Sickle cell anemia is an autosomal linked recessive trait disorder. It is controlled by a single allele - Hbs . The defect arises as the amino acid valine (neutral amino acid) replaces the normal chain amino acid glutamic acid (acidic amino acid) at the 6th position of the hemoglobin's beta chain. This change takes place due to the single base substitution of the sixth codon for beta-globin gene.