X-chromosome

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, which resulted in the naming of its counterpart Y chromosome, for its similarity to the letter "Y".

Structure

The X chromosome in humans spans about 155 million base pairs (the building material of DNA). It represents approximately 5% of the total DNA in women's cells, containing about 2000 genes, compared to the Y chromosome containing just 78 genes.

Genetic disorders

Certain genetic disorders can be traced to the X chromosome. These are known as X-linked disorders. The most common is hemophilia, a disorder that affects the blood's ability to clot. Other X-linked disorders include Duchenne muscular dystrophy, Fragile X syndrome, and X-linked agammaglobulinemia.

Inheritance

In mammals, the X chromosome carries a larger number of genes than the Y chromosome. This means that any gene on the X chromosome can be present in two copies in females (XX) but only one copy in males (XY). This difference in gene dosage is compensated by 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 choice of which X chromosome will be inactivated is random in placental mammals.

See also

• Y chromosome
• Sex-determination system
• Genetic disorder
• X inactivation

References

X-chromosome Resources
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