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

16 thoughts on “Percent of Shared Cousins Indicates Relationship

  1. Hello,

    I don’t know if what I’ve been attempting to do fits the type of scenario you have presented in both this blog post and the last one, “Testing a guess with Teamwork’. This is what I have…

    I noticed matches to the surname BROOKOVER in both my test and the tests of a couple third cousins (siblings) that descend through a common ancestor, James Marshall (our 4XGGF). Like your BAKER search, I decided to do a BROOKOVER surname search for the three of us. I came up with BROOKOVER match amounts of 67 (mine), 49 and 69 (the siblings). ranging between 8cM-30cM. I ended up expanding this to include eight other descendants of James MARSHALL, with him being their 3X-6XGGF. The number of BROOKOVER matches range between 10-82 for this group (with the sole 3X descendant having 82 matches between 8cM-56cM). Collectively, I have confirmed 188 individual BROOKOVER matches, researching and documenting all of them back to one common ancestor-Johann (Jacob) BROOKOVER (1738-1827). There are 8 children attributed to Jacob BROOKOVER. We have matches to descendants of seven of them. Most of these matches fall in the 5th-6th cousin range for me and Jacob BROOKOVER would be my 5XGGF, based on the hypothesis I’m trying to prove.

    I hypothesize our connection to the BROOKOVER line lays with either my 3rd or 4th great-grandmother, both MNU. Based on the DOB for my 3XGGM, Elizabeth (1790), , she may be the granddaughter of Jacob BROOKOVER. If the connection lays with my 4XGGM, Mary, (born c.1760-1764), she would likely be the daughter of Jacob BROOKOVER. Mary is the wife of James Marshall, the common ancestor for all eleven cousins I have analyzed. Unfortunately, it is confounded as to which of these women might be the connection as Elizabeth is married to Mary’s son, Eli, so either scenario gives the same “distance” to Jacob BROOKOVER as one of my 3rd great-grandparents would be his grandchild, if that makes sense.

    So, I’m wondering if my work can be used for what you are proposing. My “Thomas BROWN” is Jacob BROOKOVER and I’m trying to figure out how he fits with me and the other MARSHALL cousins. I have all my information in a spreadsheet that I will gladly share if it is useful. Thank you for your time.

    Cheers, Julie Stanton Glick

    Liked by 1 person

    • Julie, Thanks for your great work and input. You are on exactly the right track. If you can get invited to one of your many distant cousin’s AncestryDNA kit, you can check their Matches. A 4XG would be at the 5C level, and you’d probably match about 10% of their Matches (and vice versa). At least this might tell you which generational level you are working on. I’m on a hunt for all of my BROWN cousins in a huge Cluster – so far a high percentage are tying in…. Jim


      • Hi Jim,

        So, I want to get the test results of a BROOKOVER match? I know a 4XG should yield about 10% matches in common for a 5th cousin, but if both of us are 5XG from Jacob BROOKOVER, I would only expect about 2% the same because we would be 6th cousin?

        I’m not sure if this would point me into which generation it is because in either scenario it still results in Jacob BROOKOVER as a 5XG to me. Along with access to the tests of the 11 MARSHALL cousins I used, I do have access to three other tests of people that descend from James MARSHALL, the common ancestor, but through a son from a different mother. The BROOKOVER in their results seem incidental-only a few and they are of a closer generation. I don’t know if that points more to my 4X being the possible connection. I do know that of the 11 cousins I analyzed, the nine that descend from Eli (4XG’s son) and Elizabeth’s (3XG) have more matches than the two that descend from Eli’s siblings, which in my mind, points to the 3XG.

        Would you have a few minutes to look over the spreadsheet I’ve done if I send it to you? You might be able to glean some info to point me in the right direction. I appreciate any help you can give me. In the meantime, I will contact a few of the BROOKOVER matches and beg for access. 🙂

        Julie Glick

        Liked by 1 person

      • Julie, You can send the spreadsheet to me and I’ll take a look. The process I outlined gets pretty iffy at the 6C level – the DNA *is* random and doesn’t always do what we want. Jim


      • I don’t know how what I’ve done in this spreadsheet compares to what you are doing. I tried to “follow the DNA” as someone suggested to me. It shows which of my cousins matches a particular match. As I said, for me most fall in the 5C-6C range so our range is that, give or take a generation or two.

        A little explanation about the spreadsheet…
        All Marshall cousins descend from James MARSHALL and Mary MNU, who is one of the two women who possibly descend from Jacob BROOKOVER. Which of James and Mary’s children they descend through is below their names.

        All matches have been traced and documented as descendants of JACOB BROOKOVER. The “raw total” at the bottom is the number of BROOKOVER matches that came up with the surname search. The “individual total” was the number of matches I was able to document. I excluded private trees, trees with unverified or unknown BROOKOVERs (when their tree didn’t go back far enough to find an identified BROOKOVER), or if they had obvious errors (such as my research shows different parents, etc).

        The matches are grouped by which child of Jacob they descend from. For ease in following the line of descent, I also included the next two generations, giving us the grandchild and great-grandchild to Jacob. Individuals not showing a surname are all BROOKOVER. I put surnames on the ones that didn’t carry the BROOKOVER surname and in the case of females, I put their married name in parentheses. The names of the matches are mostly as listed on their tests, with periods omitted for those with just initials and managed tests showing “man” for “managed by” in the user name. A few are cut off because the column isn’t wide enough, but the spreadsheet is already dauntingly wide and I can supply the info if needed. I hope it is mostly self-explanatory.

