X chromosome


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.

Discovery

It was first noted that the X chromosome was special in 1890 by Hermann Henking in Leipzig. Henking was studying the testicles of Pyrrhocoris and noticed that one chromosome did not take part in meiosis. Chromosomes are so named because of their ability to take up staining. Although the X chromosome could be stained just as well as the others, Henking was unsure whether it was a different class of object and consequently named it X element, which later became X chromosome after it was established that it was indeed a chromosome.
The idea that the X chromosome was named after its similarity to the letter "X" is mistaken. 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. It is entirely coincidental that the Y chromosome, during mitosis, has two very short branches which can look merged under the microscope and appear as the descender of a Y-shape.
It was first suggested that the X chromosome was involved in sex determination by Clarence Erwin McClung in 1901. After comparing his work on locusts with Henking's and others, McClung noted that only half the sperm received an X chromosome. He called this chromosome an accessory chromosome, and insisted that it was a proper chromosome, and theorized that it was the male-determining chromosome.

Inheritance pattern

noticed that a number of possible ancestors on the X chromosome inheritance line at a given ancestral generation follows the Fibonacci sequence. A male individual has an X chromosome, which he received from his mother, and a Y chromosome, which he received from his father. The male counts as the "origin" of his own X chromosome, and at his parents' generation, his X chromosome came from a single parent. The male's mother received one X chromosome from her mother, and one from her father, so two grandparents contributed to the male descendant's X chromosome. The maternal grandfather received his X chromosome from his mother, and the maternal grandmother received X chromosomes from both of her parents, so three great-grandparents contributed to the male descendant's X chromosome. Five great-great-grandparents contributed to the male descendant's X chromosome, etc.

Humans

Function

The X chromosome in humans spans more than 153 million base pairs. 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, 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. 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.

Genes

Number of genes

The following are some of the gene count estimates of human X chromosome. Because researchers use different approaches to genome annotation their predictions of the number of genes on each chromosome varies. Among various projects, the collaborative consensus coding sequence project takes an extremely conservative strategy. So CCDS's gene number prediction represents a lower bound on the total number of human protein-coding genes.
Estimated byProtein-coding genesNon-coding RNA genesPseudogenesSourceRelease date
CCDS804——2016-09-08
HGNC8252606062017-05-12
Ensembl8416398712017-03-29
UniProt839——2018-02-28
NCBI8744948792017-05-19

Gene list

The following is a partial list of genes on human chromosome X. For complete list, see the link in the infobox on the right.

Structure

It is theorized by Ross et al. 2005 and Ohno 1967 that the X chromosome is at least partially derived from the autosomal 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 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 as well as in the human testis.

Role in disease

Numerical abnormalities

Triple X syndrome :
Turner syndrome:
was first discovered in insects, e.g., T. H. Morgan's 1910 discovery of the pattern of inheritance of the white eyes mutation in Drosophila melanogaster. Such discoveries helped to explain x-linked disorders in humans, e.g., haemophilia A and B, adrenoleukodystrophy, and red-green color blindness.

Other disorders

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 is an extremely rare disease of cornea associated with Xq25 region. Lisch epithelial corneal dystrophy is associated with Xp22.3.
Megalocornea 1 is associated with Xq21.3-q22
Adrenoleukodystrophy, a rare and fatal disorder that is carried by the mother on the x-cell. It affects only boys between the ages of 5 and 10 and destroys the protective cell surrounding the nerves, myelin, in the brain. The female carrier hardly shows any symptoms because females have a copy of the x-cell. This disorder causes a once healthy boy to lose all abilities to walk, talk, see, hear, and even swallow. Within 2 years after diagnosis, most boys with Adrenoleukodystrophy die.

Role in mental abilities and intelligence

The X-chromosome has played a crucial role in the development of sexually selected characteristics for over 300 million years. During that time it has accumulated a disproportionate number of genes concerned with mental functions. For reasons that are not yet understood, there is an excess proportion of genes on the X-chromosome that are associated with the development of intelligence, with no obvious links to other significant biological functions. In other words, 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. There has also been interest in the possibility that haploinsufficiency for one or more X-linked genes has a specific impact on development of the Amygdala and its connections with cortical centres involved in social–cognition processing or the ‘social brain'.

Cytogenetic band

Chr.ArmBandISCN
start		ISCN
stop		Basepair
start		Basepair
stop		StainDensity
Xp22.330323gneg
Xp22.32323504gpos50
Xp22.31504866gneg
Xp22.28661034gpos50
Xp22.1310341345gneg
Xp22.1213451448gpos50
Xp22.1114481577gneg
Xp21.315771784gpos100
Xp21.217841862gneg
Xp21.118622120gpos100
Xp11.421202430gneg
Xp11.324302624gpos75
Xp11.2326242948gneg
Xp11.2229483129gpos25
Xp11.2131293206gneg
Xp11.132063297acen
Xq11.132973491acen
Xq11.234913620gneg
Xq1236203827gpos50
Xq13.138274137gneg
Xq13.241374292gpos50
Xq13.342924447gneg
Xq21.144474732gpos100
Xq21.247324809gneg
Xq21.3148095107gpos100
Xq21.3251075184gneg
Xq21.3351845430gpos75
Xq22.154305701gneg
Xq22.257015843gpos50
Xq22.358436050gneg
Xq2360506322gpos75
Xq2463226619gneg
Xq2566197059gpos100
Xq26.170597253gneg
Xq26.272537395gpos25
Xq26.373957602gneg
Xq27.176027808gpos75
Xq27.278087886gneg
Xq27.378868145gpos100
Xq2881458610gneg