View source for X chromosome

{{Infobox chromosome
| name = Human X chromosome
| image = Human male karyotpe high resolution - X chromosome cropped.png
| caption = Human  X chromosome (after [[G banding|G-banding]]).
| image2 = Human male karyotpe high resolution - Chromosome X.png
| caption2 =  X chromosome in human male [[karyogram]].
| length_bp = 156,040,895 bp<br/>([[GRCh38]])<ref name="National Center for Biotechnology Information 2017">{{cite web | title=Human Genome Assembly GRCh38 - Genome Reference Consortium | website=National Center for Biotechnology Information | date=2013-12-24 | url=https://www.ncbi.nlm.nih.gov/grc/human/data?asm=GRCh38 | language=en | accessdate=2017-03-04}}</ref>
| genes = 804 ([[Consensus CDS Project|CCDS]])<ref name="CCDS"/>
| type = [[Allosome]]
| centromere_position = [[Centromere#Submetacentric|Submetacentric]]<ref name="StrachanRead2010">{{cite book|author1=Tom Strachan|author2=Andrew Read|title=Human Molecular Genetics|url=https://books.google.com/books?id=dSwWBAAAQBAJ&pg=PA45|date=2 April 2010|publisher=Garland Science|isbn=978-1-136-84407-2|page=45}}</ref><br/>(61.0 Mbp<ref name="850bphs">Genome Decoration Page, NCBI.   [ftp://ftp.ncbi.nlm.nih.gov/pub/gdp/ideogram_9606_GCF_000001305.14_850_V1 Ideogram data for Homo sapience (850 bphs, Assembly GRCh38.p3)]. Last update 2014-06-03. Retrieved 2017-04-26.</ref>)
| ensembl_id = X
| entrez_id = X
| ncbi_id = X
| ucsc_id = X
| refseq_id = NC_000023
| genbank_id = CM000685
}}
The '''X chromosome''' is one of the two [[sex determination system|sex-determining]] [[chromosome]]s ([[allosome]]s) in many organisms, including mammals (the other is the [[Y chromosome]]), and is found in both males and females. It is a part of the [[XY sex-determination system]] and [[X0 sex-determination system]]. The X chromosome was named for its unique properties by early researchers, which resulted in the naming of its counterpart Y chromosome, for the next letter in the alphabet, after it was discovered later.<ref name="nyt-angier">{{cite web
| url = https://www.nytimes.com/2007/05/01/science/01angi.html
| title = For Motherly X Chromosome, Gender Is Only the Beginning
| last = Angier
| first = Natalie
| publisher = New York Times
| date = 2007-05-01
| accessdate = 2007-05-01}}</ref>

==Discovery==

It was first noted that the X chromosome was special in 1890 by [[Hermann Henking]] in Leipzig.  Henking was studying the testicles of [[Pyrrhocoris apterus|''Pyrrhocoris'']] and noticed that one chromosome did not take part in [[meiosis]].  Chromosomes are so named because of their ability to take up [[staining]].  Although the X chromosome could be stained just as well as the others, Henking was unsure whether it was a different class of object and consequently named it ''X element'',<ref name=Schwartz/> which later became X chromosome after it was established that it was indeed a chromosome.<ref>David Bainbridge, 'The X in Sex: How the X Chromosome Controls Our Lives'', pages 3-5, Harvard University Press, 2003 {{ISBN|0674016211}}.</ref>

The idea that the X chromosome was named after its similarity to the letter "X" is mistaken.  All chromosomes normally appear as an amorphous blob under the microscope and only take on a well defined shape during mitosis.  This shape is vaguely X-shaped for all chromosomes.  It is entirely coincidental that the Y chromosome, during mitosis, has two very short branches which can look merged under the microscope and appear as the descender of a Y-shape.<ref>Bainbridge, pages 65-66</ref>

It was first suggested that the X chromosome was involved in sex determination by [[Clarence Erwin McClung]] in 1901 after comparing his work on [[locust]]s with Henking's and others.  McClung noted that only half the sperm received an X chromosome.  He called this chromosome an ''accessory chromosome'' and insisted, correctly, that it was a proper chromosome, and theorized, incorrectly, that it was the male determining chromosome.<ref name=Schwartz>James Schwartz, ''In Pursuit of the Gene: From Darwin to DNA'', pages 155-158, Harvard University Press, 2009 {{ISBN|0674034910}}</ref>

==Inheritance pattern==
[[File:X chromosome ancestral line Fibonacci sequence.svg|thumb|360px|The number of possible ancestors on the X chromosome inheritance line at a given ancestral generation follows the Fibonacci sequence. (After Hutchison, L. "Growing the Family Tree: The Power of DNA in Reconstructing Family Relationships".<ref name="xcs"/>)]]
[[Luke Hutchison]] noticed that a number of possible ancestors on the X chromosome inheritance line at a given ancestral generation follows the [[Fibonacci sequence]].<ref name="xcs">{{Cite journal|last=Hutchison|first=Luke|date=September 2004|title=Growing the Family Tree: The Power of DNA in Reconstructing Family Relationships|url=http://fhtw.byu.edu/static/conf/2005/hutchison-growing-fhtw2005.pdf|journal=Proceedings of the First Symposium on Bioinformatics and Biotechnology (BIOT-04)|volume=|issue=|doi=|pmid=|access-date=2016-09-03|via=}}</ref> A male individual has an X chromosome, which he received from his mother, and a [[Y chromosome]], which he received from his father. The male counts as the "origin" of his own X chromosome (<math>F_1=1</math>), and at his parents' generation, his X chromosome came from a single parent (<math>F_2=1</math>). The male's mother received one X chromosome from her mother (the son's maternal grandmother), and one her father (the son's maternal grandfather), so two grandparents contributed to the male descendant's X chromosome (<math>F_3=2</math>). The maternal grandfather received his X chromosome from his mother, and the maternal grandmother received X chromosomes from both of her parents, so three great-grandparents contributed to the male descendant's X chromosome (<math>F_4=3</math>). Five great-great-grandparents contributed to the male descendant's X chromosome (<math>F_5=5</math>), etc. (Note that this assumes that all ancestors of a given descendant are independent, but if any genealogy is traced far enough back in time, ancestors begin to appear on multiple lines of the genealogy, until eventually, a [[Founder effect|population founder]] appears on all lines of the genealogy.)

