If you think about it, the Sharingan can't be a recessive trait. Because that would mean that the Entire Uchiha clan is incredibly inbread, and all of the Uchihas would have extra limbs or something. They would be freaks, albeit freaks with really cool eyes.

So, maybe the Sharingan gene shows up in the regular people from time to time, but never manifests itself because it is a recessive gene. If thats true, then Uchihas can marry outside the clan, but only to those who have a recessive Uchiha gene, to prevent inbreeding. The gene must be somewhere in the regular population, because otherwise the Uchiha clan could of never come into existence.