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+5 votes
I recently discovered a possible ancestor for one of my brick walls.  I threw him up on an Ancestry tree, and waited to see what I would get.   I got a lot.

5 DNA match hits with decent documentation to his children.

Not all were to the same child, the five hits were to 3 different children, and none were to my Ancestor.

So, Does this count as a valid Triangulation?
in Genealogy Help by Wendy Fromme G2G6 Mach 2 (24.6k points)

3 Answers

+6 votes
Best answer

Nope; sorry, Wendy. First up, unfortunately, AncestryDNA can't be used for triangulation purposes, whether for WikiTree or for your own research...that is, not unless the raw data files are transferred to a different service provider, like GEDmatch, where shared segments can be compared. WikiTree does accept Ancestry DNA matching if the test-takers are 3rd cousins or closer and the AncestryDNA relationship estimate is in line with that, but Ancestry don't allow us to see any matching segment detail at all, so the benefits of using AncestryDNA for comparisons stops there.

As genetic genealogist Jim Bartlett frequently points out, triangulation groups are built around segments, not people. Even though there is no "industry" accepted standard or practice when it comes to autosomal DNA triangulation, one thing that everyone agrees on is that you absolutely positively need to see complete detail about those mutually-shared segments: the chromosome, the estimated size in centiMorgans, and the predicted start and stop points using the base pair number (also known as the reference cluster). "In common with," or the fact that you match a couple of other people--even if you know where on a chromosome--isn't good enough. If A matches B and C, you also then have to know exactly how B matches C: it isn't uncommon that A will match B and C only to find that B and C aren't a match at all on that segment. 

It also helps greatly if the comparisons all use the same toolset and the same version of the human genome map (for example, if all test-takers involved are on GEDmatch and you can be assured you're comparing apples to apples). The Human Genome Consortium is currently on version 38 (technically GRCh38.p12), but to my knowledge only YSEQ in Germany has moved to that version; GEDmatch uses version 36, as do most major testing companies for the sake of compatibility. I personally also recommend that all matching segments be compared to known population pile-up regions before even starting triangulation work; can save some time by back-shelving those matches to a low or no priority. Pile-up regions are chromosomal areas that display much higher rates of commonality across large population bases, indicating they carry forward anthropologically, not with a genealogically useful reference.

Second, triangulation has to work in lockstep with the traditional genealogical paper trail. When you're working with test-takers that are 4th cousins or beyond, things get really tricky. In fact, with our current technology, it's believed that only 46% of your 4th cousins will share any detectable DNA with you at all. And at that degree of distance refining the data can get a bit squirrelly because that farther back you go, the more biological ancestors are in the mix (including possible if not likely pedigree collapse)...and the job then isn't just to accept the first line of inheritance that seems to fit, but to rule out lines of inheritance that might also fit. That last bit isn't in the WikiTree guidelines, but the underlying necessity for the paper trail is. It's awfully difficult to use autosomal triangulation to work backward to 3g-grandparents or farther without having a solid paper-trail hypothesis before you start.

Last up, "triangulation" was adopted from the land surveyor's lexicon, and was first applied to genetic genealogy for yDNA, I believe in 2002 or 2003. For autosomal DNA, the term is more than a bit unfortunate. Again, not a WikiTree guideline or policy, but autosomal triangulation should never mean three people tested and you're done. If you can get all five of those Ancestry hits over to GEDmatch for comparison, you'll have something goin' on. But in general, if you're trying to get to, say, 4g-grandparents and the 6g-grandparents were 2nd cousins, you may need to see test results from 6, 8, or more cousins before drawing any reasonably accurate conclusions. That's why the aforementioned Jim Bartlett talks about triangulation groups in terms of segments, not people. Jim wrote just last month that he now has over 385 triangulation groups consisting of over 10,000 shared segments...so he averages about 26 segments in each triangulation group. So...ABT: Always Be Triangulatin'!  smiley

by Edison Williams G2G6 Pilot (351k points)
selected by Kay Wilson
It's really difficult to get people from Ancestry over to Gedmatch.

