# 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?

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.

The child of your sister's son is your great niece or nephew. The cM range from the Shared cM Project for this relationship is quite large: 251-2108 cM. Your match, at 1124 cM, is well within this.

If the child is who you think, your secondary relationship is as your first cousin twice removed (your first cousin's grandchild). That range is 43-531 cM. It's not an issue that your match is far above that range, because this is not your primary relationship.

Because of the additional relationship, you would expect your total cM to be higher than it would be with just the great niece/nephew relationship. But you can't predict what that extra cM would be. It's not additive because these are not completely separate lines. The child given up for adoption had birth parents who share a set of great grandparents. This means that it's likely that some of the segments the birth parents passed on to the child were duplicates (and only get counted once).

But yes, your match is still strongly within the range for a great niece/nephew who is the product of second cousins.