Sickle-cell anemia is caused by: Group of answer choices a substitution of a serine residue for a glutamic acid residue at the beta-6 position a substitution of glutamic acid residue for histidine at the C terminus of the alpha chain a substitution of a glutamic acid residue for a phenylalanine residue at the beta-6 position a decreased production of alpha chains of hemoglobin polymerization of hemoglobin