X chromosome
Encyclopedia
Chromosome
A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...
s in many animal species, including mammals (the other is the Y chromosome
Y chromosome
The Y chromosome is one of the two sex-determining chromosomes in most mammals, including humans. In mammals, it contains the gene SRY, which triggers testis development if present. The human Y chromosome is composed of about 60 million base pairs...
) and is common in both males and females. It is a part of the XY sex-determination system
XY sex-determination system
The XY sex-determination system is the sex-determination system found in humans, most other mammals, some insects and some plants . In this system, females have two of the same kind of sex chromosome , and are called the homogametic sex. Males have two distinct sex chromosomes , and are called...
and X0 sex-determination system
X0 sex-determination system
The X0 sex-determination system is a system that hymenopterans, grasshoppers, crickets, cockroaches, and some other insects use to determine the sex of their offspring. In this system, there is only one sex chromosome, referred to as X. Males only have one X chromosome , while females have two...
. 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.
Function
The X chromosome in humans spans more than 153 million base pairs (the building material of DNADNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
). It represents about 2000 out of 20,000 - 25,0000 genes. Normal human females have 2 X-chromosomes (XX), for a total of nearly 4000 "sex-tied" genes (many of which have nothing to do with sex, other than being attached to the chromosome that is believed to cause sexual bimorphism (or polymorphism if you count more rare variations). Men have, depending on the study, 1400-1900 fewer genes, as the Y chromosome is thought to have only 45 - 78 remaining genes down from an estimated 1438 -~2000 (again, depending on source).
Each person normally has one pair of sex chromosomes in each cell. Females have two X chromosomes, whereas males have one X and one Y chromosome
Y chromosome
The Y chromosome is one of the two sex-determining chromosomes in most mammals, including humans. In mammals, it contains the gene SRY, which triggers testis development if present. The human Y chromosome is composed of about 60 million base pairs...
. 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.
Identifying genes
Gênes
Gênes is the name of a département of the First French Empire in present Italy, named after the city of Genoa. It was formed in 1805, when Napoleon Bonaparte occupied the Republic of Genoa. Its capital was Genoa, and it was divided in the arrondissements of Genoa, Bobbio, Novi Ligure, Tortona and...
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 genes compared to the Y chromosome containing 78 genes, out of the estimated 20,000 to 25,000 total genes in the human genome. Genetic disorder
Genetic disorder
A genetic disorder is an illness caused by abnormalities in genes or chromosomes, especially a condition that is present from before birth. Most genetic disorders are quite rare and affect one person in every several thousands or millions....
s that are due to mutation
Mutation
In molecular biology and genetics, mutations are changes in a genomic sequence: the DNA sequence of a cell's genome or the DNA or RNA sequence of a virus. They can be defined as sudden and spontaneous changes in the cell. Mutations are caused by radiation, viruses, transposons and mutagenic...
s in genes on the X chromosome are described as X linked.
The X chromosome carries a couple of thousand genes but few, if any, of these have anything to do directly with sex determination. Early in embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
nic development in females, one of the two X chromosomes is randomly and permanently inactivated in nearly all somatic cells (cells other than egg
Ovum
An ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...
and sperm
Spermatozoon
A spermatozoon is a motile sperm cell, or moving form of the haploid cell that is the male gamete. A spermatozoon joins an ovum to form a zygote...
cells). This phenomenon is called X-inactivation
X-inactivation
X-inactivation is a process by which one of the two copies of the X chromosome present in female mammals is inactivated. The inactive X chromosome is silenced by packaging into transcriptionally inactive heterochromatin...
or Lyonization, and creates a Barr body
Barr body
A Barr body is the inactive X chromosome in a female somatic cell, rendered inactive in a process called lyonization, 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...
. 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
Barr body
A Barr body is the inactive X chromosome in a female somatic cell, rendered inactive in a process called lyonization, 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...
may be more biologically active than was previously supposed.
