If a Sharingan user and a Byakugan user had a baby what would the bloodline trait be?

Since we don't know the genetic traits of either ability, there's no way to know. With Naruto being a rather unimaginative shonen, I'd guess they'd end up with eyes that look hyuga when the power isn't active, but have one eye be Byakugan and one eye be Sharingan when turned on, or be able to chose either power at will.