Do you understand autosomal triangulation?

+14 votes
in The Tree House by Peter Roberts G2G6 Pilot (725k points)
Where does one find the Tier 1 Triangulation tool referred to?
Gedmatch has the triangulation tool. It's $10 a month to use.
There had to be a catch!
Considering that the site is free and run by volunteers the occasional $10 to use the Tier 1 tools for 30 days is not excessive at all.
For those with more time then money - You can do triangulation with the free one to many, and the one to one utilities
OK, so I took the hint, donated $10 and downloaded the triangulation data. Now I have to figure out to make use of it. My initial thought is to bring the data into a database and to sum the cMs for each triangulation pair. That would give an initial priority list. Has anyone established a good structured way of analysing the data? If there is/are sets of steps that can be automated I can create a spreadsheet that people can paste their data into and get the results.
Chris, I use GenomeMate Pro to analyze and map chromosomes. It's a free software program that you download onto your computer. You then import data from Gedmatch.
Thanks, I'll give that a shot.

4 Answers

+8 votes
Best answer


Will you clarify a point from the article please? 

From the article -

  • Triangulation – a process or method by which three or more people all sharing an overlapping segment of DNA in common compare their family trees in order to identify a common ancestor or ancestral couple shared by all.
  • Triangulation Group (“TG”) – a group of three or more people who all share an overlapping segment of DNA in common with each other. Members of a TG can work together to identify a shared ancestor that might explain the shared overlapping segment of DNA. This is NOT to be confused with an ICW group!

Triangulation cannot exist when siblings match a parent and child on the same segment without the third "point" of the triangle even though there are "three or more people who share an overlapping segment of DNA in common with one another"  because the siblings and the parent/child are too closely related, as I understand the definition, correct?

by Elizabeth Townsend G2G6 Mach 2 (22.2k points)
selected by Lynden Rodriguez

Ann Turner commented "I might just add that the three people should have independent routes to the MRCA. If two of them are siblings, or parent/child, they are essentially saying the same thing twice."

Basically they would triangulate automatically but it wouldn't tell you anything new.  What would you be asking?
Siblings as one leg and parent child as another leg don't triangulate automatically even though they are "more than three people" because in each case there are two "pairs" that are not far enough apart to create the triangulation unfortunately.
For those who are new both to DNA and who are trying to triangulate it reads as though triangulation is much easier than it is, or did to me  when I read it.  Hence my asking Peter to give some clarification.
+5 votes

Not entirely yet, no. Thank you for the link to the article.

I still have a question that perhaps you can help with:

I have two distant cousins A&B  (6th and 5th, respectively) and we triangulate through GEDmatch on C13. With cousin A, our MRCA is MRCA1. With cousin B, our MRCA is the son of MRCA-1 (let's call him MRCA-2). It is appropriate to triangulate all three of us to MRCA-1, correct?  Even though two of us get to MRCA-1 through MRCA-2.


EDITED TO CORRECT: Well, I was completely wrong about these two distant cousins being in my GEDMatch report as triangulating together. Although both match me on C9, they do not triangulate together (so no conflict between my report and what was being said here). I am no longer confused. Just embarrassed.

by Maxine McReynolds G2G4 (4.6k points)
edited by Maxine McReynolds
I think the MCRA-1 to MCRA-2 (son) is the same situation as I described; MCRA-1 and MCRA-2 are not far enough apart to create separate 'legs' of the triangle.  Siblings and parent/child is too close.  

If I have this piece of it now, for you to use another direct relative it would need to be either a grandparent or first cousin in order to be far enough apart for triangulation.

I understand this for near relationships but thought it was different for distant ones for some reason. So this means that if there are only two children for several generations on a particular branch, you could never triangulate that branch pas that.  For example:

Generation 10 - 1 child

Generation 9 - 1 child

Generation 8 - 2 children

Generation 7 - 2 children

Generation 6 - 1 child

Generation 5 - 1 child

Generation 4 - 7 children

Generation 3 - 20 grandchildren from Generation 5

Generation 2 - 70 great grandchildren from Generation 5

Generation 1 - 160 great, great grandchildren from Generation 5

Generation 0 - 400 third great grandchildren from Generation 5

In this example, this branch of the family tree could only be confirmed through triangulation back to Generation 5 given that there is no possible way to create separate "legs" of the triangle past G5 (assuming that anything beyond Generation 10 would be impossible to triangulate back to). Is this how triangulation works?

Another way to say it is that, you must always have completely independent paths back to an MRCA in triangulation?

Thank you for your patience. I want to make sure that I am understanding this right!

+6 votes
In case you missed it, Roberta Estes has started a new concept series working through the 'how to' analyse autosomal results in a very practical way.  I'm sure it will eventually get onto triangulation.  Here is part 1 -
by Veronica Williams G2G6 Pilot (220k points)
+7 votes

Interesting article about IBD and Triangulation.

by Maggie N. G2G Astronaut (1.3m points)
Nice Maggie, thanks for posting this.. Mags
Good article. After I learned about stacking as well, I realized that unless segments are suitably large, which would be very rare beyond a handful of generations back, triangulation really isn't rock-solid proof. DNA circles may actually be more useful.

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