X chromosome
Females have two X chromosomes, males have one X and one Y. An egg always carries a single X, while sperms carry either an X or a Y. That is how sex is determined in humans, and in most other mammals. Although females have two X chromosomes, each cell can only have one X chromosome active. During early development, a process called X-inactivation occurs. In each cell one of the X chromosomes randomly deactivates. This causes different parts of the body to have different X-linked genes, and is responsible for the black and orange coloration of calico cats.