In ankylosing spondylitis (AS), several parts of the lower spine become inflamed, including the vertebrae (bones in the spine) and spinal joints.

Over time, this can damage the spine and lead to the growth of new bone, which in some cases can cause parts of the spine to join up (fuse) and lose flexibility. This is known as ankylosis.

It is not known exactly what causes AS, but in many cases there seems to be a link with a particular gene known as HLA-B27.

HLA-B27 gene

Research has shown that more than 9 out of 10 people with AS carry a particular gene known as human leukocyte antigen B27 (HLA-B27).

Having this gene does not necessarily mean you will develop AS. It is estimated that 8 in every 100 people in the general population have the HLA-B27 gene, but most do not have AS.

It is thought that having this gene may make you more vulnerable to developing AS, and the condition is triggered by one or more environmental factors – although it is not known what these are.

Testing for this gene may be carried out if AS is suspected. However, this test is not a very reliable method of diagnosing the condition because some people can have the HLA-B27 gene but not have ankylosing spondylitis.

Read about how ankylosing spondylitis is diagnosed.

Can ankylosing spondylitis be inherited?

AS can run in families, and the HLA-B27 gene can be inherited from another family member.

If you have AS and tests have shown you carry the HLA-B27 gene, the chance of any children you have developing the condition is less than 20%. If you have AS but don't carry the HLA-B27 gene, the chance of any children you have developing the condition is less than 10%.

If you have a close relative who has AS, such as a parent or a sibling (brother or sister), you are three times more likely to develop the condition compared with someone who does not have a relative with the condition.