Those who carry two SS alleles suffer from sickle cell anemia (Human Genome Project Information, 2008; Genetics, 2000; Gene Gateway- Exploring Genes and Genetic Disorders, 2005; Gene Gateway- Exploring Genes and Genetic Disorders, 2003). Inheritance pattern of sickle cell trait/disease When both the parents are carriers of the sickle cell disease (carry one normal and one abnormal allele), each of their children have a 25% chance of inheriting two normal genes; 50% chance of inheriting one normal and one abnormal gene and thereby would only be carriers of the sickle cell trait. There is a 25% chance that each child might inherit both the abnormal genes and become individuals with the sickle cell disease (Gene Gateway- Exploring Genes and Genetic Disorders, 2005; National Heart Lung and Blood Institute).
The inheritance pattern for the sickle cell disease can be illustrated as follows In the mutated S allele, a point mutation results in the insertion of a hydrophobic valine amino acid instead of the normal hydrophilic glutamic acid in the β globin gene of the hemoglobin moiety (HBB), found in the short arm of chromosome 11.
The genetic location of the HBB gene is shown below This insertion results in a change in the three-dimensional structure of the hemoglobin protein upon unloading the oxygen to the tissues resulting in clumping together and sickling of red blood cells (Genetics, 2000; Gene Gateway- Exploring Genes and Genetic Disorders, 2005; Gene Gateway- Exploring Genes and Genetic Disorders, 2003). Mechanism of action The severity of the disorder is most prominent among individuals who carry the recessive allele (SS). The substitution of the hydrophobic valine amino acid results in the presence of an outer hydrophobic region that causes the adjacent hemoglobin molecules to stick to each other.
Thus the hemoglobin molecules polymerize, after unloading oxygen to the tissues, to give rise to rigid or rod-like structures that eventually leads to sickling of the cells. When these molecules return back to the lungs for oxygen uptake they depolymerize and assume their native structure. This repeated shunting in their structural configuration leads to rigidity of the cells and in this state they can cause blockages of several small blood vessels which they might enter.
This finally leads to oxygen deprivation of the tissue, pain and tissue/ organ damage. In addition to the above, sickling also results in a decrease of the life span of the red cells.
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