What are the causes of congenital absence of teeth?

So far the experts think it is hereditary. Usually a parent or other family member has this same condition. My son is congenitally missing number twenty. I have all of my natural teeth as does my husband, except for our wisdom teeth which we had extracted. So in our families case we don't know where our son's missing tooth came from in the family, if it in fact is hereditary, and not just a fluke.