ANS:A - A mutation in the beta chain

The sickle-cell disease occurs when the sixth amino acid, glutamic acid, is replaced by valine to change its structure and function; as such, sickle-cell anemia is also known as E6V. Valine is hydrophobic, causing the haemoglobin to collapse on itself occasionally.

The structure is not changed otherwise. When enough haemoglobin collapses on itself the red blood cells become sickle-shaped.