins identified by ThruLines for MATCH. This is right in line with our expectation for a 5C!  In the diagram above, Thomas would most likely be the son of Wilson, rather than a nephew (which would result in a 6C relationship between ME and MATCH). Note: we had already determined that the James BROWN 1705 line and the Thomas BROWN 1773 lines were both in Group 40 of the BROWN Y-DNA Project.

Maybe this is a fluke. I’d like to find another Match descendant of Thomas BROWN and see if I have the same ballpark results. Also, I’m still reviewing all of my AncestryDNA Matches with a BROWN Ancestor to see if there is another firm group (or Cluster) of BROWNs, so I can see if they also might descend from Wilson BROWN. Spoiler alert: I do have a very large (100 Match) Cluster that I have linked to my Triangulated Group [06F36] – so I’ve used [06F36] to tag my AncestryDNA Matches in that Cluster. As it turns out, virtually all of the Matches I have under Thomas 1773 are tagged [06F36] – another indication of the power of Clusters. I can now really dig into the other [06F36] Matches (tagged at AncestryDNA and in the [06F36] Triangulated Group with 284 Matches from 23andMe, FTDNA, and MyHeritage) to find their BROWN ancestry.

James BROWN c1705-1776 [see diagram above] left a Will naming 16 children. Other than the given names, there are very few records to tie the surviving children back to James [we are dealing mostly with burned out counties in Colonial Virginia]. Very few on-line Trees are tied back to James. However, we have found families with the same given names as the children.  Isham BROWN is an example – same given name, but no records to link him to James, just a first name. But there are 2 descendants of Isham who are in Group 40 of the BROWN Y-DNA Project who claim Isham as their Most Distant Known Ancestor. Eake BROWN is a fairly unusual given name, and we are finding some records and descendants for him – looking for a living DNA Match… In his Will James named son George and George’s sons George and Archibald – two men in BROWN Group 40 claim a George and an Archibald (independently) as their Most Distant BROWN Ancestors… Theoretically we should be sharing about 2% of our cousins at the 6C level.  Yes, it’s a stretch, but it’s doable.  With virtually no good records, it might be the best avenue we have for linking these lines.

If enough folks try this process, we might get enough data to build probability curves and averages for the percent of shared cousins at different cousinship levels – a parallel to the Shared cM Project.


1. If you and a DNA Match can share your lists of Matches from a potential Common Ancestor, percent of Match overlap may indicate the cousinship level.

2. This takes work and time – I used it as a last resort, when my Ancestor left no records of children.

3. This is best done at AncestryDNA, with ThruLines, and therefor limited to 6C relationships, or closer.

CODICIL: In my excitement here I have presumed [06F36] is a BROWN Cluster or Segment. Not necessarily! I have concluded that [06F36] goes back to the Wilson BROWN couple – that [06F36] segment could have come from or through either Wilson OR his wife. It’s a 50/50 probability either way. I must do a lot of other analysis to figure that out.

[23_98Mb] Segment-ology: Percent of Shared Cousins Indicates Relationship by Jim Bartlett 20230315

Testing a Guess with Teamwork!


This is a developing story about a Brick Wall I’ve had for 48 years. My mother was a BAKER, and I know her ancestry back to the “Gunsmith” BAKERs in Pennsylvania in the late 1600s. My mother’s brother did Y-DNA to prove this line. One ancestor in the line was Elliott BAKER c1775-1836 who married Keziah BROWN in 1801 Prince Edward Co, VA. Keziah named her father as Wilson BROWN, Dec’d on the marriage license. In the 1850 census she stated she was born in adjacent Buckingham Co, VA. Sure enough in the 1782 to 1792 Buckingham Personal Property Tax Lists, there was a Wilson BROWN. In the 1793 PPTL, Wilson BROWN Estate was listed – Wilson had died. Skipping  over a lot of research and steps, I know: no one else has Wilson BROWN in their Tree(except my line); there are several different BROWN Trees in Buckingham Co, VA; adjacent to Wilson in most PPTL listings was an Isham BROWN (some DNA Matches, indicating he was probably a sibling, but no genealogy help); Wilson BROWN left no Will (or any documents naming wife or children). No known male-line descendants for Y-DNA. Dead end – Brick Wall.

