Interpreting DNA matches from endogamous communities?

Question

Mystery match, does not know father. Matches me on mother's paternal side.

Me and match: 376.3 cM,15 matching segments, largest segment 47.2cM
Me and his daughter: 84.7cM, 4 matching segments, largest segment 30.0cM
My mother and him: 824.7cM, 24 matching segments, largest 93.5cm
My mother and his daughter: 333.5cM, 12 matching segments, largest 63.1cM

All are Ashkenazi Jewish, mystery cousin and I are half, my mother 100% and his daughter is 18% or something like that.

We also have more shared DNA, with my great grandfather (J) and his brother (J) married H and her sister R. So if he is a 2C1R, then his grandmother would be R, which is my great grandmothers sister.

Very confused because of endogamy. I'm wondering if this match could be a double 2C1R, or triple?

Answer 1

Those are very close matches, even for an endogamous population.

A match of 824.7 cM for your mother with him would normally indicate (in a non-endogamous population) that she might be his great-aunt or 1st cousins.

Lara Diamond just wrote a post: Endogamy in Practice: Updated where she compared all her 100% Askenazi Jewish matches with their ISOGG expected shared cM. For relationships of Great Aunt or farther, the match cM was never more than 260 cM more than the expected value.

Lara's 2C1R matches are 164 cM and 196 cM compared to the average 106 cM. If he is a 2C1R of yours, he could be a double 2C1R, i.e. 2 times 196 = 392 cM.

But that would mean that your mother would have to be a double 1C2R to him, i.e. Lara's maximum for a 1C2R is 353 cM. Doubling that gives 706 cM and even that is a bit short of the 824.7 of your mother and him.

I think it's more likely that your mother is at least a 1C1R with him making you a 2C.

If you know the age of your mystery match, it will help place his relationship. If he's about your age, then you and he usually will be a degree of cousins without any removals, and he'd then be 1R from your mother. If he's your mother's age, then he'd likely be a cousin without removal to her, and 1R to you.