SICKLE CELL ANEMIA
Hemoglobin S
Jeanette Becker -Greenbelt Middle School
Summary
Sickle cell anemia is a genetic disorder most commonly found in those individuals of African descent. Those individuals who have this disorder all have a mutation on the same gene. The mutation affects the protein within the hemoglobin of the red blood cell. This mutation is noted as HbS. One in every 400 African-Americans is affected with the gene inherited from each parent thus, causing Sickle Cell Disease.
Inheritance of Genetic Traits
In order for an individual to inherit the Sickle Cell Disease they must inherit the sickle cell trait from each parent. The child must receive an affected allele from each parent. The parent having two genes controlling the production of red blood cells in his/her own body, will have one gene available in the gamete that is to be passed on to the offspring. The parent does not have to have the Sickle Cell disease in order pass the trait on to the child. In order to be positively diagnosed with the sickle cell disease a specialized blood test, the hemoglobin electrophoresis, must be performed. Hemoglobin, a protein, is therefore made from a combination of amino acids. This particular gene for the oxygen transporting protein is located on chromosome #11. The amino acid valine is found in the sixth position in an individual with sickle cell. The normal amino acid in the sixth position would be glutamic.
In determining the possibilities of reproducing a child with an inherited genetic disease, such as sickle cell, one must consider the alleles of the parents. When a parent is a carrier for HbS he/she has one normal genotype and one affected allele. (Thus a 50% chance of passing the abnormal gene on to the zygote in any conception). If both parents have this heterozygous make up then the child may indeed receive a second affected allele, thus resulting in sickle cell disease. In this heterozygous combination, neither parent is affected by the symptoms nor the complications of the disorder.
One of the characteristic symptoms of sickle cell includes the abnormal development of the red blood cells (RBC). This sickling (C-like shape) of the RBC is a result of the bundling of the hemoglobin when they lose their oxygen. In other words, when the sickled hemoglobin molecules are attached with oxygen they exist independently, but when they lose their oxygen they gather into bundles. This continuous bundling and unbundling causes the distortion in the hemoglobin and eventually the red blood cells in which they exist. The RBC then are destroyed more easily causing anemia.
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In the United States having the sickle cell trait or the disorder doesn't seem to be advantageous. This is not so in certain areas of tropical and subtropical Africa where the malaria parasite is prevalent. It seems that the mutations causing HbS are beneficial to many of the black Africans when they appear as heterozygous combinations. The mosquito responsible for transmitting the parasite for malaria to humans depends on normal red blood cells for the completion of its life cycle. When, if fact, the cells are abnormal, having valine in the sixth position it blocks the parasite from reproducing so that the process can not be completed. This natural protection against malaria in essence has saved the life of the heterozygous individual. In this situation the abnormal allele makes this a superior mutated condition compared to the normal RBC under the attack of the malarian parasite. Since the United States does not fall within the latitudinal constraints of the tropics or subtropics the presence of the mosquito causing malaria existing in the country is minimal. Therefore, in the U.S. a herterozygote has no real advantage over the homozygote dominant individual.
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