Chromosome X

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Chromosome X is one of the two sex-determining chromosomes found in humans and many other animals, the other being the Y chromosome. In a normal human female, there are two X chromosomes (these are known as homogametic), and in a normal male there is one X and one Y (these are heterogametic)[1].

An X chromosome is much larger than a Y chromosome. As human males have only one X chromosome, the likelihood of inheriting an X chromosome linked disease in males is much greater than females. 

X Chromosome related illnesses

Many X chromosome related diseases result from an abnormal number of chromosomes due to nondisjunction during meiosis. Some examples of these include:

There are also many X linked diseases which can be inherited from parents. If only the mother is affected by the disease (either genotypcially or phenotypically), a son is more likely to have the disease. This is because they only have one X chromosome so the affected gene will be expressed. A daughter will have a 50% chance of inheriting the disease. If only the father has the disease, 100% of daughters will be carriers as they inherit an X chromosome from each parent. A son will not inherit the disease as they inherit their X chromosomes maternally[5].

X chromosome Inactivation

X chromosome inactivation is the process by which all but one X chromosomes are defunctionalized in a cell and become barr bodies[6].


  1. Ross, MT et al. (2005). The DNA sequence of the human X chromosome. Nature, 434:325-337
  2., (2014). WHO | Gender and Genetics. [online] Available at: [Accessed 26 Nov. 2014].
  3., (2014). WHO | Gender and Genetics. [online] Available at: [Accessed 26 Nov. 2014].
  4., (2014). WHO | Gender and Genetics. [online] Available at: [Accessed 26 Nov. 2014].
  6. Ahn, J. and Lee, J. (2008) X chromosome: X inactivation. Nature Education, 1(1):24

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