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
Myron (2nd cousin to Ken; 1/2 second cousin to Sue)	2C1R	181.4029	232.4263	106.25
Sara (Myron's niece)	3C	95.9599	158.6909	53.13
Sue (1/2 second cousin to Ken and Myron)	Half 2C1R	134.2947	123.5351	53.13
Judith	3C	224.05	138.70	53.13
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	501.8542	433.2418	425.00
Ina (George's 1/2 1C)	Half 1C1R	326.0912	327.1629	212.50
Beth	2C	333.3482	320.6654	212.50	2 AJ grandparents
Dave (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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