John MacDonald of the University of Delaware has an excellent site on the Myths of Human Genetics.He points out that many of the examples traditionally used in high-school biology classes are not as simple as they are made out to be:
There are no common, visible human traits that have a simple one-locus, two-allele, dominant vs. recessive method of inheritance. Rolling your tongue is not dominant to non-rolling, unattached earlobes are not dominant to attached, straight thumbs are not dominant to hitchhiker’s thumb, etc. In some cases, the trait doesn’t even fall into the two distinct categories described by the myth. For example, students are told that they either have a hitchhiker’s thumb, which bends backwards at a sharp angle, or a straight thumb. In fact, the angle of the thumb ranges continuously, with most thumbs somewhere in the middle.
He gives nice explanations of what is really going on in separate posts for twelve of the myths, but he also offers hope for simple one-locus, two-allele examples that can be used in classes:
I prefer to use cat coat genetics to teach basic genetic concepts, because there are several easily visible traits whose genetics is well-established by cat breeders.
He describes several simple traits that can be used, and suggests some web-based experiments (using cat adoption pictures to sample different geographical areas). I think that the Myths of Human Genetics site is a must-read for biology teachers, and highly recommended for their students as well.