I have a cousin who is adopted. I am related on her father's side, but we don't know who he is. Ancestry says she and I share 250 cM in 12 segments, Gedmatch says 255.2 cM in 11 segments. She shows up as a 2nd cousin on Ancestry, but she only shares 48 cM in 5 segments with my nephew (my sister's son). Ancestry has him listed as her 4th cousin. I know who our shared ancestors are, but the male had 2 children by his first wife, then the rest (my line) by his 2nd.

I was just wondering if she came through the first wife and was my half 2nd cousin, and therefore my nephews half 3rd?

Would that explain the difference?

Does anyone know what the numbers are for a half 2nd cousin?

Or do you think she is a full 2nd cousin?

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    I don't see any possibility to make precise guess. Is there any alive relatives who are not tested? I think it is good idea to test them and then try to process new data Apr 22, 2018 at 19:42

1 Answer 1


According to Shared cM project ver 3.0 (august 2017) a half-second cousin (same generation) should be from 9 to 397 cm with average at 117. See particularly Fig 1


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