View Full Version : Question regarding Triangulation

01-26-2021, 11:04 AM
Hello, I have a question regarding genealogical triangulation. Let's say I have a valid shared autosomal segment (>10cM) with an unknown person on MyHeritage and we happen to triangulate on the same segment with either a close relative (parent, sibling) of mine or a close relative of theirs. Does that mean they're certainly a recent relative of mine (albeit distant) or could it still be a false positive? Thank you!

02-01-2021, 07:41 PM
I would take that threshold up to 15cM.
For "cold matching"* anyway.
https://isogg.org/wiki/Identical_by_descent shows that false positives exist up to 15cM, because they can be removed by phasing a genome by comparison with BOTH parents.
*"cold matching" is a term I just made up - to indicate there is no other evidence available.
If you also have matching between individuals on your trees,
or even just surnames at a rare location, then you have additional evidence that might give credence to a smaller match.

Sure, you can accept a 10cM match as a possible, and then go looking for other evidence.
But if there is none, my personal practice is to leave it alone and go on to something with a higher likelihood of success.
Some 10cM matches go back a very long way, so may be in effect insoluble. (Does your tree go back 10 generations? Does theirs?)
And if everything else is quiet you can work on them then. But sort out the big ones first. And the big clusters.
I also have 6cM matches for 3rd cousins with so much additional evidence that there is no doubt there is a definite connection between us.
But only if I can find some good triangulation would I say that the 6cM segment match was a genuine representation of that connection.

Jim Bartlett is a guru on all this stuff.
He has a long-running blog on triangulation at https://segmentology.org/
Some of his early posts had stuff you won't find anywhere else, and I still re-visit them from time to time.
So it's worth going back to the earliest one and starting from there.

02-03-2021, 03:12 PM
Thanks a lot, just now saw your reply. To be honest I haven't started a serious effort into building a large tree because I first need to talk to living relatives to get names and stuff and I haven't done that yet. I was more curious about some obscure matches with foreigners from Eastern Europe that were >=10cM.

I also have 6cM matches for 3rd cousins with so much additional evidence that there is no doubt there is a definite connection between us.

Do you share multiple segments with them or just the one 6cM? How much stock do you put into the number of segments?

02-03-2021, 09:25 PM
The 6cM links to 3rd cousins are the only segment I have for them.
(We start losing aDNA connections to 3C at around 1%, 4C at around 11% - that's NO DNA matches of 6cM or greater, at all.)

But I have rock solid documentary evidence for them.
Everything about them from the non-DNA side fits.
And as well, close relatives of theirs have larger DNA matches to me or to people very close to me - or both.