Hi Kaye, have a look at this:

http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=9863599&dopt=Abstract

A small sample, where 19% of cases did not fit the normal pattern of heredity. Paternity was genetically established. So perhaps both parents don't have to be carriers.

Andrew