A discipline concerned with genetically determined resemblances and differences among human beings. The idea that certain physical and mental characteristics, normal or abnormal, can “run in families” goes back to ancient times, though the mechanism by which heredity operates remained mostly unknown until the twentieth century. Formerly, genetics was thought to be concerned only with the familial transmission of rare and insignificant characteristics, but its fundamental biological role is now apparent. Genes, the units of heredity, have two unique properties: they are self-replicating, and they carry in their biochemical structure the codes for protein synthesis. Consequently, genes play the double role of transmitting genetic information from generation to generation and of governing all the activities of living cells.