PROBABILITY AND DNA TESTING
If two DNA samples are different, then they must have come from different people. Hence, DNA testing can readily exclude an individual from being suspected. But what if two DNAsamples match for whatever tests we have run? Positive identification requires the use of probability. Inclusion depends on the assumption that it is highly improbable that the DNA of the suspect and the DNA from the evidence match merely by chance. Using DNA profiling it is now possible to achieve probabilities of less than 1 in the total world population of a chance match. We should be cautious if close relatives are suspects in criminal proceedings, because the probability of a match is obviously much greater than for the general population.
The following general steps are important when determining the probability of a match:
1 . From the same population of which the suspect is a member, select a random sample of individuals.
2 . Determine the genotype of these randomly selected individuals and estimate the frequency of the alleles at the loci used in DNA typing.
3 . Calculate the probability of finding the genotype of the suspect by assuming that this individual’s alleles at each single locus represent a random selection from the population in general. (We also assume that the alleles tested are not linked but are independent of each other.)
4 . Multiply together the frequencies that are determined from the various loci. The figure obtained represents the overall probability that the suspect’s DNA would match the evidence by chance.
The details of population genetics used to establish probabilities for genetic screening, whether DNA or blood groups, are beyond the scope of this book. However, the probabilities from DNA testing are now sufficiently good in practice to make identification virtually certain, provided that the tests are carried out properly on reasonably good samples of DNA.
Interestingly, convictions have been obtained using DNA evidence where the probability of a chance match was one in 100, but with the addition of supporting evidence. However, in cases where the evidence is primarily based on DNA testing, juries more and more often expect astronomical odds such as one in a million or billion. This was the case in the notorious O. J. Simpson trial, but the DNA evidence was ignored. However good the scientific evidence, it cannot overcome the corruption of justice by wealth and politics.
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