Causes of Ankylosing Spondylitis

The cause of AS is not known, but the condition may be partly hereditary (passed down from parents to children).

One in 2000 people in the UK have AS. It is more common and often more severe in men than in women, and usually begins between the ages of 16 and 40 years.
Source

Seems this is true as I was just informed that one of my nieces' so far has the same disease now and she is in her early 30's. I wonder how the rest will fair?!