Something Not Mentioned in the Whole Fritzl thing

Most people are aware of this incredible and horrible story.  Now, I don't advocate incest any more than the next man, but something that I found mildly interesting in this whole thing was that none of the children seem unhealthy.  In fact, considering that they never saw daylight, they are "surprisingly healthy."  Isn't incest supposed to produce horribly sick and deformed children?