It makes sense that a mutated gene would not be propagated in offspring, but some genetic diseases such as sickle-cell anemia are caused by a single point mutation.
http://www.thetech.org/genetics/art04_bad.php
I actually learned about this in my High School Biology and AP Biology classes. I hope this helps:
http://ghr.nlm.nih.gov/handbook/mutationsanddisorders/possiblemutations