==Humans==

===Function===
[[Image:Sd4hi-unten-crop.jpg|thumb|Nucleus of a female amniotic fluid cell. Top: Both X-chromosome territories are detected by [[Fluorescence in situ hybridization|FISH]]. Shown is a single optical section made with a [[Confocal laser scanning microscopy|confocal microscope]]. Bottom: Same nucleus stained with [[DAPI]] and recorded with a [[Charge-coupled device|CCD camera]]. The Barr body is indicated by the arrow, it identifies the inactive X (Xi).
]]
The X chromosome in humans spans more than 153 million  [[base pairs]] (the building material of [[DNA]]).  It represents about 800 protein-coding genes compared to the Y chromosome containing about 70 genes, out of 20,000 - 25,000 total genes in the human genome.
Each person usually has one pair of sex chromosomes in each cell. Females have two X chromosomes, whereas males have one X and one [[Y chromosome]]. Both males and females retain one of their mother's X chromosomes, and females retain their second X chromosome from their father. Since the father retains his X chromosome from his mother, a human female has one X chromosome from her paternal grandmother (father's side), and one X chromosome from her mother. This inheritance pattern [[#Inheritance pattern|follows the Fibonacci numbers]] at a given ancestral depth.

[[Genetic disorder]]s that are due to [[mutation]]s in genes on the X chromosome are described as '''X linked'''.

The X chromosome carries hundreds of genes but few, if any, of these have anything to do directly with sex determination. Early in [[embryo]]nic development in females, one of the two X chromosomes is randomly and permanently inactivated in nearly all somatic cells (cells other than [[ovum|egg]] and [[spermatozoon|sperm]] cells). This phenomenon is called [[X-inactivation]] or [[Lyonization]], and creates a [[Barr body]].  If X-inactivation in the somatic cell meant a complete de-functionalizing of one of the X-chromosomes, it would ensure that females, like males, had only one functional copy of the X chromosome in each somatic cell. This was previously assumed to be the case. However, recent research suggests that the [[Barr body]] may be more biologically active than was previously supposed.<ref name="Carrel">{{cite journal |vauthors=Carrel L, Willard H |title=X-inactivation profile reveals extensive variability in X-linked gene expression in females |journal=[[Nature (journal)|Nature]] |volume=434 |issue=7031 |pages=400–4 |year=2005 |doi=10.1038/nature03479 |pmid=15772666}}</ref>

===Genes===
{{Category see also|Genes on human chromosome X}}
The following are some of the gene count estimates of human X chromosome. Because researchers use different approaches to [[genome annotation]] their predictions of the [[number of genes]] on each chromosome varies (for technical details, see [[gene prediction]]). Among various projects, the collaborative consensus coding sequence project ([[Consensus CDS Project|CCDS]]) takes an extremely conservative strategy. So CCDS's gene number prediction represents a lower bound on the total number of human protein-coding genes.<ref name="pmid20441615">{{cite journal| author=Pertea M, Salzberg SL| title=Between a chicken and a grape: estimating the number of human genes. | journal=Genome Biol | year= 2010 | volume= 11 | issue= 5 | pages= 206 | pmid=20441615 | doi=10.1186/gb-2010-11-5-206 | pmc=2898077 | url=https://www.ncbi.nlm.nih.gov/entrez/eutils/elink.fcgi?dbfrom=pubmed&tool=sumsearch.org/cite&retmode=ref&cmd=prlinks&id=20441615  }}</ref>

