PCR's specificity is given by the primers. Primers are small sequences of nucleotides, small strands of DNA, that bind to the DNA strand that we want to amplify in specific locations limiting the locus of interest. The DNA polymerase, the enzyme that will bind to the 3' end of the primer and start producing the complementary DNA strand, will only start its work where the primer bound to. This way, by engineering specific primers, we can amplify only the locus of interest rather then several loci all at the same time scattered throughout the genome.
