Q:

How many centimorgans should I share if second cousins marry?

My first cousin's daughter had a child with my sister's son. The child was given up for adoption. I'm matching someone on Ancestry.com with 1124 centimorgans. Not sure if this is the child and cannot ask too many questions as it is sensitive. Is 1124 centimorgans consistent with the family relationship?

A:

Theoretically, because you're related to this person twice, you can add the expected ranges for each relationship. I'm going to use the ranges from the Shared cM Project.

Grand-niece/nephew: 251 to 2108 cM (average: 910)
1C2R: 43 to 531 cM (average: 229)
Result: 294 to 2639 cM (average: 1139)

If the person is related to you in only one way (Shared cM Project for 1124cM), you've already got a number of potential relationships, with the most likely being:

Great-Grandparent
Great-Aunt/Uncle
Half Aunt/Uncle
1C
Half Niece/Nephew
Great-Niece/Nephew
Great-Grandchild

You can probably reasonably rule out the first and last, based on what you know of your family's history and relative ages. But keep in mind that the others require more research before they can be ruled out.

