What is the typical effect of a frameshift mutation?

• A. It introduces a new start codon early
• B. It increases gene dosage
• C. It shifts the reading frame, altering downstream amino acids
• D. It has no impact on the resulting protein

Answer: (C)

Shifting the reading frame means the grouping of nucleotides into codons changes from the mutation onward. Since codons are read in triplets, inserting or deleting bases not in multiples of three redefines every downstream codon, so the amino acid sequence after the mutation is altered. This often introduces a premature stop codon, producing a truncated protein that is usually nonfunctional. Frameshift mutations typically have a drastic impact on protein function because every downstream amino acid is changed, not just a single residue. This differs from adding a new start codon or changing gene copy number, which are separate scenarios and do not describe the characteristic effect of a frameshift.