Final answer:
The βs allele leads to the substitution of glutamic acid with valine at the sixth position of the β-globin chain, causing the hemoglobin molecule to deform and red blood cells to sickle, leading to sickle cell anemia.
Step-by-step explanation:
The βs allele causes a single amino acid substitution in the β-globin polypeptide, leading to sickle cell anemia. In normal hemoglobin, the amino acid glutamic acid at position six of the β-globin chain is hydrophilic and helps maintain the solubility of the hemoglobin molecule. However, in the presence of the βs allele, this glutamic acid is substituted by the hydrophobic amino acid valine. The substitution of hydrophilic glutamic acid with hydrophobic valine leads to a deformation of hemoglobin molecules, which in turn causes red blood cells to assume a sickle shape that is inefficient at carrying oxygen and can block blood flow.
The hemoglobin molecule is composed of two alpha and two beta chains, each chain comprising approximately 150 amino acids, resulting in a total of about 600 amino acids in the molecule. The dramatic structural and functional differences that lead to sickle cell anemia are due to this single amino acid out of the 600. Noteworthy is that three nucleotides encode each amino acid, and the mutation causing sickle cell anemia is the result of a single base change (point mutation) in one of these nucleotides.