A single gene mutation results from

• A. A change in a base sequence in DNA
• B. A change in the number of chromosomes
• C. A change in the RNA sequence
• D. A change in the cell's mitochondria

Answer: (A)

Mutations in a single gene come from a change in the DNA sequence that codes for that gene. When a pattern of bases in the gene’s DNA is altered—such as a base substitution, insertion, or deletion—the RNA transcribed from that gene can change, which may then alter the protein produced. This direct change in the gene’s DNA is what defines a single-gene mutation.

In contrast, changing the number of chromosomes affects many genes at once, so that’s a chromosomal mutation. An RNA sequence change can happen during transcription or RNA editing, but a true single-gene mutation is the DNA base sequence itself. A change in the mitochondria involves a different genome and is not the typical example of a single-gene mutation in nuclear DNA.