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Encyclopedia > X chromosome
The X chromosome
The X chromosome

The X chromosome is one of the two sex-determining chromosomes in many animal species, including mammals (the other is the Y chromosome). 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, and this resulted in its counterpart being named the Y chromosome for the next letter in the alphabet when it was discovered later [1]. Image File history File links This is a lossless scalable vector image. ... Image File history File links This is a lossless scalable vector image. ... Image File history File links This is a lossless scalable vector image. ... A sex-determination system is a biological system that determines the development of sexual characteristics in an organism. ... Figure 1: A representation of a condensed eukaryotic chromosome, as seen during cell division. ... The human Y chromosome is one of two sex chromosomes, it contains the genes that cause testis development, thus determining maleness. ... The XY sex-determination system is a well-known sex-determination system. ... The X0 sex-determination system is a system that grasshoppers, crickets, roaches, and some other insects use to determine the sex of their offspring. ...

Contents

In humans

Function

The sex chromosomes are one of the 23 homologous pairs of human chromosomes. The X chromosome spans more than 153 million base pairs (the building material of DNA) and represents about 5% of the total DNA in women's cells, 2.5% in men's. In genetics, two nucleotides on opposite complementary DNA or RNA strands that are connected via hydrogen bonds are called a base pair (often abbreviated bp). ... The structure of part of a DNA double helix Deoxyribonucleic acid (DNA) is a nucleic acid that contains the genetic instructions for the development and function of living organisms. ...


Each person normally has one pair of sex chromosomes in each cell. Females have two X chromosomes, while males have one X and one Y chromosome. The human Y chromosome is one of two sex chromosomes, it contains the genes that cause testis development, thus determining maleness. ...


Identifying genes on each chromosome is an active area of genetic research. Because researchers use different approaches to predict the number of genes on each chromosome, the estimated number of genes varies. The X chromosome contains about 2000[2] genes compared to the Y chromosome containing 78[3] genes, out of the estimated 20,000 to 25,000 total genes in the human genome. Genetic disorders that are due to mutations in genes on the X chromosome are described as X linked. A genetic disorder is a disease caused by abnormalities in genes or chromosomes. ... It has been suggested that mutant be merged into this article or section. ...


The X chromosome carries a couple thousand 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. X-inactivation ensures that females, like males, have one functional copy of the X chromosome in each body cell. It was previously assumed that only one copy is actively used. However, recent research suggests that the Barr body may be more biologically active than was previously supposed.[4] For other uses, see Embryo (disambiguation). ... A human ovum Sperm cells attempting to fertilize an ovum An ovum (plural ova) is a haploid female reproductive cell or gamete. ... A spermatozoon or spermatozoan ( spermatozoa), from the ancient Greek σπέρμα (seed) and (living being) and more commonly known as a sperm cell, is the haploid cell that is the male gamete. ... In those species in which sex is determined by the presence of the Y or W chromosome rather than the diploidy of the X or Z, a Barr body is the inactive X chromosome in a female cell, or the inactive Z in a male. ... The Lyon hypothesis states that in cells with multiple X chromosomes, all but one is inactivated during mammalian embryogenesis. ... In those species in which sex is determined by the presence of the Y or W chromosome rather than the diploidy of the X or Z, a Barr body is the inactive X chromosome in a female cell, or the inactive Z in a male (Lyon, 2003), rendered inactive in... In those species in which sex is determined by the presence of the Y or W chromosome rather than the diploidy of the X or Z, a Barr body is the inactive X chromosome in a female cell, or the inactive Z in a male (Lyon, 2003), rendered inactive in...


Structure

The X-chromosome is a remarkably gene-poor region. It is composed primarily of repeated segments of DNA which do not code for proteins or any known function. Only 1.7% of the chromosome encodes for any functional proteins at all--lowest density of genes to date--and the genes themselves are very short compared to the length of the average human gene. 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). These CT genes found on the X-chromosome are estimated to account for about 90% of all the CT genes encoded within the human genome. Due to their relative abundance, it is thus hypothesized that these genes (and thus the X-chromosome) confer evolutionary fitness to human males. [5]


It is theorized by Ross et al 2005 and Ohno 1967 that the X-chromosome is at least partially derived from the automosal (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 which 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 desease. Euchromatin is a lightly packed form of chromatin that is rich in gene concentration, and is often (but not always) under active transcription. ... The Y chromosome is one of the sex-determining chromosomes in humans and most other mammals (the other is the X chromosome). ...


Role in disease

Image File history File links Chromosome_X_Etude_Inactivation_X.PNG‎ http://fr. ...

