|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. Electrophoresis of tissue extracts from 15 different green tree frogs (Hyla cinerea) reveals 4 allelic versions of the enzyme aconitase (one of the enzymes of the citric acid cycle). The 4 alleles can be distinguished by the speed with which their protein product migrates:
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