Using gene probes to detect a mutant CF gene allele relies on what principle?

• A. The probe binds nonspecifically to DNA
• B. The probe binds to non-complementary sequences
• C. The probe binds specifically to the complementary mutant allele and radioactivity indicates presence
• D. The probe sequences the allele by polymerase extension

Answer: (C)

The principle here is sequence-specific hybridization of a labeled DNA probe to its complementary DNA sequence. A probe designed to match the mutant CF allele will bind only if that exact sequence is present in the sample, and the attached radioactivity provides a detectable signal indicating the allele’s presence. This relies on base-pairing rules and assay stringency, so true matches are detected while mismatches are not. The other options don’t fit: nonspecific binding would give false signals, binding to non-complementary sequences won’t occur, and sequencing by polymerase extension is a different method used to read DNA sequence rather than detect a specific allele with a labeled probe.