Numerical abnormalities

Klinefelter's syndrome: Not to be confused with XYY syndrome or XXX syndrome. ...

  • Klinefelter's 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.
  • Typically, males with Klinefelter's syndrome have one extra copy of the X chromosome in each cell, for a total of two X chromosomes and one Y chromosome (47,XXY). Less commonly, affected males may 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 mental retardation and other medical problems.
  • Klinefelter's 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): Testosterone is a steroid hormone from the androgen group. ... Mental retardation is a term for a pattern of persistently slow learning of basic motor and language skills (milestones) during childhood, and a significantly below-normal global intellectual capacity as an adult. ... Triple X syndrome is a form of chromosomal variation characterized by the presence of an extra X chromosome in each cell of a human female. ...

  • 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 their normal siblings is 100 [1]. Their stature on average is taller than for normal females. They are fertile and their children do not inherit the condition. [2]
  • Females with more than one extra copy of the X chromosome (48,XXXX or 49,XXXXX) have been identified, but these conditions are rare. The extra genetic material may lead to mental retardation and other medical problems.

Turner syndrome: IQ redirects here; for other uses of that term, see IQ (disambiguation). ... IQ redirects here; for other uses of that term, see IQ (disambiguation). ...

  • 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 characteristic features of the condition, including short stature and infertility (the inability to conceive a child).
  • 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).

Aneuploidy is a chromosomal state where abnormal numbers of specific chromosomes or chromosome sets exist within the nucleus. ...

Other conditions

Many diseases are caused by mutations on the X chromosome. These diseases occur far more frequently in males because they only have one X chromosome, so even if the gene is recessive, they will have the disease. Females must receive a copy of the gene from both parents to have such a disease. However, they will still be carriers if they receive one copy of the gene. Recessive genes on the X chromosome that cause serious diseases are usually passed from female carriers to their ill sons and carrier daughters. This is because males, who always have the disease and are not just carriers, would have to father a daughter to pass on the gene. This is unlikely because severe genetic diseases often cause death in childhood or early adulthood. Even those males who survive childhood are unlikely to father children because a sickly male will be less likely to find a mate. However, if the disease shows up late in life, or is not severe, he will pass the gene to all of his daughters. He can not pass it to his sons because a male receives his X chromosome from his mother. A mother with one copy of the gene has a 50% chance of passing it to her children of both sexes, but her daughters will just be carriers of the gene unless their father has it too. It has been suggested that mutant be merged into this article or section. ... In genetics, the term recessive gene refers to an allele that causes a phenotype (visible or detectable characteristic) that is only seen in a homozygous genotype (an organism that has two copies of the same allele). ... In genetics, the term recessive gene refers to an allele that causes a phenotype (visible or detectable characteristic) that is only seen in a homozygous genotype (an organism that has two copies of the same allele). ... Mate may refer to: Relationships: Mate (term), a term for a friend, especially in the United Kingdom and Australasia; also used to address strangers One of a pair of animals, sometimes also applied to a human partner; see mating Nautical: A deck officer on a merchant marine vessel, usually ranked...


Diseases well known for their X-linked recessive inheritance are hemophilia (types A and B), and color blindness. There are few examples of X-linked dominant diseases; the best known in this category is vitamin D resistant rickets. The following genetic disorders are associated with the X chromosome: Haemophilia or hemophilia is the name of any of several hereditary genetic illnesses that impair the bodys ability to control bleeding. ... Color blindness in humans is the inability to perceive differences between some or all colors that other people can distinguish. ... The word dominant has several possible meanings: In music theory, the dominant or dominant note (second most important) of a key is that which is a perfect fifth above the tonic; in just intonation the note whose pitch is 1. ... X-linked hypophosphatemia (hypophosphatemic rickets, vitamin D-resistant rickets) is a form of rickets (or osteomalacia) that differs from most cases of rickets in that ingestion of Vitamin D is relatively ineffective. ...

