Chromosome X


The human X chromosome is a submetacentric chromosome that contains ∼5% of the genetic material in the genome. The mammalian X chromosome is special in several ways: it is one of the sex‐determining chromosomes; it displays a distinctive inheritance pattern due to the XX and XY chromosomal profile distinctive of females and males, respectively; it is relatively gene rich and the majority of genes is subjected to inactivation of one of the two X chromosomes to achieve balanced gene dosage between males and females; it is characterised by two X–Y homologous (pseudoautosomal) regions that are necessary for normal meiosis. These features made the X one of the first and best‐studied chromosomes. Mapping and sequencing of this chromosome occurred quite rapidly and now is also the first chromosome for which a manually finished telomere‐to‐telomere virtually gapless sequence assembly has become available. The X chromosome contains less than 900 genes many of which have been associated with loci for disease susceptibility, tumorigenesis and monogenic disorders. In particular, X chromosome transcripts account, to date, for 15% of intellectual disabilities.

Key Concepts

  • The length of the human X chromosome is about 156 040 895 bp codifying for less than 900 transcripts.
  • In mammals, the X chromosome is one of the sex‐determining chromosomes and evolved with the Y chromosome from a pair of ancestral autosomes (proto‐X and proto‐Y)
  • The X chromosome contains two regions of homology with the Y chromosome, which are necessary for male meiosis and recombination.
  • In female mammals, one of the two X chromosomes has to be inactivated to maintain balanced expression of X‐linked transcripts between males and females.
  • The pattern of inheritance of X‐linked disorders is characteristics and easily recognisable due to the difference in X chromosome content observed in females and males.

Keywords: chromosome X; sequencing; X‐inactivation; X‐linked diseases; chromosomal abnormalities; evolution


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Further Reading

Bergero R, Gardner J and Charlesworth D (2019) Evolution of a Y Chromosome from an X Chromosome. Current Biology‐D‐19‐01074. Available at SSRN: or

Strachan T and Read AP (2018) Human Molecular Genetics, 5th edn. John Wiley: New York, NY.

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Franco, Brunella(Feb 2020) Chromosome X. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0005924.pub2]