Biology Reference
In-Depth Information
Table 12.1
A comparison of the properties of SNPs and STRs
STR
SNP
Frequency of occurrence
Once every 15 kb
Once every 500 bp
10
−
3
2
−
3
×
10
−
8
Typical rate of mutation
Typical number of alleles
Between 5 and 20
2
Potential to multiplex
Currently 15 STR loci
examined in one
reaction
Difficult to amplify more than
50 SNPs in one reaction
Number of loci required to
have a P
M
of 1 in 1
billion
10
∼
60
Method of detection
Capillary gel electrophoresis
(CGE)
CGE, microarrays, mass
spectroscopy
Automation potential
Medium
High
Artefacts
Amplification of STRs can
produce artefacts such as
stutter and split peaks
No stutter artefacts associated
with the amplification of
the SNPs
Amount of DNA required
∼
0.5 ng
−
1ng
∼
100 pg
Size of amplicon
Amplicon sizes typically
between 100 bp and 400 bp
Amplicon sizes can be less
than 100 bp
Mixtures
Interpretation of mixtures of
STR loci is possible
Mixtures of SNP loci can be
highly problematic to
interpret
Predicting geographical
origin
Limited ethnic identification
from STR loci
Some SNPs can be associated
with particular ethnic
groups
Phenotypic information
No possibility to infer
phenotype
Possible to predict some hair
colour, eye colour, skin
colour
of discrimination due to the large number of alleles they have in comparison with
biallelic SNPs. In contrast to STRs, around four times more SNPs are required to
reach the discrimination power equivalent to STR loci. Another major disadvantage
to using SNPs is that mixtures of two or more people might be either problematic or
impossible to interpret since SNPs are biallelic markers. In addition, current DNA
databases consist of profiles comprising STR loci and therefore SNPs cannot be used
in that context. At the same time it is possible to analyse hundreds of SNP loci and,
because of their structure, the amplicon size can be much smaller, typically less than
100 bp. This allows the detection of DNA templates that are highly degraded and
may generate data when standard STR typing fails to generate a result. A comparison
between STR and SNP markers is shown in Table 12.1.
In the foreseeable future, STRs will be the most commonly used genetic polymor-
phism analysed. They are tried and tested in most judicial systems and also form
the basis of most forensic DNA databases. Even so, the use of SNPs in forensic
genetics is likely to increase in the coming years and may at some point in the future
replace the analysis of STR polymorphisms. The application of SNPs to specialized
applications, for example SNP-based blood grouping [43, 53] and molecular autopsy
