Biology Reference
In-Depth Information
STR loci, can also occur; this reduces the amount of information from the profile
but does not confuse the interpretation. At present, there is no clear consensus in the
scientific community about the use of LCN PCR [14]. LCN was prominent in the
trial of the one person accused of involvement with the Omagh bombing in 1998.
Although the process was supported by a Home Office inquiry [15] and is now used
in many countries, there remain jurisdictions, including most states in the USA, which
prefer to remain with standard cycling conditions and no further enhancements.
Peak balance
STR loci that are used in forensic analysis are commonly heterozygous, producing
two peaks in the profile. In a perfect profile the two peaks that are produced are
balanced 1:1 in terms of peak height and area, but in reality this is very rare and one
peak will be larger than the other (Figure 7.8). The variations in peak height can be
due to chance events, where one allele is more efficiently amplified than another. In
good-quality DNA extracts, the smaller peak is, on average, approximately 90% the
size of the larger peak [5].
Laboratories will use different values that are based on their own validation studies
but commonly require the smaller peak of a locus to be within 60% of the larger
peak [5]. Peak imbalance can be more extreme when profiling degraded DNA and
when amplifying low amounts of template DNA. On rare occasions the mutation of
a primer binding site will reduce the efficiency of the PCR for one allele, which can
result in high levels of peak imbalance and even allele drop-out. The frequency of
these mutations is low, ranging between frequencies of 0.01 and 0.001 per locus [16].
Mixtures
Many biological samples that are recovered from a scene of crime will contain a
mixture of cellular material from more than one person. Clothing will often contain
cellular material from the wearer and may also contain material from an assailant
after an assault; the handle of a door or a steering wheel may have been handled
Amelogenin
D8S1179
D21S11
D18S51
99%
86%
85%
98%
Figure 7.8
TheprofileshowsthegreenlocifromtheSGMPlus
kit. The peak area of the smallest
peak at each locus is shown as a percentage of the larger peak. The size of the peaks is proportional
to the amount of PCR product; this can be gauged by measuring the peak height or more usually
the peak area
