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DNA Forensics

DNA Forensics. MUPGRET Workshop.

maris
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DNA Forensics

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  1. DNA Forensics MUPGRET Workshop

  2. “DNA evidence…offers prosecutors important new tools for the identification and apprehension of some of the most violent perpetrators, particularly in cases of sexual assault. At the same time, DNA aids the search for truth by exonerating the innocent. The criminal justice system is not infallible.” Janet Reno

  3. Biological Basis • The same DNA is found in virtually all cells in our bodies. • The four letters of DNA ATCG spell out instructions that give each individual their unique appearance. • The arrangement of the letters is different in each individual. • ~1 letter per 1000 is different.

  4. DNA forensics • Makes use of the similarities and differences in our DNA sequences to determine whether two biological samples come from the same individual or not.

  5. VNTR • Variable number of tandem repeats. • Type of DNA marker that is unique to each individual. • DNA of about 80 base pairs that is repeated many times in a row. • Number of repeats differs between individuals = different size DNA fragments on a gel.

  6. VNTR • By comparing across multiple VNTR genes we can get a more accurate test of whether two samples are the same or not.

  7. Probability • One gene has 20 alleles. The probability that two individuals have the same allele is 1/20. • If two genes each have 20 alleles and two individuals have the same alleles at both the probability is 1/20 x 1/20 = 1/400.

  8. Probability • People have two alleles for each gene. • If there are 20 alleles for that gene. • There are 210 possibile genotypes for that gene. • The number of possible combinations at 2 genes with 20 alleles is 2102, for 4 genes with 20 alleles is 2104, etc.

  9. Marker 1 Marker 2 Related 1 & 4 2 & 3 1 2 3 4

  10. Similar DNA Profiles • Twins from a single egg have identical profiles. • Siblings, parents, and other blood relatives share some alleles in common. The expected proportion depends on how closely related the pair of individuals are.

  11. Uses of DNA Profiling • Victim identification • September 11 • Srebrenica Massacre • Missing persons • Military personnel • Paternity Testing

  12. Use of DNA Profiling • Criminal Testing • Conviction • Exoneration • Excluding suspects • Identifying protected species

  13. Do matches establish guilt? • Not necessarily • Must have additional evidence • Chain of evidence must be preserved • Person may have had reasonable access to the crime scene

  14. Advantages of DNA Evidence • More sensitive than blood typing • More informative • More resilient

  15. How reliable is DNA profiling? • Generally, highly reliable. • Reliability is affected by methods used to collect, store and analyze samples. • Most common problem is contamination or mixture of a sample. • Standards have been established for forensic analysis.

  16. CODIS • Convicted Offender database • Contains for 44,000 DNA profiles in the forensic evidence section. • Over 5 million profiles will be entered in the next 4 years. • Contributed to matches in more 6000 cases.

  17. CODIS • Who should be included? • Currently convicted offenders are required to be registered in the UK. • CODIS rules say only convicted criminals after conviction can be entered but not all states comply with this. • Some states submit only certain types of criminals.

  18. CODIS • Some states allow use for all types of crime investigations. • Others only for sex-related or violent crimes.

  19. Problems with DNA evidence • Evidence can be degraded if not properly handled. • Backlogs in processing (16,000 rape cases backlogged in NYC). • Contamination • Human error

  20. Other questions • Should tissue samples be saved or destroyed after profiling? • Can personal or medical information be obtained from DNA profiles? • Why are innocent people convicted of crimes and then exonerated by DNA evidence?

  21. More questions • Is it possible for an innocent person to be convicted based on DNA evidence? • How has DNA evidence changed the criminal justice system?

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