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Chromosome X
The female-determining sex chromosome - one of two X’s in typical females, one of two sex chromosomes in typical males. Undergoes X-inactivation to equalise gene dosage.
Schematic of chromosome X showing the short (p) and long (q) arms. Arm ratio per GRCh38 - banding schematic.
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Overview
The X chromosome is one of the two human sex chromosomes, measuring approximately 156 million base pairs and representing roughly 5 per cent of the total genome. It carries approximately 800 protein-coding genes, a substantially higher number than its counterpart, the Y chromosome. Because typical females possess two X chromosomes (46,XX) and typical males carry one X and one Y (46,XY), evolution has equipped cells with a mechanism called X-inactivation to equalise gene dosage between the sexes.
The X chromosome is clinically significant for a wide array of conditions. Many of these follow an X-linked recessive inheritance pattern, meaning males are disproportionately affected because they lack a second X chromosome to compensate for a pathogenic variant. Disorders such as Duchenne muscular dystrophy, haemophilia A, and fragile X syndrome trace their origins to genes on this chromosome. Whole-chromosome abnormalities, including Turner syndrome (monosomy X) and Klinefelter syndrome (47,XXY), also involve the X chromosome and are among the more common sex-chromosome aneuploidies detected through prenatal screening.
The X chromosome is classified as submetacentric, meaning its centromere sits off-centre, creating a short arm (Xp) and a longer arm (Xq). The centromere plays a vital role during cell division, ensuring each daughter cell receives the correct chromosomal complement. Cytogenetically, the X chromosome is a rich landscape of genes and regulatory elements that have been extensively mapped over decades of research.
Structure
Like all human chromosomes, the X chromosome is divided into two arms by its centromere. The short arm, designated Xp, extends from the tip of the chromosome to the centromere, whilst the long arm, Xq, stretches from the centromere to the opposite telomere. The submetacentric position of the centromere means the q arm is noticeably longer than the p arm. Both arms contain genes interspersed with non-coding DNA, regulatory sequences, and repetitive elements that together contribute to chromosome stability and gene regulation.
At the molecular level, the X chromosome's 156 million base pairs of DNA are packaged into a highly organised structure. DNA wraps around histone proteins to form nucleosomes, which then coil into chromatin fibres. During cell division, chromatin condenses further to produce the familiar X-shaped chromosome visible under a microscope at metaphase. This tight packaging allows the enormous length of DNA to fit inside the cell nucleus whilst remaining accessible for transcription and replication. On the X chromosome, one additional layer of regulation exists in females: X-inactivation, a process in which one of the two X chromosomes is largely silenced early in development to prevent a double dose of X-linked gene products.
Video: What is a karyotype?
Notable regions
The cytogenetic band Xp21.2 is home to DMD, the gene encoding dystrophin. Dystrophin is a large structural protein essential for maintaining the integrity of muscle-fibre membranes during contraction. Pathogenic variants that disrupt dystrophin production lead to Duchenne muscular dystrophy, a severe progressive muscle-wasting disorder, or the milder Becker muscular dystrophy, depending on whether the variant abolishes protein function entirely or permits partial activity. Because DMD is X-linked recessive, the condition predominantly affects males, although female carriers may occasionally exhibit mild symptoms due to skewed X-inactivation.
Further along the long arm, the region Xq27.3 contains FMR1, a gene that undergoes an unusual form of mutation involving expansion of a CGG trinucleotide repeat. When the repeat expands beyond 200 copies (the threshold for full mutation), the gene becomes silenced, resulting in fragile X syndrome, the most common inherited cause of intellectual disability. Males with the full mutation typically show moderate to severe learning difficulties, characteristic facial features, and behavioural traits such as social anxiety. Females with the full mutation often have milder presentations due to the presence of a second, typically functioning X chromosome. Intermediate-sized expansions, known as premutations (55-200 copies), can cause fragile X-associated tremor or ataxia syndrome in older adults and fragile X-associated primary ovarian insufficiency in females.
