Biology Reference
In-Depth Information
From a forensic point of view there is very little rationale in analysing the 99.9%
of human DNA that is common between individuals. Fortunately, there are well-
characterized regions within the genome that are variable between individuals and
these have become the focus of forensic genetics.
The genome and forensic genetics
With advances in molecular biology techniques it is now possible to analyse any
region within the 3.2 billion bases that make up the human genome. DNA loci
that are to be used for forensic genetics should have some key properties; they
should ideally:
•
be highly polymorphic (varying widely between individuals);
•
be easy and cheap to characterize;
•
give
profiles
that
are
simple
to
interpret
and
easy
to
compare
between
laboratories;
•
not be under any selective pressure; and
•
have a low mutation rate.
Tandem repeats
Two important categories of tandem repeat have been used widely in forensic genet-
ics: minisatellites, also referred to as variable number tandem repeats (VNTRs);
and microsatellites, also referred to as STRs. The general structure of mini- and
microsatellites is the same (Figures 2.5 and 2.6). Variation between different alleles
is caused by a different number of the repeat unit, which in turn results in alleles
that are of different lengths; it is for this reason that tandem repeat polymorphisms
are also known as length polymorphisms.
A-type CCCTATCCA
B-type CCCTCTCCA
Other repeat variant
C-type CCCTGTCCA
K-type CCCTAACCA
Figure 2.5
The structure of two minisatellite alleles found at the D1S7 locus [28]. The alleles are
both relatively short containing 104 and 134 repeats; alleles at this locus can contain over 2000
repeats. The alleles are composed of several different variants of the 9 bp core repeat; this is a
common feature of minisatellite alleles
