No two humans—not even monozygotic twins—are genetically identical. Genes and environment influence human biological variation in visible characteristics, physiology, disease susceptibility and mental abilities. The exact influence of genes and environment on certain traits is not well understood. Compared to the great apes, human gene sequences—even among African populations—are remarkably homogeneous. On average, genetic similarity between any two humans is 99.5%-99.9%. There is about 2–3 times more genetic diversity within the wild chimpanzee population than in the entire human gene pool.