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Sickle cell anaemia is caused by a mutation in the β-globin chain of hemoglobin, replacing glutamic acid with less polar valine at the sixth amino acidIn chemistry, an amino acid is any molecule that contains both amino and carboxylic acid functional groups. In biochemistry, this shorter and more general term is frequently used to refer to alpha amino acids: those amino acids in which the amino and carb position. The association of two wild typeIn biology, a wild type is one of the major genotypes of a species that occur in nature, in contrast to induced mutations or artificial cross-breeding. Classical genetics. α-globin subunits with two mutant β-globin subunits forms hemoglobin S , which polymerises under low oxygen conditions causing distortion of red blood cells and a tendency for them to lose their elasticity.
At the onset of the disease, red blood cells are capable of regaining their original shape and elasticity when oxygen concentration increases. However, with repeated bouts of low oxygen conditions, red blood cells permanently lose their elasticity. These rigid red blood cells are unable to flow through narrow capillaries, causing vessel occlusion and ischemia.
The alleleAn allele is any one of a number of alternative forms of the same gene occupying a given locus (position) on a chromosome. An example is the gene for blossom color in many species of flower a single gene controls the color of the petals, but there may be responsible for sickle cell anaemia is incompletely (autosomal) recessiveIncomplete dominance and the closely related co-dominance are terms in genetics that refer to the situation where an organism inherits a blended or combined phenotype instead of just the dominant trait, when both the dominant and recessive alleles are pre. A person who receives the defective gene from both father and mother develops the disease; a person who receives one defective and one healthy allele remains healthy, but can pass on the disease and is known as a carrierAn asymptomatic carrier (or carrier , is a person who is infected with the agent of an infectious disease, or carries the abnormal gene of a recessive genetic disorder. Although unaffected themselves, a carrier can transmit it to others. Mary Mallon, know. If two parents who are carriers have a child, there is a 1-in-4 chance of their child developing the illness and a 1-in-2 chance of their child just being a carrier.
The gene defect is a known mutation of a single nucleotideA nucleotide is an organic molecule consisting of a nitrogenous heterocyclic base (a purine or a pyrimidine), a pentose sugar ( deoxyribose in DNA or ribose in RNA), and a phosphate or polyphosphate group. A nucleoside is similar, except that it contains (U to A) of the β-globin gene, which results in glutamic acid to be substituted for valine at position 6. Hemoglobins with this mutation are referred to as HbS, as opposed to the more normal adult HbA. This is normally a benign mutation, causing no apparent effects on the secondary, tertiary, or quaternary structure of hemoglobin. What it does allow for, under conditions of low oxygen concentration, is the polymerization of the HbS itself. In people heterozygous for HbS (carriers), the polymerization problems are minor. In people homozygous for HbS, the presence of long chain polymers of HbS distort the shape of the red blood cell, from a smooth doughnut-like shape to ragged and full of spikes, making it fragile and susceptible to breaking within capillaries. Carriers only have suymptoms if they are deprived of oxygen (for example, climbing a mountain) will they develop symptoms.
The sufferers of the illness usually die young and yet the disease is not eliminated from the gene pool by natural selection. This is because carriers are relatively resistant to malaria. Carriers of the allele have an unsymptomatic condition called sickle cell trait. Since the gene is incompletely recessive, carriers have a few sickle red blood cells at all times, not enough to cause symptoms, but enough to give resistance to malaria. Because of this, heterozygotes have a higher fitness than either of the homozyogotes. This is known as heterozygote advantage.
The malaria parasite has a complex life cycle and spends part of it in red blood cells. In a carrier, the presence of the malaria parasite causes the red blood cell to rupture, making the plasmodium unable to reproduce. Further, the polymerization of Hb affects the ability of the parasite to digest Hb in the first place. Therefore, in areas where malaria is a problem, people's chances of survival actually increase if they carry sickle cell anaemia.
Due to the above phenomenon, the illness is still prevalent, especially among people with recent ancestry in malaria-striken areas, such as Africa, the Mediterranean, India and the Middle East. In fact, sickle-cell anaemia is the most common genetic disorder among African Americans; about 1 in every 13 is a carrier.
The evolution of sickle-cell anaemia is probably an example of Baldwinian evolution, whereby humans modify their environment and thus change the selective pressures. As humans in tropical areas in Africa and elsewhere developed agriculture and animal husbandry, they expanded the niche for Anopheles mosquitoes that could transmit the malaria parasite.
It is interesting that in the USA, where there is no endemic malaria, the incident of sickle cell anaemia amongst people of African descent is much lower than in West Africa and falling. Without endemic malaria from Africa, the condition is purely disadvantageous, and will tend to be bred out of the affected population.