The distal long arm at Xq28 houses F8, which encodes coagulation factor VIII, and nearby at Xq27.1 sits F9, encoding factor IX. Pathogenic variants in F8 cause haemophilia A, whilst those in F9 cause haemophilia B, also known as Christmas disease. Both disorders impair the blood-clotting cascade, leading to prolonged bleeding after injury and spontaneous bleeding into joints and soft tissues. Haemophilia A is more common than haemophilia B, but both follow X-linked recessive inheritance, affecting mainly males and requiring lifelong management with clotting-factor replacement therapy.
One further region of note is Xq13.2, the location of XIST, a long non-coding RNA gene that orchestrates X-inactivation. XIST is transcribed from the X chromosome destined to be silenced and coats that chromosome in cis, recruiting protein complexes that modify chromatin and switch off most genes along its length. This process ensures that females do not produce twice as much X-linked gene product as males, maintaining dosage balance across the sexes.
Notable genes
Beyond the genes already mentioned, the X chromosome carries a wealth of clinically important loci. DMD is particularly notable not only for its size, one of the largest human genes spanning more than 2 million base pairs, but also for the profound impact its loss has on muscle function. GLA, located at Xq22.1, encodes alpha-galactosidase A, a lysosomal enzyme; pathogenic variants cause Fabry disease, a metabolic storage disorder that can affect the kidneys, heart, and nervous system. OTC at Xp11.4 produces ornithine transcarbamylase, a urea-cycle enzyme; deficiency leads to potentially life-threatening hyperammonaemia, particularly in newborn males.
Several genes on the X chromosome contribute to neurodevelopmental conditions. IQSEC2, HUWE1, PHF8, SYN1, and USP9X all play roles in brain development and synaptic function, and pathogenic variants in these genes are associated with X-linked intellectual disability, epilepsy, or autism spectrum features. KDM6A, also X-linked, encodes a histone demethylase implicated in Kabuki syndrome, a multisystem disorder characterised by distinctive facial features, growth delays, and intellectual disability. The breadth of phenotypes linked to X-chromosome genes underscores the chromosome's importance in development, metabolism, and tissue maintenance.
Chromosomal disorders
Turner syndrome arises from complete or partial absence of one X chromosome, most commonly resulting in a 45,X karyotype. Females with Turner syndrome typically present with short stature, ovarian dysgenesis leading to infertility, and an increased risk of congenital heart defects and renal anomalies. Some individuals have mosaic karyotypes, in which only a proportion of cells lack the second X chromosome, leading to milder or more variable features. Diagnosis is often prompted by prenatal ultrasound findings, such as increased nuchal translucency or cystic hygroma, and confirmed by karyotype or chromosomal microarray. Management involves growth hormone therapy, oestrogen replacement at the time of expected puberty, and monitoring for cardiovascular and metabolic complications.
Klinefelter syndrome, in contrast, results from the presence of an extra X chromosome in males, yielding a 47,XXY karyotype. The additional X chromosome is usually a consequence of non-disjunction during parental meiosis. Affected males often have tall stature, reduced testosterone production, small testes, and infertility due to impaired spermatogenesis. Learning difficulties and language delays are common but not universal. Diagnosis may occur in adolescence or adulthood when concerns about development or fertility arise, though increasingly it is identified prenatally through non-invasive prenatal testing or after incidental findings on karyotype. Testosterone replacement therapy can improve some physical and psychological features, whilst fertility treatment options are evolving.
Fragile X syndrome, although not a whole-chromosome disorder, represents a prominent example of a structural abnormality on the X chromosome. The CGG-repeat expansion in FMR1 silences the gene and eliminates production of fragile X mental retardation protein, which is crucial for synaptic plasticity and brain development. The clinical picture includes intellectual disability, behavioural challenges, characteristic facial features such as a long face and large ears, and in some individuals, features overlapping with autism spectrum disorder. Fragile X syndrome follows X-linked inheritance but exhibits non-Mendelian expansion dynamics, as the repeat can grow across generations, a phenomenon known as anticipation. Female carriers of premutations may remain unaffected or develop milder cognitive issues, but they are at risk of transmitting an expanded full mutation to their children.
