Okay, the title is exaggerating slightly but you get the idea.

Say a Nord and a Darl Elf have a baby, what race would the son be? If an Imperial and a Redguard had a child, what would happen?

I say this because I'm sure this has happened in the past, but I've never seen a half-breed in-game.

http://www.uesp.net/wiki/Skyrim:Racial_Phylogeny might hold some answers for you.