Polycythemia vera is caused by a genetic mutation, but it is not inherited in the traditional sense. Nearly all cases arise from a mutation that develops spontaneously during a person’s lifetime, not one passed down from a parent. That said, the full picture is more nuanced: certain inherited gene variants can make a person more likely to develop the disease in the first place, and polycythemia vera does cluster in families at higher rates than chance alone would explain.
An Acquired Mutation, Not an Inherited One
Polycythemia vera is classified as an acquired clonal blood disorder. The key mutation happens in a single blood stem cell in your bone marrow, which then multiplies and produces too many red blood cells (and often too many white blood cells and platelets). Because this mutation occurs in a body cell rather than in an egg or sperm cell, it cannot be passed directly to your children.
About 95% of people with polycythemia vera carry a specific mutation called JAK2 V617F. The remaining 5% have a different mutation in another region of the same gene, known as exon 12. Both mutations affect the JAK2 gene, which helps control how blood stem cells grow and divide. In a healthy cell, this gene’s signaling pathway only switches on when the body sends a chemical signal (like erythropoietin) requesting more blood cells. The mutation essentially jams that switch in the “on” position, so cells keep multiplying without ever receiving the signal to do so.
Why It Still Runs in Families
Even though the disease-causing mutation itself isn’t inherited, polycythemia vera does appear more often in certain families than you’d expect by random chance. A large Swedish epidemiological study found that first-degree relatives of people with myeloproliferative neoplasms (the broader category that includes polycythemia vera) have a five- to sevenfold increased risk of developing one themselves.
The explanation lies in inherited gene variants that act as a “fertile background.” Everyone carries slightly different versions of the JAK2 gene. Some of these inherited variants appear to make the gene region more fragile and prone to acquiring the somatic mutation that triggers disease. Two hypotheses explain how this works. The first, called the hypermutability hypothesis, suggests that certain inherited versions of the chromosome are more susceptible to DNA copying errors, making the disease-causing mutation more likely to occur. The second, the fertile background hypothesis, proposes that the mutation happens at the same rate in everyone, but when it lands on a chromosome that already carries these predisposing variants, the mutant cell gains a stronger growth advantage and is more likely to expand into a detectable disease. Both mechanisms may contribute, and researchers consider polycythemia vera a unique model for studying how inherited and acquired genetic events converge.
How the JAK2 Mutation Drives the Disease
The JAK2 gene encodes a protein that sits inside blood-forming cells and relays signals from receptors on the cell surface to the cell nucleus. Normally, when erythropoietin or thrombopoietin docks on the outside of a blood stem cell, JAK2 activates a chain of signaling proteins that tell the cell to grow and divide. Once the hormone detaches, signaling stops.
The V617F mutation changes a single amino acid in a regulatory portion of the JAK2 protein that normally keeps its signaling activity in check. With that brake removed, JAK2 fires continuously. Downstream signaling proteins stay permanently active, driving the cell to proliferate regardless of whether the body actually needs more blood cells. This is why people with polycythemia vera typically have abnormally low levels of erythropoietin: the body senses it already has too many red blood cells and stops requesting more, but the mutant cells ignore that silence.
How Testing Confirms the Mutation
Genetic testing for the JAK2 mutation is a central part of diagnosing polycythemia vera. A standard blood draw is usually sufficient. The lab analyzes your DNA for the V617F mutation first, and if that comes back negative, a second test checks for exon 12 mutations. In some cases, a bone marrow biopsy is also performed to examine cell growth patterns directly.
Under current diagnostic criteria, a confirmed JAK2 mutation is one of three major requirements for diagnosis. The other two are elevated hemoglobin or hematocrit levels (above 16.5 g/dL or 49% in men, above 16.0 g/dL or 48% in women) and a bone marrow biopsy showing increased production across all three blood cell lines. If all three major criteria are met, the diagnosis is confirmed. If the bone marrow biopsy isn’t performed, a low erythropoietin level can substitute as a minor criterion alongside the mutation and elevated blood counts.
How This Differs From Secondary Polycythemia
Not everyone with too many red blood cells has polycythemia vera. Secondary polycythemia is a reactive condition where the body overproduces red blood cells in response to something else, like chronic low oxygen from lung disease, sleep apnea, or living at high altitude. In these cases, erythropoietin levels are typically elevated because the body is genuinely signaling for more oxygen-carrying capacity. There is no JAK2 mutation involved, and the bone marrow itself is functioning normally.
The JAK2 test is what cleanly separates these two conditions. A positive result points to polycythemia vera. A negative result, combined with high erythropoietin, redirects the investigation toward secondary causes.
Treatment Targets the Faulty Signaling
Because the genetic mechanism is well understood, treatment can target it directly. For people whose disease isn’t adequately controlled with standard approaches like blood removal (phlebotomy) or low-dose aspirin, a class of drugs called JAK inhibitors can dial down the overactive signaling pathway. These medications block both JAK1 and JAK2, reducing the constant “grow” signal that the mutant protein sends. This helps lower red blood cell counts, shrink an enlarged spleen, and relieve symptoms like severe itching and fatigue that result from excess inflammatory proteins released by the overactive pathway.
Polycythemia vera affects roughly 65,000 people in the United States, with an annual incidence of 0.5 to 4.0 new cases per 100,000 people. It remains a chronic condition without a cure for most patients, but understanding its genetic basis has transformed both diagnosis and management over the past two decades.