Re: Triangulated DNA matches and Pile-up Areas #dna

Adam Cherson

"The same question seems to come up over and over again among those new to autosomal DNA testing. If I match A and B on the same segment why is that not enough to prove they match each other and we have a common ancestor?

The reason the ancestor is not proven is that you have two strands of DNA on each chromosome (remember there are 23 pairs of chromosomes) and the testing mechanism cannot differentiate between the two of them. So A could match the piece from your mother and B could match the piece from your father or one of them could even be a false match to a mix of alleles from both parents (see my post on IBC for more on that concept)" from (this is an old post so ignore the techniques shown on the rest of the post)

In the first example, every member of the group matches you but not each other. In the second example the group matches you and they all match each other.

To do triangulation manually you need to see not only who matches you, but then also compare them to each other and see if they match at the same locations. The 3-D Chromosome Browser on Gedmatch gives you a table showing the internal matching of every kit you put into the group (note that the table duplicates each match by reversing the kit order). If you sort this output by chromosome number and then by location you can then see quickly whether there are any identical positions of internal matching between more than any one pair in the group. If you have Tier1 tools there is an automated triangulation app available, which is even faster, and safer. The MyHeritage triangulator also works well with up to seven. I'm not sure what the number limit is on Gedmatch. I do not believe FTDNA, Ancestry, or 23 have triangulation, although I am not up on the latest platform upgrades.
Adam Cherson

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