Sunday, May 1, 2016

Endogamy In Action; Sibling Edition

Earlier, I looked at how endogamy made me share much more DNA with known relatives than would be expected.  While this can help to guide those with endogamous backgrounds to know what varying amounts of shared DNA may mean about actual relationships, it's important to remember that the amount can significantly vary by individuals.  Each of my parents has a brother who has tested, and my parents have both been tested as well.  While each of my parents and their brothers are related to other tested relatives in the same way, in practice, they sometimes share a very different amount of DNA with known relatives.

Name Actual Relationship Shared cM w/mom Shared cM w/uncle ISOGG average Comments
Ruth Aunt 1771.4380 1725.0190 1700.00
Scott 1C 1013.0784 732.1877 850.00
Ken 2C1R 281.4508 243.9142 106.25
(2nd cousin to Ken; 1/2 second cousin to Sue)
2C1R 181.4029 232.4263 106.25
(Myron's niece)
3C 95.9599 158.690953.13
(1/2 second cousin to Ken and Myron)
Half 2C1R 134.2947 123.5351 53.13
Judith 3C 224.05 138.7053.13
Jonathan (Judith's son) 3C1R 160.0293 79.4094 26.56
Pat 2C 181.9369 221.3467 212.50
Ben 2C1R, 3C1R 127.7907 143.6804 106.25+ Has 2 Supkoff great grandparents
Table showing shared DNA between my mother and maternal uncle and other known relatives

 In the above chart, the columns represent the following:
  1. The first column is the relative who has tested
  2. The second column is the testee's known relationship to my mother and her brother; xCyR means that the person is an xth cousin, y times removed.
  3. The third column is how many cM of DNA my mother shares with the individual
  4. The fourth column is how many cM of DNA my maternal uncle shares with the individual
  5. The fifth column is how much my mother and uncle would be expected to share with that individual, per ISOGG's Autosomal DNA statistics.
  6. Finally, the sixth column has additional information about the individual which may be relevant.

Even at the first cousin level, the difference in amounts of shared DNA is pretty extreme--and likely not due (much) to endogamy at that point.  My uncle and his first cousin Scott share 723cM--less than the ISOGG expected amount, while my mother shares quite a bit more--1013cM.

All of the other relatives on my mother's side share more DNA with both my mother and uncle than the ISOGG average (yay endogamy!), but there are still huge differences in shared amounts between my mother and uncle.

For example, cousin Jonathan is my mother's third cousin once removed, so he'd be expected to share about 26.56cM with the siblings.  My uncle shares much more DNA--79.4094cM, likely due to endogamy.  But my mother shares a full 160.0293--more than double that Jonathan shares with my uncle!  Lesser differences can be seen between other relatives and my mother and uncle.

How about on my father's side?  Fewer known relatives have tested, but here's what I have:

Name Actual Relationship Shared cM w/dad Shared cM w/uncle ISOGG average Comments
George 1C1R
433.2418 425.00
(George's 1/2 1C)
Half 1C1R 326.0912 327.1629 212.50
Beth 2C 333.3482 320.6654 212.50 2 AJ grandparents
(Beth's nephew)
2C1R 372.6452 241.0107 106.25 1 AJ grandparent
Table showing shared DNA between my father and paternal uncle and other known relatives

Cousins George and Ina and my paternal grandfather all shared the same grandfather; my grandfather and George also shared the same grandmother.  While Ina shares almost exactly the same amount of DNA with both my father and uncle, there's a much larger difference in shared DNA when looking at George--my father shares nearly 70cM more DNA with George than does my uncle.

The other two people in the table are related to one another; Beth is second cousin to both my father and uncle, and Dave is her nephew.  Even though Beth has only 2 Ashkenazic Jewish grandparents and Dave has one, the amounts of DNA they share with both my father and uncle are well above the ISOGG average.  And while the amounts Beth shares with my father and uncle are pretty close, Dave shares quite a bit more with my father than he does with my uncle--over 130cM more!  And even though Dave is a generation further removed from our shared ancestors than is Beth, my father actually shares more DNA with Dave than he does with Beth.

So while others' shared length of DNA segments can be a guide towards your relationship with someone with similar amounts of shared DNA, remember that it's just that--a guide.  The shared amount can even vary between siblings.  I wanted to demonstrate that my numbers shouldn't be taken as absolutes for how much DNA would be shared for a particular relationship in an endogamous situation.

Note:  I'm on Twitter.  Feel free to follow me (@larasgenealogy).

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  1. Your data fits with my observations that close relationships (out to 1st cousins) are not that different in endogamous populations but from 2C1R and 3rds on out one starts to share significantly more than the expected amount of DNA. Have you seen the Paul, Tannenbaum et al paper on this done with family tree DNA data?

    1. I've been noticing that as well, that not much difference. But further out you go, endogamous or not the amount can vary as I found out on both my endogamous and non-endogamous side.

      I put both sides of my family in there so you don't know which are the endogamous relatives.

      It can get a bit more complicated with half-cousins, half-siblings sometimes too as I had two of my half-brothers tested and a half-aunt.

  2. More evidence of how confusing the use of DNA is for Ashkenazi Jews. I admire your persistence in using it and in investing so much in tests. I wish I had more faith in the whole endeavor.