DNA (deoxyribonucleic acid) is a substance containing genetic information that is found inside the center (nucleus) of most body cells. DNA determines what features a person inherits from his or her parents, such as blood type, hair color, eye color, and other characteristics.

Chromosomes are parts of body cells that carry the DNA. The DNA of each chromosome is divided up into genes. Each person's DNA contains tens of thousands of genes.

Changes (mutations) can occur in the DNA of genes or chromosomes. These changes may be harmless and undetectable. Or the changes may affect certain body processes or functions to cause genetic diseases, such as hemophilia, cystic fibrosis, or Down syndrome.