Figure 1.

Positional cloning, exemplified here by the stages used in the cloning of the Lps locus, consisted of genetic mapping, physical mapping, and candidate and mutation identification steps. In genetic mapping, the phenotype was proved to emanate from a point between a proximal and distal marker on the chromosome, based on phenotypic documentation of crossover(s) between each of these markers and the causative mutation. In physical mapping, all of the genomic DNA was captured in a set of overlapping BAC and/or YAC clones spanning the critical region. In the candidate identification step, these clones were examined for gene content. Ultimately, the causative mutation was identified because it was present only in the mutant strain, and not in control strains.