Well - I think it would be mostly the same every generation, maybe with some small fluctuations. Take cancer, for example. Although cancer doesn't always run in families, you are certainly more likely to get it if a family member had it, but a family member having had it doesn't guarantee that you will by any means. Cancer rates, despite this, aren't to my knowledge skyrocketing, though (at least, not singlehandedly due to genetics). I think colourblindness works the same way. This is how I'm assuming the metamorphmagi-gene works.