In sickle cell anaemia, glutamic acid is replaced by 

Sickle cell anemia is a genetic trait for which the newborns are screened. It is an autosomal recessive disorder which means both copies of the gene in each cell must have mutations to express the symptoms. The homozygous recessive ($H{b}^s$$H{b}^s$) are not viable and, hence, the disease is transmitted by heterozygous individuals. Hemoglobin has four protein subunits, two $?$-globin and two $?$-globin. The $H{b}^b$ gene codes the $?$-chain. Missense mutation, replacement of A by T at the 17th nucleotide of the $H{b}^b$ gene changes the codon GAG (glutamic acid) to GTG (which encodes valine). The mutated alllele $H{b}^s$ encodes the abnormal hemoglobin molecules which stick to one another and cause stiffness and sickle shape of red blood cells. The sickle shaped RBC block and damage the vital organs and tissue.