What type of mutation occurs when a codon for an amino acid changes to a codon for the same amino acid?

A mutation in which a codon for an amino acid changes to a codon for the same amino acid is referred to as a silent mutation. This type of mutation occurs due to the redundancy in the genetic code, where multiple codons can encode for the same amino acid. For example, the codons GAA and GAG both encode for the amino acid glutamic acid. If a mutation changes GAA to GAG, there is no change in the amino acid sequence of the resulting protein, thus the functionality of the protein remains unchanged.

Silent mutations typically arise during DNA replication or DNA repair processes. They can also result from single nucleotide polymorphisms (SNPs) that change a base pair but do not affect the overall protein product. Because there is no impact on the amino acid sequence, silent mutations generally do not affect an organism's phenotype, making them particularly interesting in studies of genetics and evolution.

This distinguishes silent mutations from other types such as frameshift mutations, nonsense mutations, and missense mutations, which result in altered protein sequences or functions, leading to potentially significant biological consequences.