ASSESSMENT OF STR PROFILES
Figure 7.8 The profile shows the green loci from the AmpFlSTR® SGM Plus® 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
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 . 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 . 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 .
Manybiologicalsamplesthatarerecoveredfromasceneofcrimewillcontainamixture 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 by several people – there are many circumstances when mixtures of material can be collected. A mixture in a DNA profile can be recognized by the presence of more than two alleles at any locus within the profile, normally there will be several loci that have three or four alleles present and a loss of peak balance. Having determined that the profile is mixed, the first task is to assess how many contributors are represented in the profile. Two person mixtures are most commonly seen in forensic casework; with a two person mixture a maximum of four alleles will be present at any locus whereas three person mixtures will contain up to six alleles