{| class="wikitable" style="text-align:right"
| Estimated by || [[Protein-coding genes]] || [[Non-coding RNA|Non-coding RNA gene]]s || [[Pseudogene]]s || Source || Release date
|-
| [[Consensus CDS Project|CCDS]] || 804 || - || -
|style="text-align:center"| <ref name="CCDS">{{cite web | title=Search results - X&#91;CHR&#93; AND "Homo sapiens"&#91;Organism&#93; AND ("has ccds"&#91;Properties&#93; AND alive&#91;prop&#93;)  - Gene | website= NCBI |version = CCDS Release 20 for ''Homo sapiens'' | url=https://www.ncbi.nlm.nih.gov/gene?term=X%5BChr%5D%20AND%20%22Homo%20sapiens%22%5BOrganism%5D%20AND%20%28%22has%20ccds%22%5BProperties%5D%20AND%20alive%5Bprop%5D%29&cmd=DetailsSearch |date=2016-09-08 | accessdate=2017-05-28}}</ref>
| 2016-09-08 
|-
| [[HUGO Gene Nomenclature Committee|HGNC]]|| 825 || 260 || 606
|style="text-align:center"| <ref name="HGNC20170512">{{cite web | title=Statistics & Downloads for chromosome X | website=HUGO Gene Nomenclature Committee | url=http://www.genenames.org/cgi-bin/statistics?c=X  |date=2017-05-12 | accessdate=2017-05-19}}</ref>
| 2017-05-12 
|-
| [[Ensembl genome database project|Ensembl]] || 841 || 639 || 871
|style="text-align:center"| <ref name="Ensembl Release 88">{{cite web | title=Chromosome X: Chromosome summary - Homo sapiens | website= Ensembl Release 88 | url=http://mar2017.archive.ensembl.org/Homo_sapiens/Location/Chromosome?r=X |date=2017-03-29 | accessdate=2017-05-19}}</ref>
| 2017-03-29 
|-
| [[National Center for Biotechnology Information|NCBI]] || 874 || 494 || 879
|style="text-align:center"| <ref name="NCBI coding">{{cite web | title=Search results - X&#91;CHR&#93; AND "Homo sapiens"&#91;Organism&#93; AND ("genetype protein coding"&#91;Properties&#93; AND alive&#91;prop&#93;)  - Gene | website=NCBI | date=2017-05-19 | url=https://www.ncbi.nlm.nih.gov/gene?term=X%5BCHR%5D%20AND%20%22Homo%20sapiens%22%5BOrganism%5D%20AND%20%28%22genetype%20protein%20coding%22%5BProperties%5D%20AND%20alive%5Bprop%5D%29&cmd=DetailsSearch | accessdate=2017-05-20}}</ref><ref name="NCBI noncoding">{{cite web | title=Search results - X&#91;CHR&#93; AND "Homo sapiens"&#91;Organism&#93; AND ( ("genetype miscrna"&#91;Properties&#93; OR "genetype ncrna"&#91;Properties&#93; OR "genetype rrna"&#91;Properties&#93; OR "genetype trna"&#91;Properties&#93; OR "genetype scrna"&#91;Properties&#93; OR "genetype snrna"&#91;Properties&#93; OR "genetype snorna"&#91;Properties&#93;) NOT "genetype protein coding"&#91;Properties&#93; AND alive&#91;prop&#93;) - Gene | website=NCBI | date=2017-05-19 | url=https://www.ncbi.nlm.nih.gov/gene?term=X%5BCHR%5D%20AND%20%22Homo%20sapiens%22%5BOrganism%5D%20AND%20%28%28%22genetype%20miscrna%22%5BProperties%5D%20OR%20%22genetype%20ncrna%22%5BProperties%5D%20OR%20%22genetype%20rrna%22%5BProperties%5D%20OR%20%22genetype%20trna%22%5BProperties%5D%20OR%20%22genetype%20scrna%22%5BProperties%5D%20OR%20%22genetype%20snrna%22%5BProperties%5D%20OR%20%22genetype%20snorna%22%5BProperties%5D%29%20NOT%20%22genetype%20protein%20coding%22%5BProperties%5D%20AND%20alive%5Bprop%5D%29&cmd=DetailsSearch | accessdate=2017-05-20}}</ref><ref name="NCBI pseudo">{{cite web | title=Search results - X&#91;CHR&#93; AND "Homo sapiens"&#91;Organism&#93; AND ("genetype pseudo"&#91;Properties&#93; AND alive&#91;prop&#93;)  - Gene | website=NCBI | date=2017-05-19 | url=https://www.ncbi.nlm.nih.gov/gene?term=X%5BCHR%5D%20AND%20%22Homo%20sapiens%22%5BOrganism%5D%20AND%20%28%22genetype%20pseudo%22%5BProperties%5D%20AND%20alive%5Bprop%5D%29&cmd=DetailsSearch | accessdate=2017-05-20}}</ref>
| 2017-05-19 
|}

