sex chromosome sex chromosome a sex chromosome (also referred to as an allosome, heterotypical chromosome, gonosome, 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. human male xy chromosomes after g-banding part of a series onsex biological terms sexual dimorphism sexual differentiation feminization virilization sex-determination system xy x0 zw z0 temperature-dependent haplodiploidy heterogametic sex homogametic sex sex chromosome x chromosome y chromosome testis-determining factor hermaphrodite sequential hermaphroditism simultaneous hermaphroditism intersex (biology) mating type sexual reproduction evolution of sexual reproduction anisogamy isogamy germ cell meiosis gametogenesis spermatogenesis oogenesis gamete spermatozoon ovum fertilization external fertilization internal fertilization sexual selection plant reproduction fungal reproduction sexual reproduction in animals sexual intercourse copulation human reproduction lordosis behavior pelvic thrust sexuality plant sexuality animal sexuality human sexuality mechanics differentiation activity sexual system dioecy gonochorism hermaphrodite nettie stevens and edmund beecher wilson both independently discovered sex chromosomes in 1905. however, stevens is credited for discovering them earlier than wilson. differentiation 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 23rd pair of chromosomes are called allosomes. these consist of two x chromosomes in most females, and an x chromosome and a y chromosome in most males. females therefore have 23 homologous chromosome pairs, while males have 22. the x and y chromosomes have small regions of homology called pseudoautosomal regions. an x chromosome is always present as the 23rd chromosome in the ovum, while either an x or y chromosome may 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 deactivated: in some cells, the x chromosome inherited from the mother deactivates; in other cells, it is the x chromosome inherited from the father. this ensures that both sexes always have exactly one functional copy of an x chromosome in each body cell. the deactivated x chromosome is silenced by repressive heterochromatin that compacts the dna and prevents expression of most genes. this compaction is regulated by prc2 (polycomb repressive complex 2). sex determination schematic karyogram of a human, showing the sex chromosomes in green box at bottom right. the x chromosome is part of chromosome group c, and the y chromosome is part of group g. bands and sub-bands are annotated to the right of each chromosome (or chromosome pair), and the gene for the sex-determining region y protein is located at yp11.2.further information: karyogram