Biology Reference
In-Depth Information
7
Assessment of STR profiles
DNA profiles generated from casework samples require some experience to interpret.
Guidelines have evolved to assist with the interpretation of STR profiles, ensuring
that the results are robust and consistent; this is especially important when dealing
with samples that contain very small amounts of DNA, degraded DNA or mixtures of
profiles that come from two or more individuals - all situations that complicate inter-
pretation. This chapter explores a number of artefacts that can occur in DNA profiles.
Some casework scenarios that can lead to complex profiles are also considered.
Stutter peaks
During the amplification of an STR allele it is normal to generate a stutter peak, that
is one repeat unit smaller or larger that the true allele; smaller alleles are formed
in the majority of cases [1]. Stutter peaks are formed by strand slippage during the
extension of the nascent DNA strand during PCR amplification (Figure 7.1) [2, 3].
Even in good-quality profiles there will be some stutter peaks; these are recogniz-
able and do not interfere with the interpretation of the profile. Threshold limits are
normally used to aid in the identification and interpretation of stutter peaks, so, for
example although the degree of stutter varies between loci, they are typically less
than 15% of the main peak [4, 5]; understanding stutter peaks is especially important
when interpreting mixtures.
Different STR loci have varying tendencies to stutter. This is dependent on the
structure of the core repeats: di- and trinucleotide repeats are more prone to stutter
than are tetra- and pentanucleotide repeats, and this is one of the reasons that all
the autosomal STRs that have been adopted by the forensic community have tetra-
and pentanucleotide core repeats (Figure 7.2). STRs with simple core repeats tend
to have higher stutter rates than compound and complex repeats.
Split peaks (
±
N
)
The
Ta q
polymerase that is used to drive the PCR adds nucleotides to the newly
synthesized DNA molecule in a template-dependent manner.
However, it also has an activity, called terminal transferase, whereby it adds a
nucleotide to the end of the amplified molecule which is non-template-dependent [6].
Approximately 85% of the time an adenine residue is added (Figure 7.3).
