Genes: A Very Short Introduction by Jonathan Slack

Author:Jonathan Slack [Slack, Jonathan]
Language: eng
Format: epub
Tags: Genetics & Genomics, Life Sciences, Science
ISBN: 9780199676507
Google: nk9jBAAAQBAJ
Amazon: 019967650X
Publisher: Oxford University Press
Published: 2014-09-14T15:00:00+00:00

Forensic identification

Any of the many types of genetic variation could in principle be used for individual identification. However, methods designed for use in criminal proceedings have to be very robust and reliable, and a high degree of standardization is necessary to create national databases and to make comparisons possible on a large scale. For these reasons forensic science has focused on a type of marker known as the simple tandem repeat (STR). STRs are very simple DNA sequences, three to five nucleotides in length, which are repeated several times as copies lying next to each other. At a particular locus there might typically be 5 to 30 copies arranged end to end (i.e. in tandem). The number of repeats at a particular locus is very variable between individuals and is inherited reliably. Over evolutionary time periods, the frequency of changes (mutations) in copy number at such loci is quite high and so there is a lot of genetic variation in the human population. An individual might, for example, have a paternal chromosome carrying 10 copies of a 4 nucleotide STR at a particular locus and a corresponding maternal chromosome with 12 copies. This means that his DNA sample will contain equal amounts of sequences of 40 (4 × 10) and 48 (4 × 12) nucleotides in length. The 40- and 48-length variants are both inherited in a simple Mendelian manner. Other individuals might have different length variants at the same locus, for example 8 and 9, or 13 and 16, or 2 copies of 10. The measurement of the length of the STRs at this particular locus in the genome is done by a technique known as the polymerase chain reaction (PCR). Here short synthetic sequences of DNA complementary to the non-repeated DNA on either side of the locus are prepared by chemical synthesis. These can initiate DNA replication by a DNA polymerase enzyme plus the four deoxynucleoside triphosphates. The PCR reaction involves repeated cycles of DNA synthesis, strand separation and reattachment of the synthetic primers. The end result is the synthesis of a large number of copies of the DNA at the locus of interest. The reaction products are separated by a suitable technique, such as capillary electrophoresis, and the molecular sizes of the products obtained indicate the number of repeats at this locus in this DNA sample.

If just one locus is examined, many individuals will share the same markers, but if several loci are looked at, the probability of a random match for all of them between any two individuals falls until it becomes millions to one against. The technology of DNA-based forensic identification changed rapidly every few years when it was first introduced, but in the 2000s it became standardized on the basis of a certain number of loci containing different STRs. In the USA, databases are compiled using a set of 13 loci. In the UK it is ten, in Germany eight. A standard set of seven loci is used for comparing results between different European databases. In addition to the STRs, an actual gene is also included in the standard test to determine sex.

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