Biology Reference
In-Depth Information
5
′
-GATGGCA-3
′
allele G
*
5
′
-GATAGCA-3
′
allele A
Figure 2.7
A single nucleotide polymorphism (SNP). Two alleles are shown which differ at one
position indicated by the star: the fourth position in allele G is a guanine while in allele A it is an
adenine. In most cases, the mutation event at the specific locus that creates a SNP is a unique
event and only two different alleles (biallelic) are normally found
STRs satisfy all the requirements for a forensic marker: they are robust, leading
to successful analysis of a wide range of biological material; the results generated in
different laboratories are easily compared; they are highly discriminatory, especially
when analysing a large number of loci simultaneously (multiplexing); they are very
sensitive, requiring only a few cells for a successful analysis; it is relatively cheap
and easy to generate STR profiles; and there is a large number of STRs throughout
the genome that do not appear to be under any selective pressure.
Single nucleotide polymorphisms
The simplest type of polymorphism is the single nucleotide polymorphism (SNP):
single base differences in the sequence of the DNA. The structure of a typical SNP
polymorphism is illustrated in Figure 2.7.
SNPs are formed when errors (mutations) occur as the cell undergoes DNA replica-
tion during meiosis. Some regions of the genome are richer in SNPs than others [34].
SNPs normally have just two alleles, for example one allele with a guanine and
one with an adenine. This is a purine for a purine, other common changes are
between cytosine and thymine, both of which are pyrimidines. SNPs therefore are
not highly polymorphic and do not fit with the ideal properties of DNA polymor-
phisms for forensic analysis. However, SNPs are so abundant throughout the genome
that it is theoretically possible to type hundreds of them. This can result in very high
combined power of discrimination. It is estimated that to achieve the same dis-
criminatory power that is achieved using 10 STRs, 50 - 80 SNPs would have to be
analysed [35, 36]. With current technology, this is much more difficult than analysing
10 to 15 STR loci.
With the exception of the analysis of mitochondrial DNA (see Chapter 13), SNPs
have not been used widely in forensic science to date, and the dominance of tandem
repeated DNA will continue for the foreseeable future [37]. SNPs are however finding
a number of niche applications in forensic science (see Chapter 12).
WWW resources
The Human Genome Project Information: a website funded by the U.S. Department of Energy
which along with and the National Institutes of Health coordinated the project. Contains
resources on all aspects of the Human Genome Project. http://www.ornl.gov/sci/techresources/
HumanGenome/home.shtml
