and laboratory information management systems, including bar coding of samples, to reduce operator
errors. A typical electropherogram output is illustrated (Figure 5.12).
A ‘second-generation multiplex’(SGM) has further included a PCR assay targeted at the XY-homologous
amelogenin genes^17 to reveal the sex of a sample donor. An additional four loci were added to the multi-
plex, now renamed ‘SGM Plus10’ (Figure 5.12), giving it a match probability of less than 10–13.
Although some differences in practice between individual national jurisdictions remain, there has been
rapid development and near-universal acceptance of this new DNA-based technology in forensic genetics.^18
5.5.4 DNA Microarrays
DNA microarrays are now one of the most widely used tools in functional genomics.^19 They are providing
biology with the equivalent to the chemist’s periodic table – a classified inventory of all the genes for a
living organism. Oligonucleotide microarrays, also known as DNA chips, are miniature parallel analytical
devices containing libraries of oligonucleotides robotically spotted (printed) or synthesised in situ on solid
supports (glass, coated glass, silicon or plastic). The major DNA-chip technologies are distinguished by
the sizes of the DNA fragments arrayed, by methods of arraying, by their chemistry and linkers for attach-
ing DNA to the chip, and by hybridisation and detection methods.
Microarrays work by exploiting the ability of a given cDNA or mRNA test sample to hybridise to the DNA
template from which it originated. By use of a two-dimensional (2-D) array containing very many DNA sam-
ples, the expression levels of hundreds or thousands of genes within a cell can be determined quickly by
measuring the amount of cDNA or mRNA bound to individual sites on the array. The precise amount of
mRNA bound to each locus gives a profile of gene expression in the cell. Alternatively, comparative binding
184 Chapter 5
Figure 5.12 Electropherograms illustrating autosomal short tandem repeat (STR) profiles. (a) An electropherogram of
the second-generation multiplex ‘SGM Plus’profile from a male, including X- and Y-specific amelogenin
products of 106 and 112 bp, respectively. Most short tandem repeats (STRs) are heterozygous and the
alleles are evenly balanced. Numbers beneath STR peaks indicate allele sizes in repeat units. The STR
profile (displayed here as red, black and grey) uses a four-colour fluorescent system, with the fourth chan-
nel being used for a size marker (not shown). (b) A typical mixture from two individuals (red channel
only shown). Mixtures can only be identified if the alleles of the minor component are above the back-
ground ‘noise’in an electropherogram (in practice a ratio of 1:10) and can usually be resolved by
inspection. In this example, the contributions are in even proportions – for example, D21S11 shows four
alleles where the peaks are approximately equal in height, whereas D18S51 shows two peaks in a 3:1
ratio. The X- and Y-specific amelogenin peaks are of approximately equal height, indicating that this is
a mixture from two males
(Adapted from Ref. 12; © (2004), with permission of Macmillan Publishers Ltd)