Mutation is a recurrent event because a given length of DNA can undergo only a limited number of chemical changes. Mutation arising due to a change in single base pair of DNA is called point mutation.

The human disorder called sickle cell anaemia is caused by a single base substitution in a gene. This alteration results in the replacement of glutamic acid by valine at position 6 in the β polypeptide chain of haemoglobin, making the latter haemoglobin S. The disorder is manifested only in homozygous recessive state (HbS, HbS). It kills the victim by the age of 20 years.

Amino acid composition of the relevant portion of β-chain of haemoglobin : (a) From a normal individual; (b) From an individual with sickle cell anaemia showing point mutation.