Mutagens may be physical, i.e., ionizing radiations X-ray, UV rays, gamma rays, DNA reactive chemicals, i.e., hydroxyl radicals, ${\mathrm{H}}_2{\mathrm{O}}_2$, etc., or biological such as virus. 
A mutagen can induce mutation by inducing, a change in the base sequence by insertion, deletion or substitution.

e.g., a single base sequence substitution at the sixth codon of the $\mathrm{\beta }-\mathrm{globin}$gene changes the codon from GAG to GUG. This results in the substitution of glutamic acid (Glu) by valine (Val) at the sixth position to the $\mathrm{\beta }-\mathrm{globin}$ chain of the hemoglobin molecule.

The mutant hemoglobin molecule undergoes polymerization under low oxygen tension causing the change in the shape of the RBC from biconcave disc to the elongated sickle, l.e., like structure which is not functional.