I was told that if one parent had the gene then there was a 1 in 4 chance of passing it onto a child - my dad had AS however he went undiagnosed until an accident on hoiliday resulted in an MRI scan - mine was discovered at age 20 - and being female I was really unlucky as the vast majority of AS sufferes are male.
Here is a link outlining the risk assessment for children...
http://ghr.nlm.nih.gov/handbook/inheritance/riskassessment
Hope it helps...take care..C