Chromosome X
Chromosome X is one of the two sex-determining chromosomes found in humans and many other animals,(the other is a 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:
- Klinefelter syndrome - this is caused by the presence of at least one extra X chromosome in a male karyotype giving a 47,XXY karyotype or less frequently 48,XXXY, 49,XXXXY and 48,XXYY karyotypes. The extra X chromosome interferes with the males sexual development and reduces levels of testosterone.[2]
- Turner syndrome - this is caused by the presence of only one X chromosome giving a 45,X karyotype. This results in altered development, short stature and infertility.[3]
- Trisomy X - this is caused by an extra copy of an X chromosome in female cells and gives a 47,XXX karyotype. It shows no obvious characteristics different to the rest of the female population except accelerated growth and speech and language disabilities.[4]
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 is the process by which all but one X chromosomes are defunctionalized in a cell and become barr bodies.[6]
References
- ↑ Ross, MT et al. (2005). The DNA sequence of the human X chromosome. Nature, 434:325-337
- ↑ Who.int, (2014). WHO | Gender and Genetics. [online] Available at: http://www.who.int/genomics/gender/en/index1.html#KlinefelterSyndrome [Accessed 26 Nov. 2014].
- ↑ Who.int, (2014). WHO | Gender and Genetics. [online] Available at: http://www.who.int/genomics/gender/en/index1.html#Turnersyndrome [Accessed 26 Nov. 2014].
- ↑ Who.int, (2014). WHO | Gender and Genetics. [online] Available at: http://www.who.int/genomics/gender/en/index1.html#XXXFemales [Accessed 26 Nov. 2014].
- ↑ http://genome.wellcome.ac.uk/doc_WTD020851.html
- ↑ Ahn, J. and Lee, J. (2008) X chromosome: X inactivation. Nature Education, 1(1):24