why does R.B.C. become sickle shaped in sickle cell Anemia?

James Harrick was the 1st to publish a description of sickle-cell anemia, the 1st inherited human disease to be understood at the molecular level. 
In 1957 ,vernon Ingram and his colleagues when compared the aa of the alpha & beta chains found that the alpha chains of Hb A & that of Hb S were identical but the beta chain of Hb S were substituted by valine for glutamic acid at the 6th position which allowed a new bond to be formed that lead to change in the conformation of the protein and caused the aggregation of Hb molecule.thereby causing the shape of R.B.c. to change into sickle shape.
sickle cell anamia is caused because of a mutation in the Hb aa sequences i.e. Glutamic acid is replaced by valine.Actually the sequence is :- Val-His-Leu-Thr-Pro-Glu-Glu-Lys
which changes into:- VAl-His-Leu-Thr-Pro-Val-Glu-Lys.
glu is hydrophilic so, it likes to be on surface as surface is in contact with water so, it forms a spherical structure but val being hydrophobic do not like to be on surface so another Hb sticks to val to hide it so, it forms fibres i.e. sickle cell.if we see the quaternary structure of Hb, sticky protein causes sickle Hb to agglutinate and form fibres which deform the R.B.C.hence the change of one aa causes sickle cell anamia. also, we conclude that sequestering hydrophobic residues in the protein core protects proteins from hydrophobic agglutination
This change in aa sequence is due to substitution of a T:A base pair for A:T base Pair, with a T in the trancribed strand in the first case and an A in the transcribed strand in the second case.