The following are some of the genes located on chromosome X:
* [[Apolipoprotein O|APOO]]: encoding [[protein]] Apolipoprotein O
* [[ARMCX6]]: encoding [[protein]] Armadillo repeat containing X-linked 6
* [[Protein BEX1|BEX1]]: encoding [[protein]] Brain-expressed X-linked protein 1
* [[Protein BEX2|BEX2]]: encoding [[protein]] Brain-expressed X-linked protein 2
* [[BEX4]]: encoding [[protein]] Brain expressed, X-linked 4
* [[Coiled-coil domain containing protein 120|CCDC120]]: encoding [[protein]] Coiled coil domain containing protein 120
* [[CCDC22]]: encoding [[protein]] Coiled-coil domain containing 22
* [[CD99L2]]: CD99 antigen-like protein 2
* [[Chordin-like 1|CHRDL1]]: encoding [[protein]] Chordin-like 1
* [[CMTX2]] encoding [[protein]] Charcot-Marie-Tooth neuropathy, X-linked 2 (recessive)
* [[CMTX3]] encoding [[protein]] Charcot-Marie-Tooth neuropathy, X-linked 3 (dominant)
* [[Cancer/testis antigen family 45, member a5|CT45A5]]: encoding [[protein]] Cancer/testis antigen family 45, member A5
* [[CXorf36]]: encoding [[protein]] hypothetical protein LOC79742
* [[CXorf40A]]: Chromosome X open reading frame 40
* [[CXorf49]]: chromosome X open reading frame 49. encoding [[protein]]
* [[CXorf66]]: encoding [[protein]] Chromosome X Open Reading Frame 66
* [[CXorf67]]: encoding [[protein]] Uncharacterized protein CXorf67
* [[DACH2]]: encoding [[protein]] Dachshund homolog 2
* [[EFHC2]]: encoding [[protein]] EF-hand domain (C-terminal) containing 2
* [[ERCC excision repair 6 like, spindle assembly checkpoint helicase|ERCC6L]] encoding [[protein]] ERCC excision repair 6 like, spindle assembly checkpoint helicase
* [[F8A1]]: Factor VIII intron 22 protein
* [[FAM120C]]: encoding [[protein]] Family with sequence similarity 120C
* [[FAM122B]]: Family with sequence similarity 122 member B
* [[FAM122C]]: encoding [[protein]] Family with sequence similarity 122C
* [[FAM127A]]: CAAX box protein 1
* [[FAM50A]]: Family with sequence similarity 50 member A
* [[FATE1]]: Fetal and adult testis-expressed transcript protein
* [[FMR1-AS1 gene|FMR1-AS1]]: encoding a [[long non-coding RNA]] FMR1 antisense RNA 1
* [[FRMPD3]]: encoding [[protein]] FERM and PDZ domain containing 3
* [[FUNDC1]]: encoding [[protein]] FUN14 domain containing 1
* [[FUNDC2]]: FUN14 domain-containing protein 2
* [[G protein nucleolar 3 like|GNL3L]] encoding [[protein]] G protein nucleolar 3 like
* [[GPRASP2]]: G-protein coupled receptor-associated sorting protein 2
* [[GRIPAP1]]: encoding [[protein]] GRIP1-associated protein 1
* [[HDHD1A]]: encoding [[enzyme]] Haloacid dehalogenase-like hydrolase domain-containing protein 1A
* [[LAS1L]] encoding [[protein]] LAS1-like protein
* [[MAGEA2]]: encoding [[protein]] Melanoma-associated antigen 2
* [[MAGEA5]] encoding [[protein]] Melanoma antigen family A, 5
* [[Melanoma antigen family a, 8|MAGEA8]]: encoding [[protein]] Melanoma antigen family A, 8
* [[MAGED4B]]: encoding [[protein]] Melanoma-associated antigen D4
* [[RP11-217H1.1|MAGT1]]: encoding [[protein]] Magnesium transporter protein 1
* [[MBNL3]]: encoding [[protein]] Muscleblind-like protein 3
* [[MIR222]]: encoding [[microRNA]] MicroRNA 222
* [[MIR361]]: encoding [[microRNA]] MicroRNA 361
* [[MIR660]]: encoding [[protein]] MicroRNA 660
* [[MORF4L2]]: encoding [[protein]] Mortality factor 4-like protein 2
* [[Motile sperm domain containing 1|MOSPD1]]: encoding [[protein]] Motile sperm domain containing 1
* [[Motile sperm domain containing 2|MOSPD2]]: encoding [[protein]] Motile sperm domain containing 2
* [[NKRF (gene)|NKRF]]: encoding [[protein]] NF-kappa-B-repressing factor
* [[NRK (gene)|NRK]]: encoding [[enzyme]] Nik-related protein kinase
* [[Otu deubiquitinase 5|OTUD5]]: encoding [[protein]] OTU deubiquitinase 5
* [[PASD1]]: encoding [[protein]] PAS domain-containing protein 1
* [[PBDC1]]: encoding [[]] FALSE
* [[PCYT1B]]: encoding [[enzyme]] Choline-phosphate cytidylyltransferase B
* [[PIN4]]: encoding [[enzyme]] Peptidyl-prolyl cis-trans isomerase NIMA-interacting 4
* [[PLAC1]]: encoding [[protein]] Placenta-specific protein 1
* [[PLP2]]: encoding [[protein]] Proteolipid protein 2
* [[RPA4]]: encoding [[protein]] Replication protein A 30 kDa subunit
* [[RPS6KA6]]: encoding [[protein]] Ribosomal protein S6 kinase, 90kDa, polypeptide 6
* [[RRAGB]]: encoding [[protein]] Ras-related GTP-binding protein B
* [[SFRS17A]]: encoding [[protein]] Splicing factor, arginine/serine-rich 17A
* [[SLITRK2]]: encoding [[protein]] SLIT and NTRK-like protein 2
* [[SMARCA1]]: encoding [[protein]] Probable global transcription activator SNF2L1
* [[SMS (gene)|SMS]]: encoding [[enzyme]] Spermine synthase
* [[SSR4]]: encoding [[protein]] Translocon-associated protein subunit delta
* [[TAF7l]]: encoding [[protein]] TATA-box binding protein associated factor 7-like
* [[TCEAL1]]: encoding [[protein]] Transcription elongation factor A protein-like 1
* [[TCEAL4]]: encoding [[protein]] Transcription elongation factor A protein-like 4
* [[THOC2]]: encoding [[protein]] THO complex subunit 2
* [[TMEM29]]: encoding [[protein]] Protein FAM156A
* [[TMEM47]]: encoding [[protein]] Transmembrane protein 47
* [[TMLHE]]: encoding [[enzyme]] Trimethyllysine dioxygenase, mitochondrial
* [[Tenomodulin|TNMD]] encoding [[protein]] Tenomodulin (also referred to as tendin, myodulin, Tnmd and TeM)
* [[SEDLP|TRAPPC2P1]] encoding [[protein]] Trafficking protein particle complex subunit 2
* [[TREX2]]: encoding [[enzyme]] Three prime repair exonuclease 2
* [[TRO (gene)|TRO]]: encoding [[protein]] Trophinin
* [[TSPYL2]]: encoding [[protein]] Testis-specific Y-encoded-like protein 2
* [[USP51]]: encoding [[enzyme]] Ubiquitin carboxyl-terminal hydrolase 51
* [[YIPF6]]: encoding [[protein]] Protein YIPF6
* [[ZC3H12B]]: encoding [[protein]] ZC3H12B
* [[ZFP92]]: encoding [[protein]] ZFP92 zinc finger protein
* [[ZMYM3]]: encoding [[protein]] Zinc finger MYM-type protein 3
* [[Zinc finger protein 157|ZNF157]]: encoding [[protein]] Zinc finger protein 157
* [[ZNF182]] encoding [[protein]] Zinc finger protein 182
* [[Zinc finger protein 275|ZNF275]]: encoding [[protein]] Zinc finger protein 275
* [[Zinc finger protein 674|ZNF674]]: encoding [[protein]] Zinc finger protein 674

===Structure===

It is theorized by Ross et al. 2005 and Ohno 1967 that the X chromosome is at least partially derived from the autosomal (non-sex-related) genome of other mammals, evidenced from interspecies genomic sequence alignments.

