Sickle-cell anaemia is a classical example of point mutation in which amino acid valine replaces the amino acid 

A classical example of point mutation is a change of single base pair from AT to TA from GAG to GUG in the gene for beta globin chain that results in the change of amino acid residue glutamate (codon GAG)  to valine (codon GUG). It results into a diseased condition called as sickle cell anemia. Valine also has three other codons such as GUU, GUA and GUC.