DNA Forensics

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DNA Forensics

• MUPGRET Workshop

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• DNA evidenceoffers 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

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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.

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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.

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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.

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VNTR

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

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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.

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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.

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Related 1 4 2 3
1 2 3 4
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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.

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Uses of DNA Profiling

• Victim identification
• September 11
• Srebrenica Massacre
• Missing persons
• Military personnel
• Paternity Testing

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Use of DNA Profiling

• Criminal Testing
• Conviction
• Exoneration
• Excluding suspects
• Identifying protected species

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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

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Advantages of DNA Evidence

• More sensitive than blood typing
• More informative
• More resilient

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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.

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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.

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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.

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CODIS

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

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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

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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?

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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?