Positional cloning. (A–C) Scaffold walking. (A) The mutant gene and closest marker reside in different scaffolds. (B) The mutant gene is not on the scaffold (between the scaffolds). (C) The mutant gene and the closest marker reside in the same scaffold. (D) Narrowing down the target region in the scaffold. *, mutation site; CLM, closely linked marker; SEM, scaffold end marker; SIM, scaffold internal marker. Denominator: observed recombination. Numerator: the number of recombinants.