It is estimated that humans have between 20,000 and 25,000 genes. Every person has two copies of each gene, one inherited from each parent.

Most genes are the same in all people, but a small number of genes (less than 1 percent of the total) are slightly different between people. These differences can contribute to each person's unique characteristics, and risk for diseases such as prostate cancer. A genetic mutation is a change in a gene that leads to diseases, such as prostate cancer.