The X chromosome is notably larger and has a more active [[euchromatin]] region than its [[Y chromosome]] counterpart. Further comparison of the X and Y reveal regions of homology between the two. However, the corresponding region in the Y appears far shorter and lacks regions that are conserved in the X throughout primate species, implying a genetic degeneration for Y in that region. Because males have only one X chromosome, they are more likely to have an X chromosome-related disease.

It is estimated that about 10% of the genes encoded by the X chromosome are associated with a family of "CT" genes, so named because they encode for markers found in both tumor cells (in cancer patients) as well as in the human [[testis]] (in healthy patients).<ref name="ross">{{cite journal  |vauthors=Ross M, etal |title=The DNA sequence of the human X chromosome |journal=Nature |volume=434 |issue=7031 |pages=325–37 |year=2005 |url=http://www.nature.com/nature/journal/v434/n7031/full/nature03440.html |doi=10.1038/nature03440 |pmid=15772651 |pmc=2665286}}</ref>

===Role in diseases===
====Numerical abnormalities====
[[Klinefelter syndrome]]:
* Klinefelter syndrome is caused by the presence of one or more extra copies of the X chromosome in a male's cells. Extra genetic material from the X chromosome interferes with male sexual development, preventing the testicles from functioning normally and reducing the levels of [[testosterone]].
* Males with Klinefelter syndrome typically have one extra copy of the X chromosome in each cell, for a total of two X chromosomes and one Y chromosome (47,XXY). It is less common for affected males to have two or three extra X chromosomes (48,XXXY or 49,XXXXY) or extra copies of both the X and Y chromosomes (48,XXYY) in each cell. The extra genetic material may lead to tall stature, learning and reading disabilities, and other medical problems. Each extra X chromosome lowers the child's [[IQ]] by about 15 points,<ref>{{cite web|url=http://emedicine.medscape.com/article/945649-overview#a0104|author1=Harold Chen |author2=Ian Krantz |author3=Mary L Windle |author4=Margaret M McGovern |author5=Paul D Petry |author6=Bruce Buehler |date=2013-02-22 | accessdate=2014-07-18 | title=Klinefelter Syndrome Pathophysiology |work=Medscape}}</ref><ref>{{cite journal |vauthors=Visootsak J, Graham JM |title=Klinefelter syndrome and other sex chromosomal aneuploidies |journal=Orphanet J Rare Dis |volume=1 |issue= |pages=42 |year=2006 |pmid=17062147 |pmc=1634840 |doi=10.1186/1750-1172-1-42 }}</ref> which means  that the average IQ in Klinefelter syndrome is in general in the normal range, although below average.  When additional X and/or Y chromosomes are present in 48,XXXY, 48,XXYY, or 49,XXXXY, developmental delays and cognitive difficulties can be more severe and mild [[intellectual disability]] may be present.
* Klinefelter syndrome can also result from an extra X chromosome in only some of the body's cells. These cases are called mosaic 46,XY/47,XXY.

[[Triple X syndrome]] (also called 47,XXX or trisomy X):
* This syndrome results from an extra copy of the X chromosome in each of a female's cells. Females with trisomy X have three X chromosomes, for a total of 47 chromosomes per cell. The average [[IQ]] of females with this syndrome is 90, while the average [[IQ]] of unaffected siblings is 100.<ref>{{cite book|vauthors=Bender B, Puck M, Salbenblatt J, Robinson A |title=Cognitive development of children with sex chromosome abnormalities |editor=Smith S |location=San Diego |publisher=College Hill Press |year=1986|pages=175–201}}</ref> Their stature on average is taller than normal females.  They are fertile and their children do not inherit the condition.<ref>{{cite web|url=http://ghr.nlm.nih.gov/condition/triple-x-syndrome | title=Triple X syndrome | work=Genetics Home Reference | date=2014-07-14 | accessdate=2014-07-18}}</ref>
* Females with more than one extra copy of the X chromosome (48, [[XXXX syndrome]] or 49, [[XXXXX syndrome]]) have been identified, but these conditions are rare.

[[Turner syndrome]]:
* This results when each of a female's cells has one normal X chromosome and the other sex chromosome is missing or altered. The missing genetic material affects development and causes the features of the condition, including short stature and infertility.
* About half of individuals with Turner syndrome have [[monosomy]] X (45,X), which means each cell in a woman's body has only one copy of the X chromosome instead of the usual two copies. Turner syndrome can also occur if one of the sex chromosomes is partially missing or rearranged rather than completely missing. Some women with Turner syndrome have a chromosomal change in only some of their cells. These cases are called Turner syndrome mosaics (45,X/46,XX).

====Other disorders====
{{Further information|X-linked recessive|X-linked dominant}}
[[XX male syndrome]] is a rare disorder, where the [[SRY]] region of the Y chromosome has recombined to be located on one of the X chromosomes. As a result, the XX combination after fertilization has the same effect as a XY combination, resulting in a male. However, the other genes of the X chromosome cause feminization as well.

[[X-linked endothelial corneal dystrophy]] is an extremely rare disease of cornea associated with Xq25 region. [[Lisch epithelial corneal dystrophy]] is associated with Xp22.3.

