Biology Reference
In-Depth Information
Figure 7.10
The SGM Plus
profile appears normal, except for the presence of a third allele at the
D16S539 locus
Multiple peaks at one locus
Some STR profiles appear normal, apart from the appearance of an extra allele at one
locus (Figure 7.10). In these circumstances it is very unlikely that the third allele is
because of a mixture, as we would expect to see three of four alleles at several loci.
The triallelic patterns can be classified into two types: Type 1 patterns contain
alleles with different levels of intensity, and occur as a result of a somatic mutation;
Type 2 patterns are the result of localized chromosomal rearrangements, resulting
in the duplication of the locus, and result in alleles of approximate equal intensity
[23 - 26]. In some rare cases several loci may display more than two alleles [27, 28].
Degraded DNA
Many samples that are collected from a crime scene may have been exposed to the
environment for hours, days or even longer if the crime scene has gone undetected.
When DNA analysis is being used to identify human remains, the remains may
be several years old before they are analysed or may have been exposed to severe
environmental insult, such as high temperatures. In all these circumstances the DNA
in the cellular material will not be in pristine condition and will have degraded. This
leads to a characteristic DNA profile with over-amplification of the smaller loci; the
successful amplification declines with the size of the alleles. Figures 7.11 and 7.12
show two examples of degraded DNA sample; the first one is from a bone sample
that had been in water for 30 years. The small loci have over-amplified, whereas the
