Chromosomes are cell structures made up of genetic material (DNA). They are a part of most types of cells in the body.

Humans have 46 chromosomes (23 pairs). Half of a person’s chromosomes come from the mother and half from the father. One of the 23 pairs determines a person’s gender. The sex chromosomes are called X and Y. For a child to be female, she must inherit an X chromosome from each parent (XX). For a child to be male, he must inherit an X chromosome from his mother and a Y chromosome from his father (XY).

The DNA of the chromosomes is divided up into genes. The genes determine the features a person inherits from his or her parents, such as blood type and other characteristics, including risks for developing certain diseases. Changes in chromosomes or genes 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. Gene changes can be passed from parents to children or can occur through a new mutation.