Sex chromosomes

An allosome (also referred to as a sex chromosome, heterotypical chromosome, heterochromosome, or idiochromosome) is a chromosome that differs from an ordinary autosome in form, size, and behavior. The human sex chromosomes, a typical pair of mammal allosomes, determine the sex of an individual created in sexual reproduction. Autosomes differ from allosomes because autosomes appear in pairs whose members have the same form but differ from other pairs in a diploid cell, whereas members of an allosome pair may differ from one another and thereby determine sex.

In humans, each cell nucleus contains 23 pairs of chromosomes, a total of 46 chromosomes. The first 22 pairs are called autosomes. Autosomes are homologous chromosomes i.e. chromosomes which contain the same genes (regions of DNA) in the same order along their chromosomal arms. The chromosomes of the 23rd pair are called allosomes consisting of two X chromosomes in women, and an X chromosome and a Y chromosome in men. Women therefore have 23 homologous chromosome pairs, while men have 22. The X and Y chromosomes do have small regions of homology called pseudoautosomal regions.

The X chromosome is always present as the 23rd chromosome in the ovum, while either an X or a Y chromosomes can be present in an individual sperm. Early in female embryonic development, in cells other than egg cells, one of the X chromosomes is randomly and permanently partially inactivated (X-inactivation): In some cells the X chromosome inherited from the mother is inactivated, while in others the X chromosome from the father is inactivated. This ensures that both sexes always have one functional copy of the X chromosome in each body cell.

...
Wikipedia