When two genetic entities to be identified have very close DNA, it is impossible to find specific primers to everyone. 
Common primers to both entities are then used to generate a DNA fragment copy but the identification is done by the reading of small differences in the genetic code. 
DNA sequencing can be used to determine the sequence of individual genes, large gene regions, complete chromosomes or whole genomes of any organism. DNA sequencing has become a key technology in many areas of biology.