X-linked

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An X-linked gene is a gene which is coded for on the X chromosome. Mutations of said gene are therefore more likely to be presented in males as males have only one copy of the gene.

X-linked diseases are a caused by a single gene disorder, cause by the sex chromosomes which are passed down through generations. Males have the sex chromosomal configuration of XY whereas females have the configuration of XX. 

In recessive X-linked diseases, females have to inherit two copies of the mutant X chromosome from their parents before they will show any characteristics of the diseases, due to them having two X chromosomes. If they only inherit one, the other X chromosome will be able to mask this mutation and instead will just be a carrier of the disease. As males only carrying one X chromosome, they are more likely to inherit the disorder as their Y chromosomes may not be able to mask the disorder. This is due to their Y chromosome being a lot smaller than the X chromosome, containing less genes than the X chromosome, and therefore less likely that the Y chromosome will mask the recessive mutation.

Recessive X-linked disorders include: red-green blindness, haemophilia, Duchenne Muscular Dystrophy and Becker Muscular Dystrophy [1].

Duchenne Muscular Dystrophy starts to affect boys when they are very young, which causes progressive muscle weakness.[2]

Haemophilia is a very serious recessive X-linked disorder as it caused by the failure of the clotting proteins. Therefore causing any injured person with the disease to bleed to death. This disease has been associated with royalty, as England's Queen Victoria was a carrier and she passed this disease down through the generations, causing multiple problems mainly amongst her male descendants [3].

Dominant X-linked diseases are not as common as recessive X-linked disorders. The number of females and males affected by dominant X-linked diseases are very similar, as only one X chromosome has to be inherited for the traits of the disease to be expressed.

Some dominant X-linked diseases include: some form of rickets, Coffin-Lowry syndrome and Hereditary nephritis [4].

Patterns of inheritance in X-linked diseases are complicated by the fact that fathers always pass their X chromosome to their daughters where-as mothers pass the X chromosome to daughters and sons equally.

References

  1. http://genome.wellcome.ac.uk/doc_WTD020851.html
  2. http://www.geneticdisordersuk.org/aboutgeneticdisorders/inheritance/xlinked
  3. http://genome.wellcome.ac.uk/doc_WTD020851.html
  4. http://www.rightdiagnosis.com/x/x_linked_dominant_genetic_diseases/intro.htm
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