Which of the following is a classic example of point mutation?

Mutation that arises due to change in a single base pair of DNA is known as point mutation. A classic example of such a mutation is sickle cell anaemia. Sickle cell anaemia is caused by a single base substitution in the beta globin gene, which leads to the substitution of Valine for Glutamic acid at the sixth position of the beta globin chain in the haemoglobin molecule. This mutation causes the haemoglobin molecule to polymerise under low oxygen tension, leading to a change in the shape of red blood cells from a biconcave disc to an elongated, sickle-like structure.

Night blindness is due the deficiency of vitamin A. 

Down’s syndrome and Turner’s syndrome are chromosomal disorders.