That said, with those standards I can't even match four people I know for a fact are 3rd cousins...and I have their DNA on Gedmatch ; )

Thanks for the best-answer star, Kay.

And Wendy, everything's on a spectrum of acceptable accuracy. What's acceptable is entirely up to you (barring WikiTree's guidelines if you want to mark ancestors "confirmed with DNA"). It's still the genealogical proof standard in play; it's just that using DNA has a very different knowledgebase and set of requirements than traditional genealogy. In the end, inadequately researched DNA evidence is no more authoritative than finding and copying a mostly-unsourced section of a found public tree. People tend to think, because DNA looks all sciencey, that it's very binary: something is either a fact, or it isn't; something is either true or false, no gray area. But the biology is squishy and random, and then we convert it to digital data and pile-on a whole boatload of assumptions and mathematical probabilities and genotyped models.  wink

Hey Edison, I understand that.   Thanks for your comment btw.

The way I understand Ancestry Shared Matches though is that is, in Ancestry's way, a chromosome match.   Their Shared Matches are people who have overlap on chromosomes.   So even though Ancestry won't let us browse, you can figure out who shares chromosome data.

And I have been the cause of some misinformation on Ancestry myself, to my chagrin, I noticed people copied my speculations as fact.  So my tree is private.   So I am aware of how that happens.

But I am also aware sometimes it means somewhere, someone Knew something : )
Hi Wendy, one thing you need to remember about Ancestry shared matches is that, even though you and your match have a shared match, it doesn't mean you all share a common ancestor nor a common segment.  For example, you (Person A) might share a 3x gggm Smith with a match (Person B), and you look at shared matches, and Joey shows up as a shared match.  That doesn't mean that Joey shares a Smith with you.  All it means is that Person A and Person B each share DNA with Joey.  But the DNA that person A shares with Joey can be a different segment (and thus relative) than the DNA shared between Person B and Joey.
+4 votes
Um, not sure.  Wouldn't think so.  You said yourself "none were to my ancestor". It's only when the DNA is to your ancestor as well as theirs that it is triangulation.  I think.  I get so confused with DNA.
by Ros Haywood G2G Astronaut (1.5m points)
I mispoke.  My ancestor that I know of with paper trail was a brick wall.  I threw up two people I suspected were his parents.  When I said none were to my ancestor, I meant that the matches were to 3 of children of the couple I suspected of being his parents.

I am not going to get matches to that ancestor.  Other than me, his line appears to be extinct.
+3 votes
By the definition of triangulation, your case fits. The next part is by how much is the match. That is, how many centimorgans do you match by?
by Doug McCallum G2G6 Pilot (449k points)
Thanks for the moral support Doug!

It ranges from 10 cMs to 6 cMs.   The good news is shared matches gave me a nice list of people without trees that might help out and some of them are much closer matches.   It may not be proof enough for the science types, but I think it's a good indication I found the right parents.

I don't have anything in the way of a paper trail.   I was tracking down Lake Eerie Fisherman in the 1830s (no easy task) when I stumbled on the possible parents.   I found another tree with no supporting documentation that listed a child with the same name and birth month/year as my ancestor.   The matches on Ancestry have more solid paper trails back to the possible common ancestor.  And there don't seem to be any other trails on their trees to my ancestor, no alternate possible histories, he just disappears.

My ancestor ran off to be a Ship's Captain.   I can't even find where or when he died, because I suspect he went out on the lake and drowned himself.

You still need the paper trail. The results might help you  do that. Ancestry trees are notoriously void of any real sources. Never trust them without verifying. For example, one of the Ancestry trees for my 3 greats grandfather uses an 1841 UK (England) census record to prove parents in Scotland for someone who died in 1824 in Canada. Every source provided needs to be verified and sanity checked. I wouldn't say you have full triangulation but you have enough to do some additional work.
I would probably ignore any matches less than 7cM. Its possible but that starts getting on the small side and increases the likelihood of bad matches.
Thanks Doug, I was really excited to finally get a breakthrough here!   At least now I have a direction to hunt that might not prove to be a total waste of time!

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