At which position in a codon are mutations most likely to be selectively neutral?

Mutations are most likely to be selectively neutral at the third position of a codon due to the properties of the genetic code and the phenomenon known as codon degeneracy. Each codon, which consists of three nucleotides, corresponds to a specific amino acid or a stop signal during protein synthesis. The genetic code is often described as being redundant or degenerate, meaning that several different codons can code for the same amino acid.

At the third position of a codon, changes in nucleotide sequence frequently do not lead to a change in the amino acid that is incorporated into the protein. This is because multiple codons can encode the same amino acid, and substitutions or mutations that occur at this position are less likely to affect the overall function of the protein. Thus, mutations here are less likely to be harmful or advantageous, rendering them selectively neutral.

In contrast, mutations at the first or second positions of a codon are more likely to result in a different amino acid being incorporated into the protein, which can significantly impact the structure and function of the resultant protein. This change can lead to a fitness consequence, making these mutations less likely to be neutral. Consequently, the third position is recognized as the most common site for selectively neutral mutations in the