Females have two X chromosomes in each cell. One X chromosome is inherited from the mother, and the other X chromosome is inherited from the father. In contrast, males have one X chromosome and one Y chromosome. The X and Y chromosomes are the sex chromosomes that determine the biological sex of an individual. The X chromosome is larger than the Y chromosome and carries more genes. In females, one of the X chromosomes is randomly and permanently inactivated in cells other than egg cells, a phenomenon called X-inactivation or lyonization. X-inactivation ensures that females, like males, have one functional copy of the X chromosome in each body cell.