There is a strong familial tendency with >90% of the risk of disease determined genetically.
A strong association with HLA-B27 exists, particularly in white western European populations. Approximately 1-2% of all people who are positive for HLA-B27 develop ankylosing spondylitis (AS). This increases to 15-20% if they also have an affected first-degree relative. However, despite much study, it is not clear how the association with HLA-B27 determines disease susceptibility.
In AS, the major histocompatibility complex (MHC) – a cell surface molecule encoded by a large gene family which mediates the actions of leukocytes – accounts for nearly half of the disease susceptibility.
Other associated genetic factors include HLA-B60 and HLA-B39 (in HLA-B27-negative patients).

Source: PatientPlus