Genetics includes the study of how human characteristics are inherited from one's parents. It explains how traits as simple as eye color or as complex as susceptibility to diseases run in families. Genes are units of heredity passed from parents to offspring and are contained in a person's cells—every human cell contains about 20 000 to 25000 genes. Genes vary greatly from person to person and influence personality, intelligence, physical appearance, and other traits to a certain extent, but learning and environment play important roles as well. The March 19, 2008, issue of JAMA is a theme issue on genetics. This Patient Page is adapted from one published in the November 14, 2001, issue of JAMA.