[[Megalocornea|Megalocornea 1]] is associated with Xq21.3-q22{{Medical citation needed|date=November 2015}}

[[Adrenoleukodystrophy]], a rare and fatal disorder that is carried by the mother on the x-cell. It affects only boys between the ages of 5 and 10 and destroys the protective cell surrounding the nerves, [[myelin]], in the brain. The female carrier hardly shows any symptoms because females have a copy of the x-cell. This disorder causes a once healthy boy to lose all abilities to walk, talk, see, hear, and even swallow. Within 2 years after diagnosis, most boys with Adrenoleukodystrophy die.

====Role in mental abilities and intelligence====

The X-chromosome has played a crucial role in the development of sexually selected characteristics for over 300 million years. During that time it has accumulated a disproportionate number of genes concerned with mental functions. For reasons that are not yet understood, there is an excess proportion of genes on the X-chromosome that are associated with the development of intelligence, with no obvious links to other significant biological functions.<ref name=":0">{{Cite journal|title = X-linked genes and mental functioning|journal = Human Molecular Genetics|date = 2005-04-15|issn = 0964-6906|pmid = 15809269|pages = R27-32|volume = 14 Spec No 1|doi = 10.1093/hmg/ddi112|first = David H.|last = Skuse}}</ref><ref>{{Cite journal|title = A systems biology approach to identify intelligence quotient score-related genomic regions, and pathways relevant to potential therapeutic treatments|journal = Scientific Reports|date = 2014-02-25|issn = 2045-2322|pmc = 3933868|pmid = 24566931|volume = 4|doi = 10.1038/srep04176|first = Min|last = Zhao|first2 = Lei|last2 = Kong|first3 = Hong|last3 = Qu}}</ref> .In other words, a significant proportion of genes associated with intelligence are passed on to the male offspring from the maternal side and to the female offspring from either/both maternal and paternal side.There has also been interest in the possibility that haploinsufficiency for one or more X-linked genes has a specific impact on development of the [[Amygdala]] and its connections with cortical centres involved in social–cognition processing or the ‘social brain'.<ref name=":0" /><ref>{{Cite journal|title = Variation in the X-linked EFHC2 gene is associated with social cognitive abilities in males|journal = PLOS ONE|date = 2015-01-01|issn = 1932-6203|pmc = 4481314|pmid = 26107779|pages = e0131604|volume = 10|issue = 6|doi = 10.1371/journal.pone.0131604|first = Carla M.|last = Startin|first2 = Chiara|last2 = Fiorentini|first3 = Michelle|last3 = de Haan|first4 = David H.|last4 = Skuse}}</ref>{{Clarify|date=November 2015}}

