Detection of Single-Nucleotide Polymorphisms (SNPs)
Differences
between the DNA sequences of individuals and species are
due to alterations of the four-letter ACGT code. Most such alterations are
not newly-arisen "mutations" between the previous and
present generations, but rather persistent variation present in
populations, called single-nucleotide
polymorphisms or SNPs. The image is a portion of an Applied
Biosystem Automated DNA sequencer chromatogram.The
top window shows an 80-nucleotide stretch of DNA from a
group of eight Atlantic Cod. Black dots in the upper
window flag positions at which one individual differs by a
single SNP from the others. The grey bracket
frames an eight-base region with two SNPs, for which the
chromatogram data are shown in the lower window.
Most individuals have the sequence CACATAGG: individuals BS02
& BS11 share a CT SNP variant at the third
position (CATATAGG)
and individual BS03 has a unique AG SNP at the sixth position (CACATGGG).
Identification of such genetic differences among individuals or
species permits inferences about relationships among individuals
in time (genealogy) and space (phylogeography),
and the evolutionary history (phylogeny) of species. For
example, reconstruction of the pattern of SNP variation shows
that individuals BS02 and
BS03 are less
closely related to each other than either is to individual BS01.