A mutant phenotype may revert back to normal phenotype either due to reverse mutation at the same locus or due to a supressor mutation at another locus. For instance a mutant a^– may give rise to a⁺ by reverse mutation, but the same effect may be obtained due to change in another mutable unit as follows:


where sup^– is a suppressor mutation suppressing the effect of a^– mutation. Mutations which are due to base substitutons, and lead to either a premature termination of polypeptide chain or to insertion of a wrong amino acid, are sometimes corrected by suppressors. Frame shift mutations resulting due to deletions or additions of base pairs may also be suppressed by suppressor mutations. A detailed discussion of these mutations at the molecular level requires knowledge of protein synthesis and the genetic code. Therefore, these mutations and the mechanisms of their induction will be discussed in The Genetic Code dealing with the Genetic Code. It will be seen that these suppressor mutations originate due to alterations in base sequences of either tRNAs or rRNA or sometimes due to alterations in ribosomal proteins.