Which statement could explain why a particular point mutation had no effect on the polypeptide expressed by a gene?

The statement that a point mutation could have no effect on the polypeptide expressed by a gene is most accurately explained by the substitution of a single nucleotide. This phenomenon occurs when a nucleotide in the DNA sequence is replaced with another nucleotide without altering the overall protein sequence. Such substitutions can be silent mutations, meaning they do not change the amino acid that is produced due to the redundancy in the genetic code. This redundancy exists because multiple codons can specify the same amino acid, allowing for certain mutations to occur without affecting the resulting polypeptide's composition or function.

In the context of the other choices, a deletion of a single nucleotide can lead to a frameshift mutation, potentially altering the reading frame of the gene and affecting all subsequent amino acids. The addition of an additional stop codon would truncate the polypeptide, usually leading to a nonfunctional protein. The addition of a single nucleotide can also cause a frameshift mutation if it interrupts the reading frame, thereby changing the sequence of amino acids downstream. Therefore, the nature of a single nucleotide substitution allows for stability in the protein produced, making it the option that explains a lack of effect on the polypeptide.