substitution mutation definition
A substitution is a mutation that exchanges one base for another (i.e., a change in a single chemical letter such as switching an A to a G). Such a substitution could: 1  change a codon to one that encodes a different amino acid and cause a small change in the protein produced. 2  For example, sickle cell anemia is caused by a substitution in the beta-hemoglobin gene, which alters a single amino acid in the protein produced.