Sickle cell disease is the most prevalent genetically based disease in the United States. Approximately 1 in 12 Americans of African descent are carriers, having one allele coding for HbS and one gene for HbA. About 1 in 375 Americans of African descent are homozygous for HbS and have the active disease. High occurrence of the HbS allele also occurs in people who live, or whose ancestors lived, in certain parts of Asia, the Mediterranean, and the Middle East.
The alpha chain gene is found on chromosome 11. Each gene is made up of a very long strand of nucleotides. In sickle cell disease, there is a change in only one nucleotide in the sequence that codes for the beta chain: A thymine is substituted for an adenine.
Genes code for proteins. Because of that change in one nucleotide, a slightly different protein is produced. HbS differs from HbA by only one amino acid: Glutamic acid in HbA is replaced by valine in the sixth position on the beta chain. The substitution does not affect the hemoglobin molecule's ability to bind with oxygen. HbS can carry oxygen just as effectively as HbA. However, glutamic acid is a hydrophilic ("water-loving") amino acid, whereas valine is hydrophobic ("water-hating"). The valine occurs on the outside of the beta chain. The hydrophobic portions of HbS molecules are attracted to each other. When the concentration of oxygen is
homologous similar in structure alleles particular forms of genes gamete reproductive cell, such as sperm or egg recessive requiring the presence of two alleles to control the phenotype homozygous containing two identical copies of a particular gene a, a nucleotide the building block of RNA or DNA
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