A DNA profile is obtained by:
* extracting the DNA from a sample (blood, saliva, semen, sweat or other biological material);
* measuring the amount of DNA obtained;
* producing multiple copies of specific areas of DNA of interest (these correspond to the ‘markers’ referred to below); and
* cataloguing the size of each marker in the particular individual from whom the DNA came.
The technique currently used for DNA profiling in the United Kingdom is SGM Plus®(SGM+). It tests for ten ‘markers’, known as short tandem repeats (STRs), and a sex marker. STRs are short sequences of DNA that are repeated in tandem several times, and the number of repeats varies between individuals. The number of repeats is recorded and thus, a DNA profile consists of 20 two-digit numbers (each person has two copies of each marker, one inherited from each parent), and a sex indicator. The probability of a chance match between unrelated individuals using SGM+ is on average less than one in a billion (1,000,000,000). The discriminatory power of the analysis decreases for related individuals. SGM, a technique used before the introduction of SGM+, analysed six of the same markers plus the gender marker and had a lower discriminatory power. A proportion of the profiles on the NDNAD are based on SGM (22 per cent of criminal justice samples and 19 per cent of crime scene samples). When a current crime scene sample matches an SGM profile, the relevant biological sample is retrieved and the profile is upgraded to SGM+. A recent retrospective upgrade of 24,000 cases from SGM to SGM+ found that there were 3,600 cases where the profiles had originally matched using SGM, but no longer matched when using SGM+ profiling. To date, there have been no reports of chance matches between full SGM+ profiles. Chance matches are, however, more likely to arise:
* with partial profiles;
* between closely related individuals;
* as the size of the NDNAD expands; and
* between individuals within an isolated or inbred population.
...and from http://www.acpo.police.uk/asp/policies/Data/dna_good_practice_manual_2005.doc
6.2 Match probability
The probability of two DNA profiles matching depends on the nature of the DNA profile and the circumstances of the case. A full SGM+ to SGM+ match is the most powerful. The strength of the match decreases if the profile is SGM or partial and there is an increased risk of the match being by chance.
Although a person’s DNA is unique (except for identical twins, triplets etc.), some individuals who are closely related e.g. siblings may appear to have a similar profile. This occurs because they have inherited their DNA in different proportions from the same parents. The value of a match in a given case may therefore be weakened if there is a possibility that the offender could have been a close relative of the suspect.
Because the match probability depends on the fullness of the profile and the circumstances of the case, a match probability figure is not provided with a match report and the police and CPS should liaise with the scientist to assess the value of the evidence.
6.3 Chance matches
A match between two profiles on the NDNAD can occur simply by chance. This risk is very low when full SGM+ profiles are involved. However, the risk increases if partial or SGM profiles are involved and therefore, there is a significant chance that a match involving a partial profile or SGM profile may not match when the profile is upgraded to SGM+. Consequently, the laboratory should be asked to upgrade matches based on partial or SGM profiles. This is normally arranged via the force DNA Submissions Unit when the match report is received.