The disease causes inflammation of the spine and large joints, resulting in stiffness and pain. In addition, bones in the chest may fuse. JAS is considered to be a multifactorial condition. Multifactorial inheritance means that many factors are involved in causing a health problem. The factors are usually both genetic and environmental, where a combination of genes from both parents, in addition to unknown environmental factors, produce the trait or condition.
Multifactorial traits do recur in families because they are partly caused by genes.
A group of genes on chromosome 6 code for HLA antigens play a major role in susceptibility and resistance to disease. Specific HLA antigens influence the development of many common disorders. Some of these disorders, such as JAS, are autoimmune related and inherited in a multifactorial manner. The HLA antigen associated with JAS is called B27. This means HLA antigen testing is not diagnostic or accurate for prediction of the condition.