Chromosomes are found within each cell of the human body. These structures are made primarily of protein, but also contain a molecule of DNA. Each parent donates 23 chromosomes to the offspring; therefore humans have 46 chromosomes total. The sex cells, the female egg and the male sperm, are unlike other cells in the body because they carry only 23 chromosomes, and not 23 pairs of chromosomes. A chromosome is either an X or a Y. When an X chromosome and a Y chromosome combine to form a pair, the resulting sex of the baby is male.

The female's eggs contain an X chromosome. However, the male's sperm can contain either an X or a Y chromosome. Therefore, the individual sperm cell that reaches the egg first to fertilize it will determine the sex of the embryo. If two X chromosomes combine, the sex is female. The Y chromosome contains specific DNA that give instructions for the male characteristics and physical features.