ABSTRACT
DNA profiling The specificity of DNA profiles is so great in statistical terms that it can be reliably specific to any individual (see p. 219). The molecule of DNA has two strands of sugar and phosphate molecules that are linked by combinations of four bases – adenine, thymine, cytosine and guanine – forming a double helix structure. Only about 10 per cent of the molecule is used for genetic coding (the active genes), the remainder being ‘silent’. In these silent zones, there are between 200 and 14 000 repeats of identical sequences of the four bases. Jeffreys found that adjacent sequences were constant for a given individual and that they were transmitted, like blood groups, from the DNA of each parent. Only uni-ovular twins have the same sequences, and the chances of two unrelated individuals sharing the same sequence is one in a billion, or higher. The statistical analysis of DNA identification is extremely complex and it is important that any calculations are based upon the DNA characteristics of a relevant population and not upon the characteristics of a ‘standard’ population somewhere else in the world. There is now no need to match blood with blood and semen with semen, as all the DNA in one individual’s body must of necessity be identical. A perpetrator leaving any of his or her cells or biological fluids at a scene is leaving proof of their presence at the scene. Comparison of DNA profiles with assumed or known family members or against known databases can ensure a person’s identity is established. If these comparisons cannot be done, other tests must be used.
