Forensic BIO

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Forensic BIO
2012-02-21 10:50:51

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  1. Antigen Polymorphism
    • ABO blood groups, Karl Landsteiner (1900): studying blood transfusion reactions
    • Four blood groups: A, B, AB, O
    • Blood type is inherited, and frequencies in the population differ
    • First polymorphic antigen marker
    • High probability of coincidental match
  2. Protein Polymorphism
    • Approximately 100 protein polymorphisms had been discovered by the 1980’s
    • Commonly used systems: erythrocyte enzymes, serum proteins, hemoglobin
    • Combining blood group typing and protein polymorphisms, the chance of coincidental match was reduced
  3. DNA Polymorphism
    • DNA fingerprinting (VNTRs – variable number tandem repeats)
    • mid-1980’s: Kary Mullis – PCR (polymerase chain reaction)
    • Single nucleotide polymorphisms (SNPs): HLA-DQA1, Polymarker (PM)
    • Amplified fragment length polymorphism (AMP-FLP): D1S80
    • Short tandem repeats (STRs): CODIS loci· Samples only Valid when reference sample is present Y-STRs
    • Mitochondrial DNA
    • SNPs
  4. Probative Evidence
    • Probative: Having the effect of proof, tending to prove, or actually proving
    • Probative evidence establishes or contributes to proof
  5. Evidence Collection
    • Priority – based on probative value
    • Corpus delecti (“body of crime”) – demonstrates a crime has occurred
    • Evidence that can establish connections between victim-perpetrator
    • connections between victim-scene, perpetrator-scene
    • Case-to-case linkage
  6. Evidence Collection
    • Begins only after entire scene is documented
    • Can be accomplished in a number of ways:
    • Removal of small items to the lab
    • Swabbing: of an item’s surface (preferred method for hard items, such as countertops or desktops)
    • Cutting: useful with soft items, such as fabric, from which areas of interest may be removed
    • Tape-lifts: areas of interest are sampled with a piece of tape to remove any cells that may be present
    • Scraping: the surface of items are scraped with a blade to remove deposited cells. A small “pile” of fiber, debris and cells is produced
  7. Evidence Collection: Considerations
    • multiple analysis of evidence (bloodstain patterns, fingerprints, etc)
    • trace evidence
    • control samples
    • Run (+) and (-) controls
    • (+) = proper technique
    • (-) = Contamination
    • size of sample
    • wet evidence
  8. Biology of Blood
    • Hematapoetic lineage cells: red blood cells, white blood cells, and platelets
    • in a protein-aqueous fluid called plasma
    • Serum: the fluid excluded from blood once it has clotted (serum is simply plasma minus the proteins responsible for the clotting process, primarily fibrinogen and fibrin)
  9. Biology of Blood: RBC
    • Red blood cells (erythrocytes)
    • Mature RBC do not have nuclei à no nuclear DNA
  10. Biology of Blood: Heme
    Heme: binds oxygen
  11. Biology of Blood: WBC
    White blood cells (leukocytes): immune cells, have nuclei, are the main source of nuclear DNA in blood
  12. Biology of Blood: Platelets
    • Platelets (thrombocytes): play a role in blood clotting
    • Do not have nuclei, therefore do not have nuclear DNA
  13. Presumptive Tests
    • Assay for the activity of the heme component of blood
    • Heme catalyzes a redox reaction (i.e. has peroxidase activity)
  14. Presumptive Tests for Blood
    • Hydrogen peroxide (H2O2) is typically used as the oxidizing agent (H2O2 is reduced)
    • Heme is the catalyst (catalyst: increases the rate of reaction by reducing activation energy, but is not itself consumed)
    • A colorless substrate is oxidized (loses electrons to H2O2) to a product with color, chemiluminescence or fluorescence
  15. Colorimetric Assays:
    • Kastle-Meyer
    • Leuchomalachite Green
    • Tetramethylbenzidine (TMB)