In January 2023 a small group of us, decided to dig in.

1. We found a hitherto unknown 1776 Will of a James BROWN in nearby Cumberland Co, VA [credit Kevin Baker]. Note: There is a point that Cumberland, Buckingham and Prince Edward counties all touch – this is now the focus location. James’s will named 16 children, including Isham and Wilson. BINGO! I could find no one with this James in their Tree, despite several of his children with given names that recur in BROWN Trees in VA, NC, TN, KY, etc. But I did find a lot of BROWN Trees that had other, undocumented Ancestors about this generation – hmmm.

2. Another important find was linking Isham BROWN to the BROWN Surname Y-DNA Project – Group 40! Some BROWN Y-DNA experts [credit Bill Davidson] helped us rule BROWN lines in and out of consideration . This included several BROWN lines in Buckingham and nearby counties.

3. Two of the 16 men in Group 40 list Isham BROWN, born 1749, as their Most Distant BROWN Ancestor. They were sure of Isham, but could not determine his father – hmmm – the recently uncovered James? I can almost guarantee that if the 1776 Will of James BROWN had been easily found, many would have latched onto his son Isham [please excuse my cynicism].

4. If Isham was a brother of Wilson, then Wilson, and his male-line descendants (none of them known at this point), would also be Group 40.

5. Within the BROWN Group 40 were several lines back to the 1700s, but brick walled – and most of them were in this general area of Virginia.  My experience with Y-DNA Projects (I’m an Admin for 3 of them) is that American Y-DNA testers who form a family group, almost always have a Patriarch in America. In other words, it’s my expectation that there is a Patriarch of Group 40, probably in Virginia [I suspect James].

6. At least two of the 16 men in Group 40 list Thomas BROWN, born 1773, as their Most Distant BROWN Ancestor. Ancestry lists over 2,000 Trees for this Thomas BROWN (and his wife Nancy LITTON). Most have additional generations back, but with very sketchy documentation – pre-Revolution War records are hard to find in these counties. Communications with several Tree owners (not necessarily DNA Matches) revealed that they were unsure of Thomas’s father…

7. As it turns out, I have already found over 30 DNA Matches to this Thomas BROWN – ranging from 8 to 26cM – and I’m not even halfway through the list of possibilities. These DNA Matches span 7 of the 10 known children of Thomas – a good indication that Thomas is a relative of mine.

8. Looking back at the list of 16 children of James BROWN, and putting all the clues together, I estimate Wilson was born c1752 (3 years after Isham); and he probably had 9 to 10 children before he died (probably unexpectedly, without a Will) in 1792. Their birth years would be roughly c1773 to c1791.

Putting all of this together, my educated *guess* is that Thomas 1773 was a son of Wilson 1752; or a nephew. As a son, DNA Matches from him (and Wilson), would be 5th cousins (5C) to me. As a nephew, we’d need to go back another generation (to James) and the DNA Matches would be 6C to me.

How to figure this out? Use Teamwork to Test a Guess!

IF the relationship is “Thomas is son of Wilson,” then my DNA Matches to descendants of Thomas would also descend from Wilson and be about 5C to me. Going the other way, those DNA Matches should also have nominal 5C Matches to other descendants of Wilson, like my ancestor Keziah who married Elliott BAKER and had 8 known, surviving children with descendants who have DNA tested.

So I’ve asked such a DNA Match to please go to their AncestryDNA Match list and search for the surname BAKER, and see if some going back to Elliott BAKER (or any BAKER in Prince Edward Co, VA – there were several generations of this line there) show up as Matches.

An alternative is for me to list my 30 DNA Matches under Thomas. We expect to Match about 10% of our true 5C. So I’d expect any DNA Match from Thomas to also match about 3 of the same Matches. I have. A different DNA Match through Thomas should also match 3 of my Matches, but probably a different 3.

No need to go through the process of “Target Testing” when we already have a lot of known testers…

Testing a Guess With Teamwork!

