Forensic Molecular Genetics Lecture 5

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Forensic Molecular GeneticsLecture 5

• Ralph Kirby

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• Amount of DNA extractable from a sample is one of
  the main driving forces in the improvement of
  forensic DNA profiling
• The second is the level of discrimination
• The third is the ease of explanation of the
  results to a judge or a jury

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• RFLP
• Easy to explain
• But needs lost of DNA
• Not very high discrimination

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• Polymerase Chain Reaction (PCR)
• Able to use very small quantities of DNA
• Amplifies specific defined DNA sequences
• Use specific short single stranded DNA primers
• Thermostable DNA polymerase (Fidelity?)
• Number of cycles
• Detection system
• Contamination can become a problem

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• DQA system
• Involves Specific genes and specific alleles
  available for such genes
• Need to avoid gel electrophoresis
• Uses dot blotting (Southern Blotting)
• Problem with similarity of markers
• Use of reverse dot blot with internal control

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• DQa system in its very early form eliminated
  suspect 1 as the rapist
• DQa system as it developed supported Quintanella
  as the rapist
• However, this still gave relatively poor
  discrimination, lt10 of the population
• Would you convict and send to jail for 99 years?

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DQa system as it became with 13 probes and 5
typed loci
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But hard to explain to a judge or jury
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• Need for an accurate but obvious system with good
  discrimination
• PCR amplification of Short Tandem Repeats
• Separated on an automatic DNA sequencer
• Labeled with multiply coloured fluorescent dyes
• Needs control

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• This is the type of control used

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• The STR system for chromosomal markers is fine
  for DNA samples from most types of cases as they
  are not mixed
• However, most DNA from rape cases is a mixture
• Can separate sperm, but PCR still amplifies
  victims markers
• Mixtures reduce discrimination and cause
  confusion
• However, in a rape case, only Y chromosome that
  is present in the sample will be from the rapist
  (Note possible exception of Y translocations and
  in utero sex change)
• Thus STRs for Y chromosome only have been
  developed
• Quite good discrimination, especially if used in
  conjuction with normal STRs

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• Extremely large number of possible STR markers
  available
• Many commercial kits available
• Some problems with STRs and these will be
  discussed later

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But what about identification from
skeletonsMitochondrial DNA