DNA: The use of shared DNA of 9 people to prove if 2 men in 1850s were brothers or cousins....

+6 votes
Four men with the same surname lived near each other in the 1850s.  We don't know if they were brothers, cousins, or other. There is not documentary evidence extant as evidence for their relationship to each other or any candidate parents.   I know the exact relationship of the 9 DNA donars to each other and to the two ancestral men.  Seven are descended from one of the 4 men and two DNA donors to a second of the four men. All 8 of the other DNA donors share DNA to varying degrees with my father's sample - they range from 1st cousins to 3c1r and 4th 1/2 cousins.  I've charted it out and noted the % of shared DNA, longest segment, total shared in cM, the # of shared segments and exact relationship to my father Is there a way that I can figure out, knowing how much each of these 9 living people share with each other if the two ancestors are cousins vs brothers?  I think it would be the difference in a generation to the MRCA.  I've mapped it all out but now it's to the point of seeing if the % DNA shared points to them as brothers vs cousins.  Any ideas?  Is there a tool that you can provide DNA info and known relationships?

If anyone would like to take a look, I can send the chart with the living people's names anonymized but keep the shared DNA info on it.

(There are two other Ezell men and two Ezell women in the area at the same time but I have yet to identify DNA descendants of them yet - hoping GedMatch and WikitTree will help solve this 40 year old mystery that traditional genealogical research could not.)



The "family" in the URL represents all the Ezells buying land and living near each other in Marie's Co., MO in the late 840s/1850s.   The relationships as sibs are not confirmed but easier to keep them together this way for now.
in Genealogy Help by Jan Anderson G2G1 (1.2k points)
edited by Jan Anderson
PS  If anyone would like the chart with the relationships of the DNA donors to each other and to the 2 common ancestors (which has there shared DNA data relative to my dad, 1 of the 9 donors) to see if it's a solvable puzzle, I can anonymize the living and send it to you.  I'm just starting with DNA genealogy and I don't know if this is a question that DNA can answer - I was hoping it would.

1 Answer

+3 votes

If you look at the Shared cM project, you can see that while there is considerable overlap between the various cousins, there is also a considerable difference in the average totals for each.  Assuming you know the exact relationships between each of the nine, you could use the table to decide for each possible pair from the nine whether it's more likely to fit the two men as brothers or as cousins.  Hopefully, that will give you a number of votes one way or the other.  Because of the overlaps between some of them, I wouldn't expect it to be a unanimous vote, but hopefully there's a clear winner.

This looks at first like 36 possible comparisons, but I assume the nine are in two groups, those associated with each of the ancestral men.  Hopefully the groups are 4 and 5, but could be 3 and 6 or 2 and 7.  Since you aren't interested in comparing people within the same group, only all combinations between the groups, there will be much less than 36 possible comparisons/votes (20 or 18 or 14 or 9).

I don't think you could say this method would 'prove' a relationship, but if the totals are strongly leaning one way, then you could say the evidence is 'strongly suggestive'.

by Rob Jacobson G2G6 Pilot (129k points)
Sorry, forgot to add the second paragraph.  (And I should add I didn't test this, so I could be missing an important factor.)
Thanks, Rob. I realized how poorly I framed the question so I've edited it. I will take a look at your reply.   It's been fun discovering the relationships among the nine donors to each other and to the two common ancestors and charting it all out with their shared DNA relative to my dad.  I just wonder if when going 3-4 generations back like this, one is able to distinguish brothers from cousins due to the range of high-low in shared DNA cM.
I'm parsing your answer and replying (again and again) as I process it for understanding. Unfortunately, there 5 donors in 1 group and only 2 in the other ---- for now until I chase down more connections.  The 2 in the one group are 4th 1/2 cousins to each other (I think that's what it is) and 3c1r to my dad.  In the other group, relative to my dad, there is a 1c, 2c and three 1c1r.

Of course I may have misunderstood the whole problem, so let's check my assumptions.

  • You have your Dad, but he's related to everybody, so probably not useful here for this.
  • You have a group of 2 people, call them Group A, and they are the only ones currently connected to ancestor A.
  • You have the bigger group of 5, call them Group B (B for big), and they are only connected to ancestor B.
What you are testing is the relationship between each individual in Group A against each person in Group B.  So you propose ancestors A and B are brothers, then work out what relationship Group A person 1 has with Group B person 1, then check the table to see what the expected total cM should be, and compare that with their actual GEDmatch comparison total.  Then propose A and B are cousins, figure out what the relationship would be for the 2 persons again, and check the total for that relationship.  Then you can determine which one was a better fit.  Then repeat for person A1 with person B2 then B3, B4, and B5, then person A2 with person B1 etc.
Remember that those low to high ranges are not linear,  they aren't an even distribution from low to high.  They are more like bell curves, with most totals near the average, most probably within the middle third of the range.  You should be able to tell if a total is a good fit, or not a fit at all, or a poor fit, near one end of the range.
Have I explained it any better?

Hi Rob -   Thanks for persevering. I realize it's so important to clearly describe things and I appreciate your efforts to that - it's a good exercise for me as I've never had to describe DNA issues before (new to genetic genealogy but not traditional genealogy.  I've prepared 2 charts but don't think I can share on WT. Ii could email them if you'd like to PM me your address.  I should correct myself and say there are 10 donors all together, but one is my dad's niece so that's not much value added there.  Two are descended from Ancestor A and 8 from Ancestor B (thru 3 of  his 9  grandchildren) I see the value in looking at all 10 together as a sort of triangulation on steroids. :)  Your outline of a plan is understandable at a conceptual level for me -  I will see if I can work through it..I just have to wrap my head around how to actually execute it.

Your assumptions with my answers:

  • You have your Dad, but he's related to everybody, so probably not useful here for this. Everyone shares DNA with all others with 1 exception: CB (Anc B) does not share with the 2 descendants from Anc A.
  • You have a group of 2 people, call them Group A, and they are the only ones currently connected to ancestor A. Correct, the only ones thru genealogical/lineage pedigree; however, 7/8 of Group B share DNA with the two in Group A
  • You have the bigger group of 5, (it's 8) call them Group B (B for big), and they are only connected to ancestor B. Yes, from a genealogical pedigree POV; however, 7/8 share DNA with the 2 descendants in Group A.

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