A gene is the part of a body cell that contains genetic material or deoxyribonucleic acid (DNA), which determines a person's physical traits. Genes, alone or in combination, determine what features (genetic traits) a person inherits from his or her parents, such as blood type, hair color, eye color, and other characteristics, including risks of developing certain diseases.

Many genes together make up chromosomes. Defects in genes or chromosomes may cause changes in certain body processes or functions. These changes may be undetectable or may cause genetic diseases, such as hemophilia or Down syndrome. Some conditions, such as juvenile rheumatoid arthritis and depression, may also be affected by genetic problems.

Defective genes can be passed from parents to children. Diseases that run in families are often caused by genetic defects. A person may be born with a certain genetic makeup that makes him or her susceptible or at risk for a certain condition.