Question

which mutation would result in the least change in the phenotype of an organism?
a deletion of a single nucleotide near the beginning of the coding sequence
a single base pair substitution
an insertion of three nucleotides in the middle of the coding sequence
a deletion of two nucleotides outside of the coding sequence

Explanation:

• Option A: Deletion of a single nucleotide near the beginning of the coding sequence causes a frameshift mutation. This changes the reading frame for all subsequent codons, leading to a significant change in the amino - acid sequence and thus a large phenotypic change.
• Option B: A single base - pair substitution may result in a silent mutation (if the new codon codes for the same amino acid), a missense mutation (if the new codon codes for a different amino acid), or a nonsense mutation (if the new codon is a stop codon). However, compared to frameshift mutations, the potential for a small phenotypic change is higher. For example, in a silent mutation, there is no change in the amino - acid sequence, so the phenotype may not change at all.
• Option C: Insertion of three nucleotides in the middle of the coding sequence adds one extra amino acid. This can disrupt the protein's structure and function, potentially leading to a noticeable phenotypic change.
• Option D: Deletion of two nucleotides outside of the coding sequence may affect regulatory regions. Regulatory regions control gene expression (e.g., when, where, and how much a gene is expressed). Changes in these regions can lead to significant phenotypic changes as they can affect the overall level of protein production.

Answer:

B. A single base pair substitution