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.
MB’s grandson and I worked on the relationship. Recently he found yet another way we are related. Here is what he told me recently.
“As it turns out, Grandma “Sande” (MB) is descended from Rasmus twice (because her parents are first cousins) but also from his brother Sakarias. So, we’re cousins through Nils Erikson Frette and his wife three ways. And cousins through Hallvard and Anna two ways because of the pedigree collapse. So that’s actually five cousin relationships on the Sande line.
The additional relationship I was referring to is on the Thompson line. (Grandma Sande’s mother). Her ancestors are about half from Skanevik and half from Rogaland.
So altogether that’s six cousin relationships!
Since I have so many Etne cousins who have tested perhaps I can better sort out which DNA comes from those Frette and Tveito farm ancestors. This will take a lot of work; perhaps after the holidays.
Another problem is how to represent the multiple ancestors in a tree much less their shared DNA. I will study Israel’s book some more for inspiration and try to make a picture of this confusing relationship in a future post.