Endogamous populations are much harder to work with in genetic genealogy because you have double and triple 6th cousins who look like 2nd to 3rd cousins when you compare their DNA to yours. Ashkenazim (see my Ashkenazi DNA post), Mennonites (see Tim Jantzen’s project), and Polynesians (See Kalani Mondoy”s project) are a few of these intermarried groups. See the ISOGG wiki for a further discussion of endogamy.
A fellow genetic genealogist, Israel Pickhotz, has written a fascinating book about how he has confirmed and refuted many genealogical connections in his extended Ashkenazi family. He did this by testing every cousin he could. That story is an inspiration to those of us frustrated by using DNA to research our jewish roots. It is as easy to read as it can be, given that genetic genealogy is not easy to understand. Lara, blogging at her blog Lara’s Family Search, wrote an excellent description of the book in her review which is hard to improve upon.
Israel’s blog continues his story: http://allmyforeparents.blogspot.com/
Another DNA expert, Jim Bartlett has just written an interesting blog post investigating the math of endogamy at his segmentology blog. As it is titled part I, I am looking forward to part II.
The problem comes when so many cousins marry each other as you go back up the tree that it gets difficult to calculate the shared DNA. Plus once you get past 3rd cousins, DNA inheritance becomes more and more random anyway.
On my Norwegian side, my Dad has a woman “MB” listed as a 2nd to 3rd cousin who upon investigation was found to be a fifth cousin three times and a sixth another time. She shares 49 cM over 4 segments with my Dad and a whopping 141 cM in 8 segments with my third cousin in Norway. That third cousin is related the same way to MB as we are, but he, like MB, descends from a cousin marriage within this group.
