AS is passed from mother to son. The most definitive blood test for you both is HLA-B27. 90% positive present with the disease.
Just to clarify:
90 to 95% of Ankylosing Spondylitis patients are HLA-b27+, but less then 1% of HLA-b27+ individuals develop AS.
HLA-b27 is a fairly normal gene shared by approximately 8% of the US population. Very few of those people develop AS.
The ratio of HLA-b27 positivity is lower in patients with related Spondyloarthropathies (Undifferentiated Spondylitis, Psoriatic Spondylitis, Reiter's Syndrome).
Ankylosing Spondylitis is largely a genetic disease, but it is not necessarily passed from mother to son. A child of either sex can inherit the genetic predisposition for the disease from either parent - or from a mixture of genes from both parents.