Umm....The argument about recessive vs. dominant is intersting, but I have a different idea. Have you heard of variable penetrance? That means that you have the genetic info needed to express some trait but whether or not you actually do so depends on what other genes you may have and what you encounter in your environment. It's like inheriting a tendancy or a possibility.

A good example of this is schizophrenia. Children of schizophenics have a good chance of developing schizophrenia, but it doesn't obey the 50% rule as for other dominant traits. These kids can go through life perfectly fine without any detectable problems, or, if they are in some kind of very stressful situation (college, problems at work) they can start hallucinating and thinking space aliens are after them. The thing is, their inherited tendancy is totally invisible until something in their environment brings it out. Alcoholism is another example.

I think this is a pretty good analogy because although Sasuke had the genetic data necessary to use the sharingan he isn't able to do so until he has the proper training and the right environmental stimuli (the fight with Haku).