ask: Is it possible for a person to still have Polcycthemia-Vera even though they don't have the JAC mutation gene?

Hi Anonymous,
A mutation, or change, in the body's JAK2 gene is the major cause of PV. The JAK2 gene makes an important protein that helps the body produce blood cells. What causes the change in the JAK2 gene isn't known. PV generally isn't passed from parent to child.
http://www.nhlbi.nih.gov/health/dci/Diseases/poly/poly_whatis.html

Mutation causes the disorder
The problem with blood cell production associated with polycythemia vera is caused by a change, or mutation, to DNA in a single cell in your bone marrow. In polycythemia vera, researchers have found this mutation to be a change in a protein switch that tells the cells to grow. Specifically, it's a mutation in the protein JAK2 (the JAK2 V617F mutation). More than 90 percent of people with polycythemia vera, and about half the people with other myeloproliferative disorders, have this mutation. Doctors and researchers don't yet understand the full role of this mutation and its implications for treating the disease.

Your DNA is a set of instructions for your cells, telling them how and when to grow and divide. The DNA mutation responsible for polycythemia vera makes blood cell production go awry. All the cells produced by that first mutant cell have the same problem, and they dominate your blood cell production.

The DNA mutation that causes polycythemia vera occurs after conception — meaning that your mother and father don't have it — so it's acquired, rather than inherited from a parent. Researchers and doctors don't know what causes the mutation.
http://www.mayoclinic.com/health/polycythemia-vera/DS00919/DSECTION=causes

The short answer is no. Everyone has the gene, JAK2, however, a mutation of that gene results in polycythemia vera.