===Cytogenetic band===
{{multiple image
 | header = G-banding ideograms of human X chromosome 
 | total_width = 400
 | image1 = Human chromosome X ideogram vertical.svg
 | width1 = 216
 | height1= 1125
 | caption1 = G-banding ideogram of human X chromosome in resolution 850 bphs. Band length in this diagram is proportional to base-pair length. This type of ideogram is generally used in genome browsers (e.g. [[Ensembl]], [[UCSC Genome Browser]]).
 | image2 = Human chromosome X - 400 550 850 bphs.png
 | width2 = 1003
 | height2= 2801
 | caption2 = G-banding patterns of human X chromosome in three different resolutions (400,<ref name="400bphs">Genome Decoration Page, NCBI. [ftp://ftp.ncbi.nlm.nih.gov/pub/gdp/ideogram_9606_GCF_000001305.14_400_V1 Ideogram data for Homo sapience (400 bphs, Assembly GRCh38.p3)]. Last update 2014-03-04. Retrieved 2017-04-26.</ref> 550<ref name="550bphs">Genome Decoration Page, NCBI.   [ftp://ftp.ncbi.nlm.nih.gov/pub/gdp/ideogram_9606_GCF_000001305.14_550_V1 Ideogram data for Homo sapience (550 bphs, Assembly GRCh38.p3)]. Last update 2015-08-11. Retrieved 2017-04-26.</ref> and 850<ref name="850bphs">Genome Decoration Page, NCBI.   [ftp://ftp.ncbi.nlm.nih.gov/pub/gdp/ideogram_9606_GCF_000001305.14_850_V1 Ideogram data for Homo sapience (850 bphs, Assembly GRCh38.p3)]. Last update 2014-06-03. Retrieved 2017-04-26.</ref>). Band length in this diagram is based on the ideograms from ISCN (2013).<ref name="Nomenclature2013">{{cite book|author=International Standing Committee on Human Cytogenetic Nomenclature|title=ISCN 2013: An International System for Human Cytogenetic Nomenclature (2013)|url=https://books.google.com/books?id=lGCLrh0DIwEC|year=2013|publisher=Karger Medical and Scientific Publishers|isbn=978-3-318-02253-7}}</ref> This type of ideogram represents actual relative band length observed under a microscope at the different moments during the [[Mitosis|mitotic process]].<ref name="SethakulvichaiManitpornsut2012">{{cite journal|last1=Sethakulvichai|first1=W.|last2=Manitpornsut|first2=S.|last3=Wiboonrat|first3=M.|last4=Lilakiatsakun|first4=W.|last5=Assawamakin|first5=A.|last6=Tongsima|first6=S.|title=Estimation of band level resolutions of human chromosome images|year=2012|pages=276–282|journal=In Computer Science and Software Engineering (JCSSE), 2012 International Joint Conference on|doi=10.1109/JCSSE.2012.6261965|url=https://www.researchgate.net/profile/Anunchai_Assawamakin/publication/261304470_Estimation_of_band_level_resolutions_of_human_chromosome_images/links/5459f7ff0cf2cf516483fffd/Estimation-of-band-level-resolutions-of-human-chromosome-images.pdf}}</ref>
}}
{| class="wikitable" style="text-align:right"
|+ [[G banding|G-band]]s of human X chromosome in resolution 850 bphs<ref name="850bphs"/>
! Chr. 
! Arm<ref>"'''p'''": Short arm; "'''q'''": Long arm.</ref> 
! Band<ref>For cytogenetic banding nomenclature, see article [[Locus (genetics)|locus]].</ref> 
! ISCN<br/>start<ref name="ISCN">These values (ISCN start/stop) are based on the length of bands/ideograms from the ISCN book, An International System for Human Cytogenetic Nomenclature (2013). [[Arbitrary unit]].</ref> 
! ISCN<br/>stop<ref name="ISCN"/>
! Basepair<br/>start 
! Basepair<br/>stop 
! Stain<ref>'''gpos''': Region which is positively stained by [[G banding]], generally [[GC-content|AT-rich]] and gene poor; '''gneg''': Region which is negatively stained by G banding, generally [[GC-content|CG-rich]] and gene rich; '''acen''' [[Centromere]]. '''var''': Variable region; '''stalk''': Stalk.</ref> 
! Density
|-
| X || p 
|style="text-align:left"| 22.33 || 0 || 323 || {{val|1|fmt=commas}} || {{val|4400000|fmt=commas}} 
| style="background:white"| gneg || 
|-
| X || p 
|style="text-align:left"| 22.32 || 323 || 504 || {{val|4400001|fmt=commas}} || {{val|6100000|fmt=commas}} 
|style="background:#979797"| gpos || 50
|-
| X || p 
|style="text-align:left"| 22.31 || 504 || 866 || {{val|6100001|fmt=commas}} || {{val|9600000|fmt=commas}} 
| style="background:white"| gneg || 
|-
| X || p 
|style="text-align:left"| 22.2 || 866 || 1034 || {{val|9600001|fmt=commas}} || {{val|17400000|fmt=commas}} 
|style="background:#979797"| gpos || 50
|-
| X || p 
|style="text-align:left"| 22.13 || 1034 || 1345 || {{val|17400001|fmt=commas}} || {{val|19200000|fmt=commas}} 
| style="background:white"| gneg || 
|-
| X || p 
|style="text-align:left"| 22.12 || 1345 || 1448 || {{val|19200001|fmt=commas}} || {{val|21900000|fmt=commas}} 
|style="background:#979797"| gpos || 50
|-
| X || p 
|style="text-align:left"| 22.11 || 1448 || 1577 || {{val|21900001|fmt=commas}} || {{val|24900000|fmt=commas}} 
| style="background:white"| gneg || 
|-
| X || p 
|style="text-align:left"| 21.3 || 1577 || 1784 || {{val|24900001|fmt=commas}} || {{val|29300000|fmt=commas}} 
|style="background:black; color:white;"| gpos || 100
|-
| X || p 
|style="text-align:left"| 21.2 || 1784 || 1862 || {{val|29300001|fmt=commas}} || {{val|31500000|fmt=commas}} 
| style="background:white"| gneg || 
|-
| X || p 
|style="text-align:left"| 21.1 || 1862 || 2120 || {{val|31500001|fmt=commas}} || {{val|37800000|fmt=commas}} 
|style="background:black; color:white;"| gpos || 100
|-
| X || p 
|style="text-align:left"| 11.4 || 2120 || 2430 || {{val|37800001|fmt=commas}} || {{val|42500000|fmt=commas}} 
| style="background:white"| gneg || 
|-
| X || p 
|style="text-align:left"| 11.3 || 2430 || 2624 || {{val|42500001|fmt=commas}} || {{val|47600000|fmt=commas}} 
|style="background:#636363; color:white;"| gpos || 75
|-
| X || p 
|style="text-align:left"| 11.23 || 2624 || 2948 || {{val|47600001|fmt=commas}} || {{val|50100000|fmt=commas}} 
| style="background:white"| gneg || 
|-
| X || p 
|style="text-align:left"| 11.22 || 2948 || 3129 || {{val|50100001|fmt=commas}} || {{val|54800000|fmt=commas}} 
|style="background:#d9d9d9"| gpos || 25
|-
| X || p 
|style="text-align:left"| 11.21 || 3129 || 3206 || {{val|54800001|fmt=commas}} || {{val|58100000|fmt=commas}} 
| style="background:white"| gneg || 
|-
| X || p 
|style="text-align:left"| 11.1 || 3206 || 3297 || {{val|58100001|fmt=commas}} || {{val|61000000|fmt=commas}} 
|style="background:#6e7f8f; color:white;"| acen || 
|-
| X || q 
|style="text-align:left"| 11.1 || 3297 || 3491 || {{val|61000001|fmt=commas}} || {{val|63800000|fmt=commas}} 
|style="background:#6e7f8f; color:white;"| acen || 
|-
| X || q 
|style="text-align:left"| 11.2 || 3491 || 3620 || {{val|63800001|fmt=commas}} || {{val|65400000|fmt=commas}} 
| style="background:white"| gneg || 
|-
| X || q 
|style="text-align:left"| 12 || 3620 || 3827 || {{val|65400001|fmt=commas}} || {{val|68500000|fmt=commas}} 
|style="background:#979797"| gpos || 50
|-
| X || q 
|style="text-align:left"| 13.1 || 3827 || 4137 || {{val|68500001|fmt=commas}} || {{val|73000000|fmt=commas}} 
| style="background:white"| gneg || 
|-
| X || q 
|style="text-align:left"| 13.2 || 4137 || 4292 || {{val|73000001|fmt=commas}} || {{val|74700000|fmt=commas}} 
|style="background:#979797"| gpos || 50
|-
| X || q 
|style="text-align:left"| 13.3 || 4292 || 4447 || {{val|74700001|fmt=commas}} || {{val|76800000|fmt=commas}} 
| style="background:white"| gneg || 
|-
| X || q 
|style="text-align:left"| 21.1 || 4447 || 4732 || {{val|76800001|fmt=commas}} || {{val|85400000|fmt=commas}} 
|style="background:black; color:white;"| gpos || 100
|-
| X || q 
|style="text-align:left"| 21.2 || 4732 || 4809 || {{val|85400001|fmt=commas}} || {{val|87000000|fmt=commas}} 
| style="background:white"| gneg || 
|-
| X || q 
|style="text-align:left"| 21.31 || 4809 || 5107 || {{val|87000001|fmt=commas}} || {{val|92700000|fmt=commas}} 
|style="background:black; color:white;"| gpos || 100
|-
| X || q 
|style="text-align:left"| 21.32 || 5107 || 5184 || {{val|92700001|fmt=commas}} || {{val|94300000|fmt=commas}} 
| style="background:white"| gneg || 
|-
| X || q 
|style="text-align:left"| 21.33 || 5184 || 5430 || {{val|94300001|fmt=commas}} || {{val|99100000|fmt=commas}} 
|style="background:#636363; color:white;"| gpos || 75
|-
| X || q 
|style="text-align:left"| 22.1 || 5430 || 5701 || {{val|99100001|fmt=commas}} || {{val|103300000|fmt=commas}} 
| style="background:white"| gneg || 
|-
| X || q 
|style="text-align:left"| 22.2 || 5701 || 5843 || {{val|103300001|fmt=commas}} || {{val|104500000|fmt=commas}} 
|style="background:#979797"| gpos || 50
|-
| X || q 
|style="text-align:left"| 22.3 || 5843 || 6050 || {{val|104500001|fmt=commas}} || {{val|109400000|fmt=commas}} 
| style="background:white"| gneg || 
|-
| X || q 
|style="text-align:left"| 23 || 6050 || 6322 || {{val|109400001|fmt=commas}} || {{val|117400000|fmt=commas}} 
|style="background:#636363; color:white;"| gpos || 75
|-
| X || q 
|style="text-align:left"| 24 || 6322 || 6619 || {{val|117400001|fmt=commas}} || {{val|121800000|fmt=commas}} 
| style="background:white"| gneg || 
|-
| X || q 
|style="text-align:left"| 25 || 6619 || 7059 || {{val|121800001|fmt=commas}} || {{val|129500000|fmt=commas}} 
|style="background:black; color:white;"| gpos || 100
|-
| X || q 
|style="text-align:left"| 26.1 || 7059 || 7253 || {{val|129500001|fmt=commas}} || {{val|131300000|fmt=commas}} 
| style="background:white"| gneg || 
|-
| X || q 
|style="text-align:left"| 26.2 || 7253 || 7395 || {{val|131300001|fmt=commas}} || {{val|134500000|fmt=commas}} 
|style="background:#d9d9d9"| gpos || 25
|-
| X || q 
|style="text-align:left"| 26.3 || 7395 || 7602 || {{val|134500001|fmt=commas}} || {{val|138900000|fmt=commas}} 
| style="background:white"| gneg || 
|-
| X || q 
|style="text-align:left"| 27.1 || 7602 || 7808 || {{val|138900001|fmt=commas}} || {{val|141200000|fmt=commas}} 
|style="background:#636363; color:white;"| gpos || 75
|-
| X || q 
|style="text-align:left"| 27.2 || 7808 || 7886 || {{val|141200001|fmt=commas}} || {{val|143000000|fmt=commas}} 
| style="background:white"| gneg || 
|-
| X || q 
|style="text-align:left"| 27.3 || 7886 || 8145 || {{val|143000001|fmt=commas}} || {{val|148000000|fmt=commas}} 
|style="background:black; color:white;"| gpos || 100
|-
| X || q 
|style="text-align:left"| [[Xq28|28]] || 8145 || 8610 || {{val|148000001|fmt=commas}} || {{val|156040895|fmt=commas}} 
| style="background:white"| gneg || 
|}

