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Every somatic cell in a female has 2 X chromosomes and one is randomly silenced ensuring that men and women have the same number of active genes.
Silencing of one X chromosome is the result of attachment of methyl groups to DNA.
X chromosome
The X chromosome is one of the two sex-determining chromosomes
It is found in both males and females.
It is a part of the XY sex-determination system and X0 sex-determination system.
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.
A male individual has an X chromosome, which he received from his mother, and a Y chromosome, which he received from his father.
The Barr body identifies the inactive X.
The X chromosome in humans spans more than 153 million base pairs of DNA.
The X-chromosome 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.
The X chromosome carries about 1500 genes, more than any other chromosome in the human body.
Individuals usually have 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.
Genetic disorders that are due to mutations in genes on the X chromosome are described as X linked.
Examples of mutations on the X chromosome include more common diseases such as color blindness, hemophilia, and fragile-X syndrome.
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.
X-linked diseases include hemophilia and red-green colorblindness.
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, that is cells other than egg and sperm cells: called X-inactivation or Lyonization, and creates a Barr body.
The partial inactivation of the X-chromosome is due to repressive heterochromatin that compacts the DNA and prevents the expression of most genes.
The X chromosome is larger and has a more active chromatin region than its Y chromosome counterpart.
Because males have only one X chromosome, they are more likely to have an X chromosome-related disease.
Klinefelter syndrome is caused by the presence of one or more extra copies of the X chromosome in a male’s cells.
The extra genetic material from the X chromosome interferes with male sexual development.
The extra genetic material from the X chromosome prevents the testicles from functioning normally and reducing the levels of testosterone.
Males with Klinefelter syndrome (47,XXY).
Less commonly 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 in an X chromosome may lead to tall stature, learning and reading disabilities, and other medical problems.
Each extra X chromosome lowers a child’s IQ by about 15 points.
The average IQ in Klinefelter syndrome is in general in the normal range, although below average.
With additional X and/or Y chromosomes as 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, and is called mosaic 46,XY/47,XXY.
Triple X syndrome, 47,XXX or trisomy X, results from an extra copy of the X chromosome in each of a female’s cells.
With trisomy X there are three X chromosomes, for a total of 47 chromosomes per cell.
The average IQ of females with trisomy x syndrome is 90, while the average IQ of unaffected siblings is 100, and their height on average is taller than normal females.
Trisomy x patients are fertile and their children do not inherit the condition.
Females with more than one extra copy of the X chromosome have been identified, but these conditions are rare.
When a female’s cells has one normal X chromosome and the other sex chromosome is missing or altered, it 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.
When chromosomal change occurs in only some of their cells, it is called Turner syndrome mosaics (45,X/46,XX).
X-linked recessive disorders include hemophilia A and B, adrenoleukodystrophy, and red-green color.
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, resulting in a male.
However, the other genes of the X chromosome cause feminization as well.
X-linked endothelial corneal dystrophy is associated with Xq25 region.
Lisch epithelial corneal dystrophy is associated with Xp22.3.
Megalocornea 1 is associated with Xq21.3-q22.
Adrenoleukodystrophy affects only boys between the ages of 5 and 10 and destroys the protective cell surrounding the nerves, myelin, in the brain, and is a fatal disorder that is carried by the mother on the x-cell.
The female carrier hardly shows any symptoms.
The child loses all abilities to walk, talk, see, hear, and even swallow, and within 2 years most boys with Adrenoleukodystrophy die.
There is an excess proportion of genes on the X-chromosome that are associated with the development of intelligence.
A significant proportion of genes associated with intelligence is passed on to the male offspring from the maternal side and to the female offspring from either/both maternal and paternal side.