        Thanks for taking a look!
        Julie Glick


      • Premise: Either Mary MNU is the daughter of Jacob B., or Elizabeth MNU is the granddaughter of Jacob B. through an undetermined child. Mary MNU is the mother of Eli, who is the spouse of Elizabeth MNU, which would mean either Eli or Elizabeth would be a grandchild of Jacob B. Eli and Elizabeth are my 3X great-grandparents, making Jacob my 5XGGF. Can I determine which woman is my connection to Jacob B.?

        Using Jim Bartlett’s blog posts, Testing a Guess with Teamwork (20230310) and Percent of Shared Cousins Indicates Relationship (20230315) I applied the principle from it to my own search, the premise above.

        I took four DNA matches that I had test access for that all descended from Jacob B and looked for matches in common with me and three other cousins that descend from Mary MNU. Three of us are 3C and the other a 3C1R. Jacob B is the 5XGGF to both John W. and Nancy and thus would be a 6C to me if Jacob B was our MRCA. Jacob B is the 4XGGF to Edwin and Mary (siblings) would be my 5C1R. Edwin is the father of John W., The four descend from Jacob B’s son, John Sr.. I came up with the following results (test user, #of matches, shared matches to the 4 cousins & percentage shared):
        John W (254 Jacob B matches): 26/10.2%; 32/12.5%; 28/10.9%; 22/8.6%
        Edwin (313 matches): 37/11.8%; 40/12.8%; 28/8.9%; 24/7.7%
        Mary (334 matches): 33/9.9%; 36/10.8%; 25/7.5%; 22/6.6%
        Nancy (252 matches) 27/10.7%; 21/8.36%; 27/10.7%; 22/8.7%

        From this, I deduced that the four people whose tests I used: John, Edwin, Mary, and Nancy; and the four cousins were a generation closer than expected and thus a closer MRCA. The percentages are more inline with John and Nancy being my 5C, and Edwin and Mary my 4C1R. This points to my Brookover connection being through Elizabeth MNU and Elizabeth MNU must be the daughter of John Sr., who must be my 4XGGF. As a sidenote, there are 188 confirmed DNA matches descending from Jacob B. between eleven cousins who all descend from Mary MNU. 143 of these matches descend from John Sr. Perhaps that was telling me something.

        I would appreciate feedback from anyone on if my assumption is plausible. Thanks!

        Liked by 1 person

      • Julie, This is great. Although there is no biological law about the percentages, IMO there is a great deal to be said for the mass of data. You did a great job of amassing that data, and it clearly supports your conclusion. This will lead you to dig into the BROOKOVER connection with much more confidence. Like you, I would invite others to comment on this process and outcome. Jim

        Liked by 1 person

      • I should hasten to add that the percentages *are* based on emperical data and are reported by the testing companies, My 90% for 3C; 50% for 4C, 10% for 5C and 2$% for 6C shouild be taken as orders of magnitude – they are in line with what the companies report, and my own observations. Jim


      • I made sure to check “all the companies” for their data and know it is in line with yours. I think my results are a good fit so far. I am hoping to find more matches through other children of Jacob Brookover to see if this holds true. The nice thing about my results thus far leading to a specific son of his is that many trees already have a daughter, Elizabeth, for that son (FamilySearch has two!), but have a spouse for her. I haven’t agreed on this spouse anyways because it is based on the assumption that her surname was misspelled on the marriage record. The research I am finding shows a family in the area by the surname on the record. So, I’m concentrating on getting rid of the spouse! Haha! Thanks for this blog. It has really aided in this search. ________________________________


      • aerowind51 – thanks for your feedback – IMO, there is a big enough gap between 3C and 4C and between 4C and 5C that this process will usually provide a good result. It would be interesting to learn of results from a highly endogamous situation. Jim

        Liked by 1 person

    • I help ThruLInes all I can. 1. I list people in my Tree with standard names and do not put Sgt, Capt. Rev. etc in the name field (they confuse the algorithm). 2. I try to plain names – John BROWN; not John James Alexander BROWN. 3. I try to list all the children and grandchildren of my Ancestors (at least out to the 6C level). 4. I add the entired line of descent down to and including the Match name (which is always living and private) – this is on the expectation that people who test often recruit other close relatives to test. I try to look at the big picture from the viewpoint of the ThruLines algorithm – what can I do to help it. BTW, ThruLines can see piople in Private, but Searchable, Trees – this is gold. Using Walk The Clusters Back, I add Cluster Notes to almost every Match over 20cM (and many below). These then indicate what Ancestor the “should” have (or has a high probability of having). I have built many Trees back to that Ancestor. It builds like a snowball rolling down hill. I have over 80,000 Matches at Ancestry – say half are false – say another fourth are too distant – that still leaves a lot of Matches who are cousins within 8C or 9C level (some would quibble over the percentage of my Matches I think are “in play” – but the number is huge – way more than I’ll ever determine. And my other advantage is Colonial Virginia ancestry – most of which goes back to the late 1600s and early 1700s – generating lots of desendants and Matches. Jim

      Liked by 1 person

      • Yes I’ve been doing those things too, I have an elusive ancestor too but he’s only my 2GGF! I’ve been following your expert guidance for years and am slowly making progress. Being in Australia, many of us only have limited number of matches to work with, Mums 4C matches are only in the 300 range (abt 35k total but 24k are small matches I preserved). I am always amazed when I share kits from the US how many have 4C’s over 1000. We also have many US matches, but we often have to try and get all of you across the pond first as our connections will usually be back in the UK! Anyway, keeps me off the streets and hopefully it helps my little grey cells continue to tick over…..

        Liked by 1 person

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