Can someone help me with my 3rd Cousin dna confirmations?

+3 votes

Triangulation time? The whole "#cousin 3x removed" thing has always confused me. Can someone walk me through this?

  • Mary Ellen Basham is my 2nd great grandmother.
  • Of her 12 siblings, I have dna matches to 7 of them.
    • 2 through Martha
    • 4 through Jonathan
    • 1 through James
    • 3 through George
    • 3 through Minerva
    • 1 through John
    • 1 through Sallie
  • See her public profile on my ancestry tree, her sibling profiles with a purplish picture denote a dna match link and you can follow a picture from the ancestor to the match.
WikiTree profile: Mary Basham
ago in Genealogy Help by Theresa Jones G2G Crew (690 points)

This helped me a lot: Triangulation

3 Answers

+5 votes
Best answer

Hello Theresa, You do not need to triangulate with 2nd great grandparents. You just need DNA matches (from up to 3rd cousins) proven with well sourced trees. I have used for this also, but when you start looking for 3rd great grandparents, you will need to triangulate. I suggest or GEDMatch to work on triangulations.

I hope this helps! I am pretty new to triangulation too.

AncestryDNA Triangulation 

AncestryDNA does not allow you to see exactly which segments of DNA are shared by your cousins. Consider uploading your results to GEDmatch to enable chromosome segment comparisons to be made.

Missy smiley

ago by Missy Berryann G2G6 Mach 3 (35.5k points)
selected ago by Theresa Jones
Thank you Missy! I wasn't sure.

You’re welcome! I am happy that I could help. wink Good luck!

Just a quick note that cousins through a 2g-grandparent MRCA could be one or more times removed due to generational differences. Each "1x removed" halves the theoretical average shared DNA from the generation before. For example, the theoretical average sharing with a 3C is about 54cM; with a 3C1R it's about 27cM; with a 3C2R it's about 13cM, and so on.

So WikiTree's "Confirmed with DNA" policy extends through to 3rd cousins only. For half third cousins, 3C1R, or any relationship farther removed, the guidelines call for triangulation evidence.
  • Mary Ellen Basham is my 2nd great grandmother.
  • Of her 12 siblings, I have dna matches to 7 of them.

So the MRCA couple must be 3rd great grandparents making the relation at least 3rd cousins something ? And if so requiring a triangulation. See Missy's answer above

AncestryDNA does not allow you to see exactly which segments of DNA are shared by your cousins. Consider uploading your results to GEDmatch to enable chromosome segment comparisons to be made.

+6 votes


What Missy says in response to your question is spot on but a little more detail may help you.

Mary Ellen is your G2 grandmother but she is not the most recent common ancestor you share with the matches you list. Those are her parents, Henry Basham and Martha Warren, your G3 grandparents.

Taking one of your matches at random, Harold Blackburn,  is your third cousin once removed. As Ed points out, that extra degree of separation does make a difference and that's where triangulation becomes necessary.

I would be very surprised if you do not have a whole series of triangulated matches within your raft of 3c1r and 4c cousins. The hang up will be if their results are only on ancestry. Ancestry will only tells you if they are matches in common with you - ICW is not the same as triangulation.

For a triangulated match you need to be able to see the matches at a chromosomal level - so gedmatch, ftDNA or MyHeritage.

If you need help with this or with writing them up in the approved wikitree format, please drop me a pm.

Well done on having so many matches at that level, I must confess to having a little cousin envy.

ago by Derrick Watson G2G6 Mach 2 (25.5k points)
+2 votes
The short answer is "Yes."

People in your generation who are descended from gt-gt grandma's siblings are your 4th cousins, so you would need to triangulate with them (since they're more distant than 3C). If they're in the generation above you, then they're still 3C1R and it's still no good.


(1) You have to keep in mind what your goal is with this. Triangulating these people will NOT DNA confirm you any further back in your pedigree, because you don't mark the MRCAs with triangulation, the way you do with 3Cs..What it WILL do, however, is DNA confirm some of those family lines, going down from those aforementioned siblings, down to the people you match. So if THAT's your goal, go ahead and start begging folks to upload to GEDmatch or some other place where you can triangulate.

To DNA confirm further back with triangulation, you'd need at last 5th cousins.

(2) If one of those matches is in your grandparents' generation, and it is strong enough, that'd be a 2C2R, which I'd argue is equivalent to a 3C, and you might be able to actually use it.

(3) A better route, however, is if there is a match on your line in your parents' generation, and also one from that generation on another line.  THEY are 3C, so if they will tell you about THEIR match to each other, maybe you can use THAT to go back a generation. If their match isn't strong enough, you could still use it without breaking the rules if you can get both parties on GEDmatch (where there's no such thing as a "predicted 3rd cousin" match). I have actually used this, myself, in a place or two.
ago by Frank Stanley G2G6 Mach 5 (55.2k points)
Oops! I didn't realize you didn't have gt-gt grandma DNA confirmed yet, so triangulation would at least get her confirmed. Stlll, #3 would ccomplish both that AND her parents, if it can be done.

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