In genetics, polymorphism (Greek polymorphismos = polymorphism) refers to the variant of a specific DNA sequence (so-called sequence variation) within a population, which occurs in > 1% of the population (otherwise it is referred to as a mutation).
Enzyme or protein polymorphism refers to a polymorphism that causes a change in the respective gene product (e.g. an enzyme or a protein/cytokine).
A well-known polymorphism is that of the TNF gene (TNF-308 G/A), in which guanosine (G) is replaced by adenine (A) (see also tumour necrosis factor-α). This can lead to significant changes in the gene product, the TNF protein. This is a "singel-base-pair polymorphism" (SNP), also known as a single nucleotide polymorphism.
Further polymorphisms are "copy number variants" and "deletions/insertions" (indels).
Genetic polymorphisms also appear to play a role in contact allergies.