DNA Triangulation with 3rd cousin once removed (Is it valid?)

+5 votes
I have done quite a bit of DNA testing but haven't done much Wikitree documentation based on the results beyond the most simple of examples.

I am making an attempt at DNA triangulation.  The three matches in question are me and 2 sisters who are 3rd cousins, once removed.  The paper trails are pretty good but I really want to check a few more DNA confirmed boxes in my family tree. :-)

The two sisters and and I share 38.31cM on Chromosone 9 as viewed on the FTDNA Chromosone browser.  Our most recent common ancestors are my 3rd great grandparents and their 2nd great grandparents.  

Is this sufficient DNA matching to qualify for checking the DNA Confirmed boxes in between us presuming I document the triangulation properly?

Does the fact that 2 of the 3 individuals involved in the triangulation are sisters have any impact (good or bad) on the validity of the triangulation?

The 2 sisters share the exact same amount of DNA on the same chromosone with my sister as well.

FTDNA indicates they are both 2nd-4th cousins.

Both sisters share additional DNA with me (116cM and 77cM) but chromosone 9 is the only one which overlaps.

Should I have posted this question on WikiTree Tech rather than WikiTree Help (or in some other category)?

Jeff Timmons

in WikiTree Help by Jeff Timmons G2G6 Mach 1 (11.5k points)

1 Answer

+5 votes
The three people required for triangulation have to be three separate lines. In other words, two of the three cannot match each other at a lower level. They match as sisters so you can only use one of them. You can use one of them and then find another match that neither of you match at a lower relationship. If you can find a 3C match (not once removed) then you will not need to triangulate.
by Virginia Fields G2G Astronaut (1.3m points)
Thank you Virginia.  Perfect explanation.  

So effectively since we descend from 2 different children of my 3rd great grandparents, I need to find a DNA match that descends from a different child of theirs who also shares the same DNA segment, right?

Fortunately I don't truly need this triangulation as a source since I have very good documentation but I really want to check more DNA confirmed boxes in my tree. :-)

Yes, as long as the third person does not match either of you as 2nd cousins or even 3rd cousins. The 3rd match needs to be 3C1R or a 4C. These are not as easy as they first sound!! I am still working on some that never seem to work as perfectly as I think they should!

https://www.wikitree.com/wiki/Help:Triangulation#The_three-legged_stool:  "Triangulation has been compared to a three-legged stool. The common ancestor or ancestral couple is the seat. The three lines to the cousins are the legs.  All three legs should meet at the seat, or if not, then no more than one generation below it."

To try to clarify, the three testers should be descendants of three different children of the MRCAs, OR two of the testers can be descended through two different children of a child of the MRCA (two different grandchildren of the MRCA); the third one through a different child of the MRCA.  It's easier to visualize than it is to explain.

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