There are several techniques for DNA-profiling. For forensic applications there are different, so-called “kits” available that simultaneously allow for multiple DNA characteristics to be defined. These characteristics can exist on the autosomal chromosomes as well as on the sex chromosomes.
The uniqueness of one DNA characteristic is by itself limited. A characteristic on a particular locus might be found in 10% of the individuals within a certain population group. If only this characteristic is present, then it does not provide support for the hypothesis that the cells from a sample come from a particular individual. However, if one would determine the characteristics of three loci, which are all found at 10% of the individuals within a certain population group, then the frequency of the appearance of the characteristics are reduced: 0.1 x 0.1 x 0.1 = 0.001 = 0.1%. The possibility that any random individual possesses these characteristics will be 1 to 1000. If a DNA-profile is obtained from 15 characteristics then the frequency will be 1 x 10-15 (= 1 in 1.000.000.000.000.000). In practice a number of correction factors apply on the calculation to correct a number of statistical uncertainties. These correction factors ensure that the evidential value will not be overstated.
If a DNA-profile of a sample corresponds with the DNA-profile that is obtained from a reference sample from a suspect, then there is very much support for the hypothesis that the cellular material examined in the sample comes from the suspect. It should be noted that the chance of finding matching DNA characteristics increases significantly if there are family ties between the potential donors of the cells. So a biological father and mother have at least half of the DNA characteristics equal to that of their child. Identical twins cannot be distinguished through DNA characteristics because they have exactly the same DNA.
It is not always possible (based on the obtained DNA characteristics) to exactly calculate the probability that any random individual has the same DNA characteristics as the ones in a sample. This may be the case when there is a complicated partial DNA mixed profile.
With the increase in matching DNA characteristics obtained from cells of a sample and that of a reference sample for a particular individual, the greater the probability that the cells in the sample come from this individual.
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