How Endogamy Looks in Practice

(Note that there is a more updated version of this post here with the benefit of another year's worth of known relatives testing.)

In response to recent posts about how I'm related to people in multiple ways, and why this expression of Ashkenazi Jewish endogamy makes using DNA to find relationships among those with Jewish ancestry much more difficult, an adoptee asked how far off from how much shared DNA would be expected my known relatives were in order to help her understand her own matches better.

We always hear about how much harder it is to use genetic genealogy to find relatives when you are from an endogamous background, and Ashkenazi Jews (AJs) are a typical example.  AJs married within the small pool of AJs for centuries, so we are all related to one another in multiple ways.  I've had enough relatives test to see how endogamy is reflected in practice.

Some of my known relatives who have tested, displayed in FamilyTreeDNA

I knew that I'd probably share more DNA with my known relatives than would be expected, but the amounts that we actually shared, especially with somewhat more distant cousins, shocked me.

Name	Actual Relationship	FTDNA's Predicted Relationship Range	Shared cM	ISOGG average	% above avg	Comments
Paternal Grandmother	Grandmother	Half Siblings, Grandparent/ Grandchild, Aunt/ Uncle, Niece/ Nephew	1327.1794	1700	-22%
Paternal uncle	Uncle	Full Siblings, Half Siblings, Grandparent/ Grandchild, Aunt/ Uncle, Niece/ Nephew	1955.2031	1700	15%
Maternal uncle	Uncle	Half Siblings, Grandparent/ Grandchild, Aunt/ Uncle , Niece/ Nephew	1848.0041	1700	8.7%
Ruth	Great Aunt	1st Cousin, Half Siblings, Grandparent/ Grandchild, Aunt/ Uncle, Niece/ Nephew	1008.4038	850	18.6%
Scott	1C1R	1st Cousin, Aunt/ Uncle, Niece/ Nephew	685.0191	425	61%
George	1C2R	1st Cousin - 3rd Cousin	308.3463	212.5	27.4%
Ina (George's 1/2 1C)	Half 1C2R	2nd Cousin - 3rd Cousin	206.0462	106.25	193%
Ken	2C2R	2nd Cousin - 3rd Cousin	108.1761	53.13	103%
Myron (2nd cousin to Ken; 1/2 second cousin to Sue)	2C2R	3rd Cousin - 5th Cousin	174.8590	53.13	229%
Sara (Myron's niece)	3C1R	3rd Cousin - 5th Cousin	89.1355	26.56	235%
Sue (1/2 second cousin to Ken and Myron)	Half 2C2R	2nd Cousin - 4th Cousin	103.5133	26.56	289%
Beth	2C1R	2nd Cousin - 3rd Cousin	232.1945	106.25	118%	2 AJ grandparents
Dave (Beth's nephew)	3C	1st Cousin - 3rd Cousin	276.0552	53.13	419%	1 AJ grandparent
Judith	3C1R	2nd Cousin - 3rd Cousin	186.3940	26.56	602%
Jonathan (Judith's son)	4C	2nd Cousin - 3rd Cousin	182.0750	13.28	1271%
Pat	2C1R	2nd Cousin - 4th Cousin	117.5067	106.25	10.6%
Ben	3C, 4C	3rd Cousin - 5th Cousin	101.6712	53.13+	~91%	Has 2 Supkoff great grandparents

Table Showing Actual & Genetic Relationships Between Myself and 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 me; xCyR means that the person is an xth cousin, y times removed.
3. The third column is FamilyTreeDNA's predicted relationship range between myself and the relative.  Note that FTDNA does try to adjust this somewhat for those with Jewish DNA.
4. The fourth columns is how many cM of DNA I share with the individual
5. The fifth column is how much I'd be expected to share with that individual, per ISOGG's Autosomal DNA statistics.
6. The sixth column is the percentage of cM shared above what would be expected for a specific relationship.
7. Finally, the seventh column has additional information about the individual which may be relevant.

Unless specifically noted, all of the individuals on this list had 4 Ashkenazi Jewish (AJ) grandparents.

With the exception of my grandmother (I guess I got more DNA from my paternal grandfather than grandmother), every individual on this list shares significantly more cM with me than would be expected for their known level of cousinship.  Even Dave, who has only 1 Jewish grandparent, shares more than four times the expected amount of DNA with me.

The "winner" in the endogamy contest is my fourth cousin Jonathan, who shares 182cM with me, as opposed to the expected 13.28cM.  While his mother (Judith) already shares much more DNA with me than would be expected, Jonathan shares almost as much, even though he's another generation removed.  So while at first glance, it would seem he just inherited the majority of what his mother shares with me, I took a closer look.

Some of the segments I share with Judith & Jonathan

In some cases (such as on chromosome 18 and a small one on chromosome 19), segments shared by Judith and myself were passed onto Jonathan intact.  In others (like on chromosome 6), a relatively large segment which I share with Judith wasn't passed to Jonathan at all.  But then there are the segments which I share with Jonathan on segments 19 and 7--segments which I do not share with Judith, so they came from Jonathan's father (to whom I have no known relationship).  This is endogamy in action and shows that some of the segments that Ashkenazi Jews have in common with known relatives are quite likely from other ancestors than the known shared ones.

Hopefully this will give more insight into what different values for shared cMs can really mean when dealing with Ashkenazi ancestry.  And good luck.  You'll need it.

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

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