All the examples above are of the protein products of alleles. Link to description of electrophoresis.
Enzymes are frequently polymorphic.
A population may contain two or more variants of an enzyme encoded by a single locus. The variants differ slightly in their amino acid sequence and often this causes them to migrate differently under electrophoresis. By treating the gel with the substrate for the enzyme, its presence can be visualized. Electrophoretic variants of an enzyme occurring in a population are called allozymes . Digestion with the enzyme then produces DNA fragments of a different length.