Chromosomes are long pieces of DNA found in the center (nucleus) of cells. DNA is the material that holds genes. It is considered the building block of the human body.
Chromosomes come in pairs. Normally, each cell in the human body has 23 pairs of chromosomes (46 total chromosomes). Half come from the mother; the other half come from the father.
Two of the chromosomes (the X and the Y chromosome) determine if you are born a boy or a girl (your gender). They are called sex chromosomes:
The mother always contributes an X chromosome to the child. The father may contribute an X or a Y. Therefore, it is the father that determines the gender of the child.
The remaining chromosomes are called autosomal chromosomes. They are known as chromosome pairs 1 through 22.
For detailed information, see heredity and disease (genetics).