1 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. 2  change a codon to one that encodes the same amino acid and causes no change in the protein produced. 3  These are called silent mutations.uch 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.