Polymorphism: a common and localized variation in the DNA code. For example, if the sequence inside a gene typically reads “…GATTACA…” but many individuals are shown to have “…GATCACA”, the “C” that is normally a “T” is a polymorphism. Polymorphisms occur frequently in the human genome: some are neutral in their effect, some are beneficial, and yet others are partly responsible for disease susceptibility and drug response.
Mutations and polymorphisms are almost the same; arbitrarily, the scientific community decided that very rare polymorphisms (seen in less than 1 percent of a given population) would be called mutations.
The word “polymorphism” is composed of two Greek roots, polus and morphe. Polus means “many”; morphs means “form”. Together, they signify the “many forms” that are seen at a particular genetic location.