Is this a kind of triangulation?

+4 votes
Howdy, just a quick question about some autosomal DNA musings.  I have a paper trail back to a probable ancestor named Dudley Street (mid 1700s to early 1800s approx.)  According to Ancestry he had a younger brother named Isaac.  Also on Ancestry, their Thrulines shows that Isaac had at least two children with a line of descendants down to two different current Ancestry members who are supposed to be some type of 6th cousin.  Ancestry says that I share 26cm on 1 segment with one of them and 9cm on 1 segment with the other.  So, my question is: Does this give some level of DNA validation to my remote Street line, or does this level of DNA sharing not prove much of anything and is a waste of my time to consider?   Thanks for reading this, I understand some basics of DNA but using it beyond a couple of generations seems a little precipitous to me.
in The Tree House by Art Black G2G6 Mach 4 (49.7k points)

2 Answers

+4 votes

The general rule is six generations I believe. Someone else just posted something about this.

I think you should rethink how you are looking at Ancestry, though. Ancestry basically just stores digital images that they let you see for a fee. They also have the thrulines which just show what other people have posted in their trees that have similar ancestors to yours. Ancestry is not doing any research, just showing you images and the research, right or wrong, that other people have done. Hint - a lot of the research other people have done is wrong.

by Lucy Selvaggio-Diaz G2G6 Pilot (668k points)
Hi there 10th cousin Lucy, (sorry, first time I've ever checked that relationship finder thing for someone to whom I was responding).  Believe me, I know the limitations and the erroneous information that comes with Ancestry family trees.  That's why I made the move to Wikitree several years ago, trying to get away from that.  I basically only use it for record images and testing some hypotheses for hints or clues every now and then.

This was something that came up while working on my mother's paternal line, where several generations back there are a set of parents that, while a nice assortment of exact dates and places for them have been known for decades, we have never been able to find the original sources for the info.  While checking Ancestry today I decided to look at Trulines to see if any of the mismatches had been cleared up and happened to come across the brothers mentioned above, which got me to wondering about the possibilities.  I had read that thread you mentioned and know it gets more limited beyond 6 generations but this is 7 generations.  Would be nice if it wasn't too much of a stretch to show a lineage to make the bridge across the ill-sourced generations.

Grasping at straws here,  starting helping my mother with genealogy in the late 1950s and we've been trying to "prove" those generations ever since.  My daughters and grandsons aren't terribly interested in genealogy, so it's up to me to get everything onto WT before I can't.  So the search for the Golden Ticket continues.  Thanks!
+6 votes
Triangulation means that a minimum of three people all share DNA on the same segment with each other.

So the smallest segment you mention is 9 cM with match B. So the question is, does the 26 cM that you share with match A include the same 9 cM with match B.

Also, does match A also share the exact same 9 cM with match B? To triangulate, all three people have to share the exact same segment with each other. So you need to be able to compare match A and match B with each other.

Also, it's preferable that the people all come from different lines. For example, using siblings as two of the legs of the triangle isn't truly triangulation.

Here is a good blog on triangulation:

And  another one:

From what I understand, Ancestry does not provide a chromosome browser to do these types of comparisons and most people need to upload their DNA to GEDMatch and ask their matches to upload their DNA to GEDMatch to do any triangulation. Maybe someone that uses Ancestry can confirm or clarify this. I've chosen not to use Ancestry, so can't help with that.

BTW, you can add your GEDMatch ID to your DNA test info here on WikiTree to help cousins check if they match with you.
by Allison Mackler G2G6 Mach 5 (58.1k points)
Howdy Allison, (14th cousins, once removed, found a new toy and just can't stop playing with it).  Hand slaps to my head as I exclaim "That's right", overlooked the problem of if the segments match up.  You're right, Ancestry doesn't have a browser as I recall, but they used to have a way to see if distant cousins showed a DNA match.  Still didn't show the segment positions though, and with all the changes they keep making they probably dropped that feature also.  But something to look into.

Also didn't think about GEDmatch.  I've had mine on here for several years but haven't checked their site in the last year or more.  It's probably a long shot, but that's all I've got at the moment.  Thanks for the wake-up call!
I love to see how I'm related to people I'm interacting with too! :)

So, yeah, it would be impossible to do a triangulation without being able to see the segment details. But it doesn't hurt to ask them if they'll upload to GEDMatch. Or even one of the other providers. They can upload to FTDNA or MyHeritage as well, they both offer chromosome browsers and segment details. MyHeritage doesn't show X matches though, so that is something to think about.

Good luck!

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