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
Euchromatin
Euchromatin is a lightly packed form of chromatin that is rich in gene concentration, and is often under active transcription. Unlike heterochromatin, it is found in both cells with nuclei and cells without nuclei...
region than its Y chromosome
Y chromosome
The Y chromosome is one of the two sex-determining chromosomes in most mammals, including humans. In mammals, it contains the gene SRY, which triggers testis development if present. The human Y chromosome is composed of about 60 million base pairs...
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).
Numerical abnormalities
Klinefelter's syndromeKlinefelter's syndrome
Klinefelter syndrome, 46/47, XXY, or XXY syndrome is a condition in which human males have an extra X chromosome. While females have an XX chromosomal makeup, and males an XY, affected individuals have at least two X chromosomes and at least one Y chromosome...
:
- 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 testosteroneTestosteroneTestosterone is a steroid hormone from the androgen group and is found in mammals, reptiles, birds, and other vertebrates. In mammals, testosterone is primarily secreted in the testes of males and the ovaries of females, although small amounts are also secreted by the adrenal glands...
. - Males with Klinefelter's 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. The average IQ in Klinefelter syndrome is in the normal range. 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 disabilityIntellectual disabilityIntellectual disability is a broad concept encompassing various intellectual deficits, including mental retardation , deficits too mild to properly qualify as MR, various specific conditions , and problems acquired later in life through acquired brain injuries or neurodegenerative diseases like...
may be present. - 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
Triple X syndrome
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. The condition always produces females, with an XX pair of chromosomes, as well as an additional chromosome, resulting in the formation of XXX. A mosaic form...
(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 their normal siblings is 100 http://www.aaa.dk/TURNER/ENGELSK/TRIEN.HTM. Their stature on average is taller than normal females. They are fertile and their children do not inherit the condition. http://www.library.nhs.uk/genepool/ViewResource.aspx?resID=93920&catID=8930
- Females with more than one extra copy of the X chromosome (48, XXXX syndromeXXXX syndromeXXXX syndrome is a rare chromosomal disorder caused by the presence of four X chromosomes instead of two X chromosomes. This condition occurs only in females, as there are no Y chromosomes present. Tetrasomy X was first described in 1961, and since then approximately 100 cases have been reported...
or 49, XXXXX syndromeXXXXX syndromeXXXXX syndrome is the presence of three additional X chromosomes. Diagnosis is done by karyotyping. Approximately 25 females have been described in medical literature worldwide with this extremely rare condition. The condition was first described in 1963...
) have been identified, but these conditions are rare.
Turner syndrome
Turner syndrome
Turner syndrome or Ullrich-Turner syndrome encompasses several conditions in human females, of which monosomy X is most common. It is a chromosomal abnormality in which all or part of one of the sex chromosomes is absent...
:
- 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 monosomyMonosomyMonosomy is a form of aneuploidy with the presence of only one chromosome from a pair. Partial monosomy occurs when only a portion of the chromosome has one copy, while the rest has two copies.-Human 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
XX male syndromeXX male syndrome
XX male syndrome is a rare sex chromosomal disorder. Usually it is caused by unequal crossing over between X and Y chromosomes during meiosis in the father, which results in the X chromosome containing the normally-male SRY gene...
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
X-linked endothelial corneal dystrophy
X-linked endothelial corneal dystrophy is a rare form of human corneal dystrophy described first in 2006. It is manifested as corneal clouding in the form of a ground glass, milky corneal tissue, and moon crater-like changes of corneal endothelium....
is an extremely rare disease of cornea associated with Xq25 region. Lisch epithelial corneal dystrophy
Lisch epithelial corneal dystrophy
Lisch epithelial corneal dystrophy is a rare form of human corneal dystrophy first described in 1992 by Lisch et al...
is associated with Xp22.3.
Megalocornea 1
Megalocornea
Megalocornea is an extremely rare nonprogressive condition in which the cornea has an enlarged diameter, reaching and exceeding 13 mm. It is noted in some patients with Marfan syndrome. It is thought to have two subforms, one with autosomal inheritance and the other X-linked...
is associated with Xq21.3-q22