There is a strong association between AS and a gene called HLA B27
About 8% of white western Europeans carry this gene.
Although HLA B27 is present in around 85% of people with AS, only about 1 in 15 people who are HLA B27 positive go on to develop AS. This means that this gene alone is not responsible for people developing AS but must contribute towards it.
A lot of genetic research is going on around the world and researchers have discovered that it is likely over 100 genes must be involved and HLA B27 is responsible for no more than 25 - 30% of the overall genetic risk. They believe that if someone also carries enough of these linked genes, their susceptibility to developing AS will be higher. One gene of interest is called ERAP1 and some variants of this gene may even protect against developing AS.
What is the risk of passing it on to my children?
If you have AS and you do carry the HLA B27 gene then there is a 50% chance your child will inherit the gene. On average the child of someone with AS has an 8% chance of developing the condition. However if the child has inherited the HLA B27 gene this risk doubles to about 15%.
The risk is much lower (probably less than 1 in 20) if it was a grandparent or uncle who was affected in the family.
Overall, there is a much greater chance that a child will not develop AS.