This is a concept I’m working on – teamwork. I know it’s hard to get Matches to respond, so I’m hoping that a very clear, short objective, coupled with a relatively easy test process, would encourage a cousin to get involved. Particularly if the result would indicate new, more probable, Ancestor for them.

BOTTOM LINE – Form an educated *guess* and think of it as a given relationship. Then get widely spread DNA Matches from that added branch to look for DNA Matches in your branch.  Using daughter’s married Surnames makes this test even more precise. If you can get several to do this, and find their Matches in your branch – this would be very powerful evidence of a genealogy link. It seems to me that this is a particularly good process for common surnames, like BROWN. If you could also find Matches with DNA segments, you’d probably have a few Triangulated Groups – but that’s another story;>j

Wilson BROWN is my Ahnentafel 98 Ancestor. I plan to update this Brick Wall story as it develops.  Think about trying to get 2,000 folks to change their Trees…

[23-98Ma] Segment-ology: Testing a Guess With Teamwork! by Jim Bartlett 20230310

Distant Common Ancestor Couples


A Segment-ology TIDBIT

There has been some recent discussion about how far back autosomal DNA is useful. Some indicate the “practical” limit to be around 2xGreat grandparents (the 3rd cousin (3C) level). I put the word practical in quotes because I don’t believe the 3C level was intended to be a rock-solid/absolute limit. I think it was intended as recommendation for many genealogists – perhaps for most genealogists just getting started.

I’ve been a genealogist for almost 50 years. I have long since researched most of the available paper records. In the late 1970s, I worked for the Smithsonian, and spent my lunch hours scrolling microfilms at the nearby National Archives; or a weekly drive over to the DAR library to roam through their stacks. I look at DNA as a new tool to add more evidence to my existing Tree and extend it even farther.

I use segment Triangulation to group Matches and to build a Chromosome Map, which also informs about the contribution to my DNA from each generation of my Ancestors. But I also value what’s called genealogy Triangulation. This is when at least three of us (me and two widely separated cousins) agree on the same Common Ancestor (CA). For almost all of my known Ancestors, I have genealogy Triangulation well beyond two other Matches.

To document, and learn from, these CAs, I developed a CA Spreadsheet. See my Common Ancestor Spreadsheet blogpost for a description, a sample and a table of the columns. My CA Spreadsheet includes thousands of DNA Matches and their CA with me. For each Ancestor Couple, this spreadsheet documents way more than two Matches for genealogy Triangulation. It usually has many DNA Match cousins, all in a large genealogy “Triangulation” for each Ancestor Couple.

This spreadsheet also includes the Shared cM amount for each DNA Match. So, it is now easy to sum up the number of DNA Matches and the average cM amount for each generation:

The takeaways here include:

1. I have many DNA Matches in 5C, 6C, 7C relationships.  These stats are from my three grandparent’s Ancestors, almost all of them from Colonial Virginia – my maternal grandmother was a recent immigrant, and I get very few Matches on her line. [My parents were not related per GEDcom.]

2. At this distance, the cM relationships trend downward, as expected.

3. The averages are below 20cM.

4. For genealogy beyond the 4C level, I agree with the general concept to ignore the segment size. This is an analysis of genealogy agreement (Triangulation), that happens to be among DNA Matches. I am not claiming that the DNA segments, individually, “prove” each relationship. However, on average, some of the segments will be Identical By Descent, and when included in such large genealogy Triangulations, they increase the confidence that the genealogy is right.

5. Where known (or imputed), I also track the Clusters and Triangulated Groups (TGs) in this spreadsheet. There is usually only one or two Clusters indicated for each Ancestral Couple; and there are usually multiple TGs for each Ancestral Couple.

6. Disclaimer: Is my CA Spreadsheet 100% accurate? NO! Is it over 90% accurate? YES, IMO! Certainly the “story” in the table above is valid.

My MAIN OBSERVATION is that atDNA “works” beyond 4C.

How far back you want to take your genealogy is a very personal decision. You get to set your objectives. This post is to let you know the ability of atDNA to help you with Ancestors back at least to the 7C level.

[22BN] Segment-ology: Distant Common Ancestor Couples TIDBIT by Jim Bartlett 20230307

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