Chromosomes
Chromosomes are threadlike structures made of DNA that are in the nucleus of all cells. A human cell's nuclei has 22 autosomes and 2 sex chromosomes. One X chromosome and one Y chromosome means the baby is male, and two X chromosomes means the baby is female.

Source: http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/SexChromosomes.html