==See also==
*[[List of X-STR markers]]
*[[Sex linkage]]
*[[X-inactivation]]
*[[Pseudoautosomal region]]
*[[Y chromosome]]

==References==<!-- CytogenetGenomeRes101:266. -->
* ''Earlier versions of this article contain material from the National Library of Medicine (https://web.archive.org/web/20081122151614/http://www.nlm.nih.gov/copyright.html), a part of the National Institutes of Health (USA,) which, as a US government publication, is in the public domain.''
{{reflist}}

==External links==
{{Commons category|X chromosomes}}
* {{cite web | author= National Institutes of Health  | title= X chromosome | work= Genetics Home Reference | url= http://ghr.nlm.nih.gov/chromosome=X| accessdate=2017-05-06}}
* {{Cite web|url=http://web.ornl.gov/sci/techresources/Human_Genome/posters/chromosome/chromoX.shtml|title=X chromosome|website=Human Genome Project Information Archive 1990–2003|access-date=2017-05-06}}

{{Chromosomes}}
{{Chromosome genetics}}
{{Sex (biology)}}

{{DEFAULTSORT:X Chromosome}}
[[Category:Chromosomes]]
[[Category:Chromosomes (human)|Chromosome X]]
[[Category:Cytogenetics]]
[[Category:Biology of gender]]
[[Category:Genes on human chromosome X|*]]