page_title: X chromosome
section_title: Function
hierarchical_section_title: X chromosome / Humans / Function
caption_attribution_description: English: Nucleus of a female amniotic fluid cell. Top: Both X-chromosome territories are detected by FISH. Shown is a single optical section made with a confocal microscope. Bottom: Same nucleus stained with Dapi and recorded with a CCD camera. The Barr body is indicated by the arrow, it identifies the inactive X (Xi). Preparation of specimen as described in: R Eils, S Dietzel, E Bertin, E Schrock, MR Speicher, T Ried, M Robert-Nicoud, C Cremer and T Cremer (1996): Three-dimensional reconstruction of painted human interphase chromosomes: active and inactive X chromosome territories have similar volumes but differ in shape and surface structure. Journal of Cell Biology, Vol 135, 1427-1440. PMID:8978813. Website. Deutsch: Kern einer weiblichen menschlichen Zelle aus Amnionflüssigkeit. Oben: Darstellung beider X-Chromosomen durch Fluoreszenz-in-situ-Hybridisierung. Gezeigt ist ein einzelner optischer Schnitt, der mit einem konfokalen Laserscanningmikroskop erzeugt wurde. Unten: der gleiche Kern mit Dapi-Färbung, aufgenommen mit einer CCD-Kamera. Das Barr-Körperchen ist hier gut zu erkennen (Pfeil) und identifiziert das inaktive X-Chromosom (Xi). Präparation wie in: R Eils, S Dietzel, E Bertin, E Schrock, MR Speicher, T Ried, M Robert-Nicoud, C Cremer and T Cremer (1996): Three-dimensional reconstruction of painted human interphase chromosomes: active and inactive X chromosome territories have similar volumes but differ in shape and surface structure. Journal of Cell Biology, Vol 135, 1427-1440. PMID:8978813. Website. 한국어: 여성 양수에 떠있는 세포의 핵. 위: FISH(Fluorescence in situ hybridization)법을 통해 두개의 X염색체를 볼 수 있다. 이 사진은 공초점 레이저 현미경에 의해 찍혔다. 아래&#160;: 같은 핵을 다피 염색(DAPI)법을통해 염색하고, CCD 카메라로 찍은 사진이다. 화살표가 가리키는 것이 바소체이고, Xi로 표시된 것이 불활성화된 X염색체이다(inactive X (Xi). 사진에 관한 내용: R Eils, S Dietzel, E Bertin, E Schrock, MR Speicher, T Ried, M Robert-Nicoud, C Cremer and T Cremer (1996): Three-dimensional reconstruction of painted human interphase chromosomes: active and inactive X chromosome territories have similar volumes but differ in shape and surface structure. Journal of Cell Biology, Vol 135, 1427-1440. PMID:8978813. Website.
context_page_description: The X chromosome is one of the two sex-determining chromosomes in many organisms, including mammals, 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, following its subsequent discovery.
context_section_description: 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 typically have two X chromosomes, whereas males typically 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 follows the Fibonacci numbers at a given ancestral depth.
  Genetic disorders that are due to mutations in genes on the X chromosome are described as X linked. If X chromosome has a genetic disease gene, it always causes illness in male patients, since men have only one X chromosome and therefore only one copy of each gene. Females, instead, may stay healthy and only be carrier of genetic illness, since they have another X chromosome and possibility to have healthy gene copy. For example hemophilia and red-green colorblindness run in family this way.
  The X chromosome carries hundreds of genes but few, if any, of these have anything to do directly with sex determination. Early in embryonic development in females, one of the two X chromosomes is randomly and permanently inactivated in nearly all somatic cells (cells other than egg and 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.
  The partial inactivation of the X-chromosome is due to repressive heterochromatin that compacts the DNA and prevents the expression of most genes.  Heterochromatin compaction is regulated by Polycomb Repressive Complex 2 (PRC2).
caption: Nucleus of a female amniotic fluid cell. Top: Both X-chromosome territories are detected by FISH. Shown is a single optical section made with a confocal microscope. Bottom: Same nucleus stained with DAPI and recorded with a CCD camera. The Barr body is indicated by the arrow, it identifies the inactive X (Xi).
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