Molecular biologist sequences

A molecular biologist sequences the two alleles that are present in a population for some gene of interest. She finds that one allele has an A nucleotide at the third position of a particular codon and the other allele has a mutation to a G at the third position of the same codon. Both codons code for the amino acid proline. Which of the following statements is most likely true of the G mutation in the second allele?