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Endogamy and DNA

As we SPRING FORWARD today, let’s look at endogamy and you.

Yes, this is about DNA. Really. No, actually, the word endogamy isn’t Latin and it has nothing to do with the law. Well, then again, it may have been the law in some cases that caused the endogamy, so…

Endogamy affects DNA results

Endogamy, drawn from the Greek (endo, meaning within1 and gamete, meaning wife2), simply means a pattern of marriages within a small population, either by custom or by operation of law.3

Here’s how it works. We all have two parents, four grandparents, eight great grandparents, 16 great great grandparents and so on — each generation doubling the number of ancestors. But pretty soon — as recently as the Middle Ages for us folks alive today — the number of ancestors in your family tree would be larger by far than the number of human beings alive on the earth at that moment.

The reason we don’t actually see this is that some of our ancestors married their cousins, so lots of individuals show up in our family trees more than once. Martin Davenport Sr. of Virginia, for example, is both my sixth and my seventh great grandfather. His grandson David Baker married a first cousin once removed, Martin’s great granddaughter Dorothy Wiseman, and that’s an example of endogamy. It was simply the custom to marry in the local community and that often meant cousins.

In some populations, however, endogamy was more than just custom: sometimes people were restricted in where they could live and who they could marry. And in those populations, because of those restrictions, the number of shared ancestors soared. It had to. You either married within the group, small as it might be, or you didn’t marry at all.

Now I knew all of this, intellectually, but it didn’t really hit me until I started managing a Family Finder project that includes kits for my uncle, my brother, and a large number of cousins ranging from first cousin to third-cousin-once-removed, in addition to my own kit. And the results in that project really show the DNA effects of endogamy.

For my uncle and many of my cousins, all 16 of their 2nd great grandparents were born in the United States and many, if not most, and sometimes all 32 of their 3rd great grandparents were born in America as well. And the further you go back towards colonial times, the smaller the available pool of marriage partners there was.

So for these members of the Family Finder project, there are on average 250-300 people in the autosomal DNA database at Family Tree DNA whose DNA matches theirs closely enough to be reported as a likely relative. On FTDNA, where there are 10 matches per results page, these folks are likely to get 25-30 pages of matches (a typical example is the first one in the chart on the left: 28 pages, 280 matches), and on average perhaps 50 will show up with a predicted relationship of fourth cousin or closer.

And a lot of those people show up as matches only because of endogamy. The chances of fifth cousins showing up as an autosomal DNA match is only about 10 percent, meaning only about one in 10 fifth cousins will have enough DNA in common with you to show up in the test.4 But if another person who’s also descended from Martin Davenport Sr. in multiple lines does the Family Finder test, we might be no more than seventh cousins but we’d likely have a better chance of more DNA in common than even fifth cousins would have… and perhaps enough that we’d be reported as a match.

And if we had lots of cousins marrying cousins, or simply neighbors marrying neighbors, we might show up as predicted third or fourth cousins when we were actually sixth or seventh cousins or even more distant than that. One test-taker in 2010 discovered that she and a match had four ancestral couples in common, were no more than seventh cousins once removed in any one line, and yet were predicted to be fourth cousins because of the amount of DNA they had in common.5

The situation is the reverse for another member of my project. All of his 2nd great grandparents (and, it appears, even all eight of his great grandparents, though there’s one we’re still chasing) were born in Europe. He has fewer than 40 FTDNA matches, and only three closer than distant cousin. (His match page results are second in the chart.) Part of that is because there are far fewer Europeans than Americans in the FTDNA database. And part of it is also because far fewer of his ancestors were geographically in a position to marry each other. So his results are the least affected by endogamy of us all.

My own results (third in the chart) are in between those two. My mother’s family has been in America as far back as I’ve been able to trace, but my father and his parents were 20th century immigrants from Germany. I have 163 matches as of March 2012. I have many fewer predicted third or fourth cousins than my all-American maternal uncle and cousins.

And then there’s my cousin Dick. Dick’s paternal ancestry is Ashkenazi Jewish. Throughout history, there have been fewer potential marriage partners for the Jewish population of Eastern Europe than for many other European population groups. Well beyond the usual tendency of people to marry within their own group, their own culture and their own religion, this group was often confined to ghettos and often restricted by law in who they could marry.6 Endogamy is a big issue in DNA testing for this population.

Dick’s match page count is on the bottom of the graphic to the left. Look at that. Eighty-four pages of results. 840 matches in all. Dozens upon dozens reported as predicted third or fourth cousins. And that’s after an adjustment by FTDNA to down-weight his matches to “prevent over predictions due to intermarriage and reflect more accurately (his) relationship to other Ashkenazi Jews” who’ve done Family Finder testing.

I know in the long run Dick will appreciate having a lot of fairly close genetic cousins. From a genealogical perspective it will be both a challenge and a joy.

But the simple fact is, the common ancestors his mother and mine shared had vastly more freedom in who they chose to marry than his father’s people could have dreamed of. And it’s absolutely chilling when you think of some of the reasons why that is so.


SOURCES

  1. Online Etymology Dictionary (www.etymonline.com : accessed 10 Mar 2012), entry for “endo”.
  2. Ibid., entry for “gamete.”
  3. Merriam-Webster Online Dictionary (http://www.merriam-webster.com/dictionary/ : accessed 10 Mar 2012), entry for “endogamy”.)

    To some degree or another, we all have endogamy in our family trees. As genealogists, we usually call this pedigree collapse.[4. Wikipedia (http://www.wikipedia.com), “Pedigree Collapse,” rev. 13 Feb 2012.

  4. “What is the probability that my relative and I share enough DNA for Family Finder to detect?”, Family Tree DNA (http://www.familytreedna.com : accessed 10 Mar 2012).
  5. Roberta Estes, “Cousinship in Endogamous Populations,” Rootsweb, Genealogy-DNA mail list, posted 17 May 2010.
  6. See e.g. the discussion of the Hapsburg “familial” laws in Heiko Haumann, A History of Eastern European Jews (Budapest : Central European University, 2002), 93-96.
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