Ankylosing spondylitis (AS) is strongly genetic. Heritability estimates put genetics at roughly 77% of the overall risk for developing the disease, making it one of the most heritable common inflammatory conditions. But having a genetic predisposition doesn’t guarantee you’ll develop AS. The interplay between specific genes, your immune system, and environmental factors like gut bacteria all determine whether the disease actually takes hold.
The HLA-B27 Gene and AS Risk
The strongest single genetic link to ankylosing spondylitis is a gene called HLA-B27, which codes for a protein on the surface of your cells that helps your immune system distinguish your own tissue from foreign invaders. More than 90% of people with AS carry this gene variant. That’s a striking number, but context matters: about 6% of the general U.S. population carries HLA-B27, and the vast majority of them never develop AS.
Among non-Hispanic white Americans, HLA-B27 prevalence is about 7.5%, while in other racial and ethnic groups combined it drops to roughly 3.5%. In Mexican Americans, it sits around 4.6%. These population-level differences partly explain why AS rates vary across ethnic groups, though the picture is more complicated than a single gene can account for.
How Ethnicity Affects the Genetic Link
The connection between HLA-B27 and AS isn’t equally strong across all populations. In white Americans with AS, about 94% carry HLA-B27. In Black Americans with AS, that number drops to 48%. This means Black patients are significantly more likely to develop AS without carrying HLA-B27 at all, which suggests other genes or immune pathways play a larger role in these populations. For clinicians, the practical consequence is that a negative HLA-B27 test is less useful for ruling out AS in Black patients than in white patients.
Among HLA-B27 positive individuals, the relative risk of developing AS is also lower in Black Americans than in white Americans. Something beyond this one gene clearly shapes who progresses to disease and who doesn’t.
Risk for Family Members
If you have AS and you’re wondering about the chances for your children or siblings, the numbers are relatively reassuring. First-degree relatives (parents, children, siblings) of someone with AS face about an 8.2% chance of developing it themselves. Parent-to-child risk is similar, at 7.9%. Second-degree relatives (grandparents, aunts, uncles) see that risk drop to about 1%, and third-degree relatives sit below 1%.
Identical twins provide the clearest window into how much genetics matter versus environment. Concordance rates in identical twins range from 25% to 75%, with one large study finding 63%. That’s high enough to confirm a powerful genetic component, but far from 100%, which proves that sharing the exact same DNA doesn’t make AS inevitable. Fraternal twins, who share about half their genes like any siblings, show concordance rates of only 4% to 15%.
Genes Beyond HLA-B27
HLA-B27 is the headline gene, but it accounts for only a portion of the genetic risk. Researchers have identified several other genes that contribute, each nudging the immune system in ways that make spinal inflammation more likely.
- ERAP1: This gene makes an enzyme that trims proteins before they’re displayed on cell surfaces for immune inspection. Variants in ERAP1 may cause the immune system to misread normal tissue as a threat, and notably, these variants only seem to increase AS risk in people who already carry HLA-B27.
- IL23R: This gene encodes a receptor involved in a key inflammatory signaling pathway. Certain variants ramp up the production of inflammatory signals that drive the kind of chronic inflammation seen in AS.
- IL12B: This gene codes for part of a signaling molecule that activates immune cells. Variants here influence how aggressively your immune system responds to perceived threats in the joints and spine.
Together, these and other non-HLA genes help explain why two people carrying HLA-B27 can have very different outcomes. The combination of variants you carry across multiple genes shapes your total risk.
How Gut Bacteria Interact With Genetics
One of the most active areas of AS research focuses on the gut. People with AS frequently have imbalances in their intestinal bacteria, and this connection turns out to be deeply tied to genetics.
The HLA-B27 protein itself appears to alter the gut microbiome. In animal studies, rats engineered to carry the human HLA-B27 gene developed both arthritis and gut inflammation more than 80% of the time when they had normal gut bacteria, but stayed healthy when raised in a germ-free environment. Even healthy humans who carry HLA-B27 without any symptoms of AS show measurable differences in their intestinal bacteria compared to people without the gene.
The mechanism ties together neatly: HLA-B27 has a tendency to misfold inside cells, which triggers a stress response that boosts production of inflammatory signals, particularly one called IL-23. The inflamed gut then becomes a breeding ground for immune cells that migrate to the spine and joints. At the same time, people with AS tend to lose beneficial bacteria that produce short-chain fatty acids (compounds that normally keep inflammation in check) while gaining an overgrowth of bacteria that actively stimulate inflammatory immune responses. Researchers have found that immune cells from the joint fluid of AS patients react strongly to common gut bacteria like E. coli, a pattern not seen in other forms of arthritis.
What Genetic Testing Can and Can’t Tell You
HLA-B27 testing is widely available and often ordered when AS is suspected, but the result is just one piece of the puzzle. A positive test in someone with typical symptoms (chronic back pain that improves with movement, morning stiffness lasting more than 30 minutes, onset before age 45) supports the diagnosis. A negative test makes AS less likely in white patients but doesn’t rule it out, especially in Black patients where nearly half of those with AS are HLA-B27 negative.
There’s no genetic test that definitively predicts whether you’ll develop AS. The disease depends on a constellation of genetic variants interacting with your immune system and gut environment. If you have a family member with AS and you’re HLA-B27 positive, your risk is elevated, but it’s still far more likely that you won’t develop the condition than that you will. The most practical use of genetic awareness is knowing to take persistent inflammatory back pain seriously rather than dismissing it, since early recognition leads to earlier treatment and better long-term outcomes for spinal mobility.