Forensic Science by Jim Fraser

Author:Jim Fraser
Language: eng
Format: epub
ISBN: 9780192571267
Publisher: OUP Oxford
Published: 2020-01-21T16:00:00+00:00

Analysis and interpretation of DNA profiles

Different alleles in DNA profiles vary in molecular weight and can therefore be analysed by the technique of electrophoresis. This separates molecules on the basis of their electrical charge and mass. There are many different types of electrophoresis, but the most widely used method for DNA analysis is capillary gel electrophoresis. Coloured dyes are attached to the DNA and these are detected by a laser to produce an electropherogram (EPG) similar to that shown in Figures 9 and 10. The peaks in the EPG represent the alleles detected. Some of the peaks indicated are size markers: fragments of DNA of known size to allow calculation of the allele sizes in the profile. From left to right on the EPG, the DNA fragments represented are larger and therefore more prone to degradation. There are also tiny peaks near the bigger peaks due to artefacts which can be safely ignored for our purposes. The first figure illustrates kinship testing using ten loci (twenty alleles) and a sex marker (amelogenin). Each locus consists of two alleles, and when they are different (heterozygous), which is often the case, two peaks will be seen in the EPG. Where the same allele is inherited from each parent (homozygous), only one peak will be detected which will be taller due to the increased signal from the two identical alleles.

9. DNA kinship testing using the SGMplus® system—an inclusion and an exclusion (from top to bottom: alleged father 1, alleged father 2, child, mother).

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