A silent mutation is

• A. A base change that causes a premature stop codon.
• B. A base change that alters the amino acid sequence.
• C. A base change that deletes entire genes.
• D. A base change that does not alter the amino acid sequence; usually no effect on phenotype.

Answer: (D)

Silent mutations arise from the redundancy of the genetic code. Because many amino acids are specified by more than one codon, a change in a DNA base can still result in the same amino acid being incorporated during translation. When the amino acid sequence of the protein remains the same, the protein’s structure and function are typically unchanged, so there is usually no visible effect on the organism’s phenotype. This is often seen when the base change occurs in the third position of a codon, the wobble position. While most silent mutations have no effect, in rare cases they can influence gene expression or splicing if they affect regulatory elements, but these are not the common outcome.