"HLA-B27 is a perfectly normal gene found in 8% of the Caucasian population. Generally speaking, no more than 2% of people born with this gene will eventually get spondylitis.
"If a parent, brother or sister has spondylitis, and I test positive for HLA-B27, what are my chances of getting the disease?If a family member had spondylitis and you test positive for the HLA-B27 gene, your chance of getting the disease increases to 20%, if you are under age 40.
If you are over 40, your chance of developing spondylitis is very low. If you have AS, the likelihood of passing it on to your children is relatively low. There is approximately a 50% chance that the child of one HLA-B27+ parent will inherit the gene, but only a small percentage of those will develop AS." [emphasis added]
"HLA-B27 probably accounts for about 40% of the overall risk, but then there are other genes working in concert with B27."
So - there is no single trigger of spondylitis and no way to know for sure if you will start to develop symptoms. If you do think you have symptoms it would be a good idea to be evaluated by a rheumatologist - maybe even your dad's rheumy if you like. But there is no need to be preoccupied with worry about it, because there is no way to prevent it.