Why Don’t the Segments Triangulate?

Have you ever used the MyHeritage chromosome browser to view triangulated segments? It’s a wonderful tool. A member of the Research Like a Pro with DNA study group asked a great question about his experience using it. He said,

I’ve compared the DNA of 3 people with my mom:

Bob, my mom’s 1st cousin on her dad’s side
two DNA cousins I’m researching, Jane and Jill (names changed)

Bob triangulates with Jane on chromosome 15. Jane triangulates with Jill on the same segment on chromosome 15. Bob does NOT triangulate with Jill on chromosome 15. Shouldn’t Jill and Bob triangulate?

Comparing Jill with Bob’s brother at the 23andMe Advanced DNA comparison chromosome browser helped figure out what was going on. Also, imputation is used for matching at MyHeritage, but possibly not for triangulation. That seems to play a role.

View the question and answer video here:

Additional Resources:

Esther, “MyHeritage DNA: Your Questions Answered,” 9 Nov 2016, MyHeritage Blog (https://blog.myheritage.com/2016/11/myheritage-dna-your-questions-answered/ : accessed 5 May 2022).

“How DNA Testing Works,” 21 May 2019, MyHeritage Knowledge Base (https://education.myheritage.com/article/how-dna-testing-works/ : accessed 5 May 2022)

Jim Barlett, “Triangulating Your Genome,” Segmentology, blog post, 29 Dec 2020, https://segmentology.org/2020/12/29/triangulating-your-genome/ : accessed 14 May 2022).

To learn more about the Research Like a Pro with DNA online, independent study course, go here: https://familylocket.com/services/research-like-a-pro-with-dna-ecourse/

Louis Kessler

· Reply

May 17, 2022 at 1:03 PM

Excellent analysis, Nichole. I really like the way you go into the nitty gritty details.

If I may add another three reasons to the three you give as to why the triangulating segments don’t start (or end) at the same point:
1. The start and stop points are fuzzy because there may be random matching (either chromosome of one matching either chromosome of the other) on each end of the match.
2. One of ancestors of one of the people along the line to the common ancestor may have a recombination which shortens the match on one side.
3. One of the people may be matching each other randomly (with either chromosome of the person matching the triangulating chromosome) and even though they are shown as triangulating, they are not.

Nicole Elder Dyer
· Reply

Author

May 17, 2022 at 2:42 PM

Thanks Louis, those points are super helpful!

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Carolyn

May 18, 2022 at 1:06 PM

Thank you so much for posting this video. I’ve been wondering why some of my triangulations are for a smaller number of cM than the actual overlap is. I suspected the differences in testing companies or tests over time on different chips caused this but it’s nice to see that I’m not the only one seeing these issues. I think I might need to do a bit of an audit now as I’m wondering if I’ve marked some matches incorrectly! Thank you again.

Stacey Cummings

August 18, 2022 at 5:23 AM

Great explanation. That helped so much! I thought I really did not understand the start and end locations. I’m using a spreadsheet of start and end locations to determine probable triangulation groups. It wasn’t working!!

I have a group of 6 matches from my paternal side that triangulate. I have my mom’s DNA on MyHeritage and none of them match her. As soon as I add match #7 (doesn’t match my mom), the group stops triangulating. The cM for #7 is 18. That’s in the middle of the group’s cM. The whole group of 7 matches start at the exact same location. There’s three shared matches in the group that #7 will triangulate with, from that I can tell #7 should triangulate with the whole group.

I understand what is going on now and can start figuring out when the imputation caused the problem.

Thank you,
Stacey

Julie John

April 15, 2024 at 7:30 AM

Thank you so much for this article and the related video. You literally saved my sanity. I was seeing exactly what you demonstrated in my MyHeritage matches between individuals that matched my father’s DNA but did not triangulate as expected. If only I had seen your video BEFORE struggling for hours with with trianglation tool and complicated what if’s that made my head hurt. : )