Gedmatch, the new comparison tool of John Ohlson offers the opportunity to triangulate ones matches, what simply means, that one can proof, if cousin A also matches cousin B. Unfortunately this seems to be more complicated than expected at first glance. When I perceived the first bud of a multiple cluster of matches in the early beginnings of Relative Finder, I immediately sent out emails to my cousins, pointing to the possibility, that they could also match. No reply! Probably I share this naïve calculation with a lot of other people – if I share a half identical region with cousin A and B, those two should share the same segment too, what is simply wrong!
Let’s pretend, we could represent an IBD segment of an MRCA, a most recent common ancestor with a simple icon and let’s have a closer look on the logical possibilities on triangulated matches.
Cousin A has on the paternal side a segment in common with cousin C, on maternal side a HIR with cousin B, but cousin B and C have nothing in common.
A similar situation. Cousin A has on the paternal side a segment in common with cousin C, on maternal side a HIR with cousin B, but now cousin B and C share a third common ancestor on different stretches of the chromosome. This looks like a perfect triangulated match, but is not.
This is a perfect triangulated match, since the homozygotous status of cousin A does not allow any uncertainty.
It seems, that a given HIR from cousin A can be traced in the complete genome even, if parts of the sequence are removed from paternal to maternal side, like the example of cousin B shows. The multiple shifting of cousin C is of course not very probable, although such events sometimes occur on the borders of a HIR ( fuzzy boundaries ). That’s why members of a younger generation can encounter a longer segment than their parents.