Prenatal screening & diagnosis
Abnormalities of the X chromosome can be detected through several prenatal pathways. Non-invasive prenatal testing analyses cell-free foetal DNA in maternal blood and can screen for common sex-chromosome aneuploidies, including monosomy X (Turner syndrome) and 47,XXY (Klinefelter syndrome), alongside the autosomal trisomies. A positive NIPT result prompts referral for diagnostic testing, typically chorionic villus sampling or amniocentesis, followed by karyotyping or chromosomal microarray to confirm the finding and clarify the exact chromosomal constitution.
Ultrasound can reveal features suggestive of Turner syndrome, such as nuchal oedema, coarctation of the aorta, or horseshoe kidney, prompting further investigation. Combined first-trimester screening primarily targets Down syndrome and other autosomal trisomies. Separate non-invasive prenatal testing can assess sex-chromosome aneuploidies. For families with a known history of X-linked conditions such as fragile X syndrome, Duchenne muscular dystrophy, or haemophilia, targeted molecular testing or preimplantation genetic testing may be offered through specialist NHS genomic services. Carrier screening for fragile X premutations is available in some contexts, particularly for women with a family history of intellectual disability or primary ovarian insufficiency.
Inheritance & recurrence
Most cases of Turner syndrome and Klinefelter syndrome arise de novo, meaning they are not inherited from a parent but result from a random error in chromosome segregation during the formation of egg or sperm. The recurrence risk for future pregnancies is therefore low, typically no higher than the population baseline, though maternal age is a recognised risk factor for non-disjunction events. Mosaic forms of these conditions, in which only some cells carry the abnormal karyotype, can occasionally arise post-fertilisation and may present with milder or more variable features.
X-linked recessive disorders such as Duchenne muscular dystrophy, haemophilia, and fragile X syndrome follow predictable inheritance patterns. A female carrier has a 50 per cent chance of passing the pathogenic variant to each child. Sons who inherit the variant will typically be affected, whilst daughters who inherit it will usually be carriers themselves, though they may exhibit mild symptoms depending on X-inactivation patterns. Males with X-linked recessive conditions cannot pass the disorder to their sons, because sons receive the father's Y chromosome, but all daughters will be obligate carriers. Recurrence-risk counselling and options for prenatal or preimplantation diagnosis are available through NHS clinical genetics services, which can provide tailored risk assessments and support family planning decisions.
Frequently asked questions
Why do males and females have different numbers of X chromosomes?
Typical females inherit one X chromosome from each parent, resulting in two X chromosomes (46,XX), whilst typical males inherit an X from their mother and a Y from their father (46,XY). This difference in sex-chromosome composition is the primary genetic basis for biological sex determination in humans.
What is X-inactivation and why does it happen?
X-inactivation is a process that occurs early in female development in which one of the two X chromosomes in each cell is randomly silenced. This prevents females from producing twice the amount of X-linked gene products compared to males, who have only one X chromosome. The inactive X chromosome remains condensed and largely transcriptionally silent throughout the life of that cell and its descendants.
Can females be affected by X-linked recessive conditions?
Females can be affected by X-linked recessive conditions, though it is less common than in males. This can occur if a female inherits pathogenic variants on both X chromosomes, if X-inactivation is heavily skewed so that the chromosome carrying the normal copy is preferentially silenced, or in the context of Turner syndrome where only one X chromosome is present. Female carriers may also experience mild symptoms in some X-linked disorders.
How are X-chromosome abnormalities detected during pregnancy?
X-chromosome abnormalities such as Turner syndrome or Klinefelter syndrome can be detected through non-invasive prenatal testing, which analyses foetal DNA in maternal blood, or through diagnostic procedures such as chorionic villus sampling or amniocentesis followed by karyotyping. Ultrasound findings, such as increased nuchal translucency or structural anomalies, may also prompt further chromosomal testing.
If my child is diagnosed with an X-linked disorder, what does that mean for future pregnancies?
The recurrence risk depends on whether you or your partner carry the pathogenic variant. For X-linked recessive conditions, a carrier mother has a 50 per cent chance of passing the variant to each child; sons who inherit it will typically be affected, whilst daughters will usually be carriers. Your NHS clinical genetics team can provide a personalised risk assessment, discuss reproductive options including prenatal diagnosis or preimplantation genetic testing, and support informed decision-making.
Launch-set genes on chromosome x
Genes on this chromosome that currently have dedicated pages in the library. This list grows as we publish more.