【单选题】Which of the following statements about sickle cell anemia is wrong?
A.
Sickle cell anemia occurs in individuals who inherit the allele for sickle cell hemoglobin from both parents. The erythrocytes of these individuals are fewer and also abnormal.
B.
In addition to an unusually large number of immature cells, the blood contains many long, thin, sickle-shaped erythrocytes. When hemoglobin from sickle cells (called hemoglobin S, or HbS) is deoxygenated, it becomes soluble and forms polymers that aggregate into tubular fibers. The insoluble fibers of deoxygenated HbS cause the deformed, sickle shape of the erythrocytes, and the proportion of sickled cells increases greatly as blood is deoxygenated.
C.
The altered properties of HbS result from a single amino acid substitution, a Val instead of a Glu residue at position 6 in the two β chains. The R group of valine has no electric charge, whereas glutamate has a negative charge at pH 7.4. Hemoglobin S therefore has two fewer negative charges than HbA (one fewer on each β chain).
D.
Replacement of the Glu residue by Val creates a “sticky” hydrophobic contact point at position 6 of the β chain, which is on the outer surface of the molecule. These sticky spots cause deoxyHbS molecules to associate abnormally with each other, forming the long, fibrous aggregates characteristic of this disorder.