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DNA evidence in courts

DNA evidence in courts . Review by Lavanya Peddibhotla And Ajay sequeira. Introduction . Human genome sequencing and mapping contributed a lot to forensic science. First developed in 1985 in England Can be performed on any nucleated cell material like blood, hair , semen etc…,

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DNA evidence in courts

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  1. DNA evidence in courts Review by Lavanya Peddibhotla And Ajay sequeira

  2. Introduction • Human genome sequencing and mapping contributed a lot to forensic science. • First developed in 1985 in England • Can be performed on any nucleated cell material like blood, hair , semen etc…, • RFLP and electrophoresis are the some of the common methods used by forensic laboratories and FBI.

  3. Methods to calculate the probability • Bayes’ theorem: Posterior odds = Likelihood ratio * Prior odds • Odd forms of Bayes’ model can be represented as: Pr(Hp|E,I) Pr(Hp|E,I) Pr(Hp|I) -------------- = --------------- * ----------- Pr(Hd|E,I) Pr(Hd|E,I) Pr(Hd|I)

  4. Bayesian Approach • Presenting the evidence in the form of likelihood ratios makes things quite complicated; instead it is easier to present the estimate of the frequency of the observed type. • Bayes’ theorem is merely a formalization of logic and common sense. • Bayes’ theorem is therefore not appropriate matter for evidence but for general education and judicial notice • Bayesian methods can be used to estimate the genotype proportions on the basis of Hardy-Weinberg law.

  5. Principles of evidence interpretation • There should be clear distinction between the role of the scientist and the role of the juror. • Scientist must always be asking questions of the kind. • Scientist must always evaluate the DNA evidence, not only under conditioning of Hp and Hd but also under the conditioning of non-DNA evidence

  6. Errors and Fallacies • Prosecution Fallacy: • Correct interpretation: Pr(Gc|Hd, I) = 1/100 2. Error in interpretation: Pr(Hd|Gc, I) = 1/100 • Defense Fallacy: 1. Defense attorney’s fallacy is to assume equal probabilities of guilt for all the people with the same genotype as of the suspect.

  7. Mixtures • Suspect and unknown person • Kevin Johnson case where the DNA profiling showed 2 out of 3 allele match. • Various explanations are possible for the above case. • The fact that 5 of the 6 loci had only 3 alleles suggest that the contributors are relatives.

  8. Uniqueness • Suppose in an N population a crime has been committed and the probability of the crime scene sample is P and the suspect has been found to have this profile. • And if profiles are found to be independent, and 1-L is the probability that no one in a population has that profile: (1-P)^N = 1-L P~L/N If L= 0.001 and N = 260 million Then P = 1/260 billion • The new policy states that if the Likelihood of a random match is less than one in 260 billion, the examiner can testify that the samples are an exact match.

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