Biology Reference
In-Depth Information
12
Single nucleotide
polymorphisms
One of the most significant outcomes of the Human Genome Project has been the
identification of large numbers of SNPs [1 - 3]. The application of SNPs to forensic
analysis is currently limited to some specialist cases. However, with advances both
in our knowledge of SNPs and in the technology used to detect the polymorphisms,
SNP analysis may play an increasingly important role in the future.
SNPs - occurrence and structure
'SNPs are single base pair positions in genomic DNA at which different sequence
alternatives (alleles) exist in normal individuals in some population(s), wherein the
least frequent allele has an abundance of 1% or greater' [4]. The structure of a SNP
is very simple, an example is shown in Figure 12.1.
Initial sequencing of the human genome identified over 2 million SNPs [5, 6].
Following the first drafts of the human genome, the International HapMap Project
was established in 2001 to catalogue common patterns of sequence variation, using
populations that represented four geographical regions (Europe, sub-Saharan Africa
(Nigeria) and Southeast Asia (China and Japan) [7]. The second phase of the project
has characterized over 3.1 million SNPs in 270 individuals from the geographically
diverse populations [8]. Other databases contain additional SNPs: the National Center
for Biotechnology Information SNP database (NCBI dbSNP) reports over 10 million
validated SNPs [9, 10]. Whole genome sequencing of individuals' genomes has
shown that they contain around 3 - 4 million SNPs relative to the NCBI's reference
human genome [11 - 16].
Forensic value of SNPs
Given the array of SNPs available in the genome, and with individuals differing at
millions of positions, the potential for SNPs to discriminate between individuals is
enormous. As SNPs are biallelic the population falls in to one of three types. Given
the example in Figure 12.1 everyone would be one of CC, CT or TT. This type of
polymorphism intrinsically limits the information that can be gained from the analysis
