PCR has many applications in biochemistry, genetics and evolution, diagnostics and forensics, see [2] [5]. We name a few of them here. PCR can be used to amplify ancient DNA such as Egyptian mummies several thousand years ago. PCR is used for direct sequencing and screening clone libraries. PCR is used for DNA typing, diagnosis of DNA, and amplification of ribosomal RNA. PCR is used for detection viruses, e.g. HIV, bacterial pathogens, and determination of familial relationships. PCR is used for amplifying human-specific DNA sequences, detecting mutations, monitoring cancer therapy, sex determination of prenatal cells, see [5].
A classical problem in genetics is the mapping of genes using recombination fractions. This is difficult to do precisely for mapping human genes because the number of offspring is low. PCR analysis of alleles in sperm offers a way of performing linkage analysis with single cells. A chromosome in a sperm is a single meiotic product, and the recombination information among different loci from a number of sperm is no less than the information contained in a family of half sibs the samd size (see [3]).
Sequence-tagged site (STSs) are unique sequence-based landmarks in the genome. They define physical locations in the genome. Some of them exhibit length or sequence polymorphism in some populations, and thus define genetic markers. The majority of genetic markers in use today are of this kind. PCR can be used to make many copies of a specific region of DNA to detect STSs.
One important class of genetic markers are microsatellites. Interspersed
DNA elements such as 
constitute one of the most abundant
human repetitive DNA families. These repeats could be bi-nucleotide,
tri-nucleotide, or tetra-nucleotides. Some blocks containing this kind
of repeats are polymorphic in
length or in number of repeats among individuals, and therefore
represent a vast new pool of potential genetic markers.
Variation in the lengths of repeat blocks could be typed
by amplifying DNA within and immediately flanking the repeat
blocks by using PCR, and then resolving the amplified DNA by using
electrophoresis, see [6].
Figure 2: The PCR cycle. The DNA sample is heated to separate the DNA
strands(initial denaturation), and then the reaction mixture goes
through repeated cycles of primer annealing, DNA synthesis, and
denaturation. The target sequence doubles in concentration for each
cycle.