I happened to come across a post by Robert Davis on the Ulster Co NY Y-DNA group at FamilyTree DNA.
Double Match Triangulator …would be of use in finding links (common matches) between two individuals that are not themselves matches. and hence the ICW tool of FF is of no use.
That was an excellent observation. And don’t talk much about non-matching people in my writeup on my DMT page or in its help file (although I do include one non-matching person in the sample files.). Using DMT on non-matching people that are possible relatives is something that you will want to do.
Why is this so? Well, it’s simply a matter of probabilities. Once you get down into 4th, 5th and 6th cousins, there is a good chance that your cousin will not reach the threshold where they will make it into you match list. Either their longest match in common does not meet FamilyTreeDNA’s threshold, or the total cM length of the common matches does not meet the threshold.
However, you may find that this person’s sibling or parent does match. Therefore you know they are related, but FamilyTreeDNA gives you no tools to check that.
So DMT to the rescue.
When you compare your Chromosome Browser Results file to someone who does not match you, you will not get any Full Triangulations. You will only get Double Matches with a Missing a-b segment. That’s okay. Go ahead and analyze those. They won’t be the same segment passed down from a common ancestor, but they could very well be two different segments from a common ancestral line. See Triangulation and Missing a-b Segments.
In fact, you may have hundreds or thousands of people who match you. Every one of those is a candidate to be Person b in your DMT runs, and you should see if they are willing to download and let you use their CBR file. But their siblings, parents, and cousins on the related side are also candidates as Person b. If you can, ask for any of the CBR files that they administer. Of course, tell them that you’ll keep their information private and not give it or disclose it to anyone, and tell them that you’ll let them know what you find.
I have received 63 Chromosome Browser Download files from possible DNA-relatives of my uncle. 37 of them show up in my uncle’s match list. Of the other 26, all but 3 have significant Double Matches with my uncle.
Take a look at the above table, which is my People file produced when I use DMT to run all 63 people against my uncle. Column A has my uncle. The yellow names at the top are some of the 26 people who don’t match my uncle who I have CBR file for. The people in Column B are the 37 people who my uncle matches to.
The values shown do not have any Triangulations. Those would show up in green and the numbers would be preceded by “T” instead of “D”.
But you can see some very significant Double Matches, such the 24.95 cM matches at the top left between Harry and Erika and Harry and Steve and Harry and Mark. You’ll also see some very useful X-Chromosome matches that are shown in red. When the numbers are the same, it is very likely they’re referring to the same segment, but you’ll have to check the Map page to be sure.
Notice Andrew’s column. He is one of the 3 that don’t Double Match anyone and can be presumed to be a non-relative. Negative information like that can also be valuable.
So I wanted to point this out and write my thoughts down before I forget. Using DMT for non-matches is yet another way that DMT can prove to be useful.