Alport syndrome is a genetic condition characterized by the progressive loss of kidney function and hearing. ... Androgen insensitivity syndrome (AIS, or Androgen resistance syndrome) is a set of disorders of sexual differentiation that results from mutations of the gene encoding the androgen receptor. ... Beckers muscular dystrophy (also known as Benign pseudohypertrophic muscular dystrophy) is an X-linked recessive inherited disorder characterized by slowly progressive muscle weakness of the legs and pelvis. ... Note: Centronuclear myopathy includes Myotubular myopathy, as outlined below. ... Charcot-Marie-Tooth disease, known also as Hereditary Motor and Sensory Neuropathy (HMSN) or Peroneal Muscular Atrophy, is a heterogeneous inherited disorder of nerves (neuropathy) that is characterized by loss of muscle tissue and touch sensation, predominantly in the feet and legs but also in the hands and arms in... Coffin-Lowry syndrome is a condition associated with mental retardation and delayed development, characteristic facial features, and skeletal abnormalities. ... Duchenne muscular dystrophy (DMD) (also known as muscular dystrophy - Duchenne type) is an eventually fatal disorder that is characterized by rapidly progressive muscle weakness and atrophy of muscle tissue starting in the legs and pelvis and later affecting the whole body. ... Fabrys disease (also known as Anderson-Fabry Disease, Angiokeratoma Corporis Diffusum and Hereditary Dystopic Lipidosis) is a lysosomal storage disease resulting from an X-linked mutation. ... Fragile X syndrome, also known as the MikyD syndrome, is a syndrome of X-linked mental retardation. ... Glucose-6-phosphate dehydrogenase (G6PD) deficiency is an X-linked recessive hereditary disease featuring nonimmune hemolytic anemia in response to a number of causes. ... Haemophilia or hemophilia is the name of any of several hereditary genetic illnesses that impair the bodys ability to control bleeding. ... Incontinentia pigmenti (IP) is a disorder that affects the skin, hair, teeth, and nails. ... -1... Menkes disease, also called the kinky hair disease or Menkes kinky hair syndrome, is a disease of abnormal copper metabolism. ... Note: Centronuclear myopathy includes Myotubular myopathy, as outlined below. ... Nonsyndromic deafness is hearing loss that is not associated with other signs and symptoms. ... Ornithine transcarbamylase deficiency (OTC), the most common of the Urea Cycle Disorders, is a rare metabolic disorder, occurring in one out of every 40000 births. ... A woman with Retts Syndrome Rett syndrome is a neurodevelopmental disorder that is classified as a pervasive developmental disorder by the DSM-IV. Many [1] argue that this is a mis-classification just as it would be to include such disorders as fragile X syndrome, tuberous sclerosis, or Down... It has been suggested that this article or section be merged into Severe combined immunodeficiency. ... X-linked agammaglobulinemia (also called X-linked hypogammaglobulinemia, XLA, Bruton type agammaglobulinemia) is a rare X-linked genetic disorder that affects the bodys ability to fight infection (origin of the name: A=no, gammaglobulin=Antibody). ... The porphyrias are inherited or acquired disorders of certain enzymes in the heme biosynthetic pathway (also called porphyrin pathway). ...

See also

It has been suggested that sex chromosome be merged into this article or section. ... The following DXS (DNA X-chromosome Segment) markers are commonly used in genealogical DNA testing. ...

References

  • Earlier versions of this article contain material from the National Library of Medicine (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.
  1. ^ Angier, Natalie (2007-05-01). For Motherly X Chromosome, Gender Is Only the Beginning. New York Times. Retrieved on 2007-05-01.
  2. ^ Macmillan Science Library (2001). Genetics on X Chromosome.
  3. ^ Richard Harris (2003). Scientists Decipher Y Chromosome.
  4. ^ Carrel L, Willard H (2005). "X-inactivation profile reveals extensive variability in X-linked gene expression in females". Nature 434 (7031): 400-4. DOI:10.1038/nature03479. 
  5. ^ Ross M et al. (2005). "The DNA sequence of the human X chromosome". Nature 434 (7031): 325-37. DOI:10.1038/nature03440. 

  Results from FactBites:
 
Chromosome - MSN Encarta (960 words)
In the cells of most organisms that reproduce sexually, chromosomes occur in pairs: One chromosome is inherited from the female parent, and one is inherited from the male parent.
Both males and females inherit one sex chromosome from the mother (always an X chromosome) and one sex chromosome from the father (an X in female offspring and a Y in male offspring).
In males, which carry an X and a Y chromosome, some genes found on the X chromosome may be missing on the Y chromosome.
X chromosome Information Center - x chromosome and y chromosome (1150 words)
Early in embryonic development picture of the x chromosome in females, one of the two X chromosomes is randomly and permanently inactivated in somatic cells (cells other than egg and sperm cells).
Chromosomal conditions involving the sex chromosomes often affect sex determination (whether a person has the sexual x chromosome disorders characteristics of a male or a female), sexual development, x chromosome abnorality genetic testing x chromosome and the ability to have children (fertility).
Recessive genes on the X chromosome that cause serious diseases are usually passed fragile x chromosome carrier from female carriers to their sons and daughters.
  More results at FactBites »

 
 

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