The severity of the deformity would be the deciding factor. If one looks at Richard III, the recent discovery of his body has proven once and for all that he did, in fact, have a slight deformity of the spine. It wasn't enough to stop him from becoming a powerful nobleman and then later a king.

That said, if you're deformed to the point of being, say, unable to ride a horse, or to wear armor, you're probably not going to be allowed to inherit your father's lands. You might be sent off to a monastery, or you might be attached to a brother's household as a sort of permanent juvenile.