Why do healthy people have children with genetic illnesses?

[Vulcano]

Hello, I am wondering why healthy people give birth to people with a dwarfism disorder?

[JackBean]

Dwarfism can be caused by about 200 distinct medical conditions
http://en.wikipedia.org/wiki/Dwarfism

[JackBean]

http://kidshealth.org/parent/growth/gro ... rfism.html (page 2)

[kehussy]

Dwarfism can and most often does occur in families where both parents are of average height. In fact, 4 out of 5 of children with achondroplasia are born to average-size parents.