A probe that changes solution color in the presence of only one of the two DNA sequences, which differ by a single nucleotide, is reported. The probe consists of two oligodeoxyribonucleotides, which form a hydrogen peroxidase-like DNA enzyme when hybridized to the abutting fragments of the complementary analyte. The active peroxidase catalyzes oxidation of colorless substrates to the colored products. The probe allows visual detection of a mutation in Alzheimer's disease related DNA.