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Gene mutation in sickle cell anaemia and chromosome mutation in Down's syndrome

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Sickle cell anaemia

Sickle cell anaemia is a part of sickle cell disease which is a genetic condition affecting the haemoglobin in our red blood cells. This impairs its function of carrying oxygen in the blood and hence can cause symptoms of anaemia such as dizziness, rapid heart rate and fatigue.

Quite rarely, a condition is caused by a simple point mutation of just one DNA base. This is the case in sickle cell anaemia. The single change, in this case a substitution, happens to result in a different amino acid being coded for altogether, as the codon the mutated base is part of codes for valine instead of glutamic acid in this case.

This results in different properties in the new haemoglobin, and as red blood cells contain millions of haemoglobin molecules, it alters the red blood cell structure and function too.

They become sticky and compromise circulation. The different shape of the cell, resembling a sickle, gives this condition its name. In areas where HIV is endemic, sickle cell disease has spread significantly. Despite it being a detrimental feature, it seems to confer some resistance if HIV infection has taken place in the same individual. In this way the condition has persisted since it is advantageous to those who also have HIV.

Down's syndrome

During meiosis, different chromosome distribution in the gametes can occur which can leave them without the expected number of chromosomes. If there are more chromosomes than expected (2 of each in humans), this is termed polysomy and can result in conditions such as Down's syndrome.

Down's syndrome involves an extra chromosome 21, and expresses itself in terms of many different features, some of which are detrimental to health. Common outcomes include unique facial features, slower overall development, higher incidence of congenital heart abnormalities, decreased or absent fertility and overall lower life expectancy.

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