Not exactly, though genes do play some role. As Charles Panati writes in Sexy Origins & Intimate Things: "Penile size, as with many male characteristics, is largely a matter of heredity. If Dad is hung, there's a good probability his sons will be too." Researchers have found that the same genes, known as Hox genes, apparently control the initial growth of the limbs and the genitalia of male and female fetuses. They documented the connection after disabling the Hox genes in mouse fetuses. In humans, a mutation in one of the Hox genes has been linked to a disorder called "hand-foot-genital syndrome." But Jeffrey Innis, one of the geneticists who conducted the research, points out that many genes ultimately determine the size and shape of hands, feet and genitals.