Final answer:
A 4-bp insertion mutation would have the least effect on gene expression if it occurred in introns, which are non-coding regions of a gene that do not contribute to the protein sequence and are removed during mRNA processing. The answer is option A.
Step-by-step explanation:
Within a gene, a 4-bp insertion mutation would have the least effect on gene expression if it occurred in introns. Introns are non-coding regions of a gene that are removed during the process of converting pre-mRNA to mature mRNA, hence they generally do not contribute to the amino acid sequence of proteins.
Since introns can vary in number and are often longer than exons, mutations within these regions are less likely to disrupt the production of functional proteins. Additionally, a mutation within an intron is less likely to cause changes in the important regulatory regions of a gene that control its expression.