A gene contains genetic information that is passed from a parent to their child. This determines the color of a person's hair, eyes, and skin. Genes affect a person's risk for getting some diseases. Genes also affect how people respond to medicines.

Genes are arranged on structures called chromosomes. Changes (mutations) in genes or chromosomes may be undetectable or may cause genetic diseases, such as hemophilia or sickle cell disease. Genetic problems may also increase the risk for some conditions, such as heart disease and juvenile idiopathic arthritis.