Fundamentals of Forensic DNA Typing - PowerPoint PPT Presentation

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Fundamentals of Forensic DNA Typing

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  1. Chapter 4 Sample Collection, etc. Fundamentals of Forensic DNA Typing Slides prepared by John M. Butler June 2009

  2. Chapter 4 – Collecting and Characterizing Biological Material Chapter Summary Biological evidence from a crime scene needs to be carefully collected, transported, and properly stored prior to examination with DNA testing methods. A chain-of-custody must be maintained for collected samples to provide confidence in correlating results for future legal proceedings. Many forensic laboratories perform presumptive tests to aid identification of the source of a crime stain, which often include blood, semen, or saliva. Recent research with ribonucleic acid (RNA) testing is expanding the capabilities of sample characterization. Reference samples from one or more suspects (with forensic cases) or biological relatives (with parentage testing or missing persons applications) are also collected for comparison purposes.

  3. Sample Collection • DNA Sample Sources • Biological Evidence at Crime Scenes • Evidence Collection and Preservation • Collection of Reference DNA Samples • Storage and Transport of DNA Evidence

  4. Crime Scene Collection of Evidence • Police officers and crime scene investigators respond to the scene of a crime to collect biological evidence to be used in forensic DNA testing • Investigators must be careful not to contaminate the evidence with their own DNA

  5. Some Sources of Biological Materials Producing Successful DNA Profiles by the Canadian RCMP Forensic Biology Laboratories John M. Butler (2009) Fundamentals of Forensic DNA Typing, Table 4.1

  6. DNA Evidence Received in the Lab • Evidentiary samples (commonly in the form of cotton swabs) are brought or shipped to the DNA laboratory after collection from the crime scene or victim • Sexual assault evidence collection kits provide swabs and bags for clothing collections from the victim

  7. DNA Collection bloodstain • Cotton swabs are commonly used to collect biological material from bloodstains or semen from sexual assault victims • The amount of DNA needed has decreased dramatically in the past decade due to sensitivity of the PCR process (which makes millions of copies of targeted regions) Cotton swab Collection Tube

  8. Sources of Biological Evidence • Blood • Semen • Saliva • Urine • Hair • Teeth • Bone • Tissue Blood Sample Only a very small amount of blood is needed to obtain a DNA profile best results with >100 cells, but DNA profiles can be recovered from as little as a single cell

  9. DNA Reference Sample from Suspect • Blood samples may be collected but require a phlebotomist to draw blood • Easier to collect a buccal swab from the inside of an individual’s mouth, which scrapes off some cheek cells

  10. Buccal Swab DNA Collection • The inside of the check is scrubbed to collect cells • Less invasive than drawing blood • Swab must be dried before storing and shipping to lab to avoid mold and bacterial growth John M. Butler (2009) Fundamentals of Forensic DNA Typing, Figure 4.1

  11. Bloodstain Card with Punch Removed for DNA Extraction and Testing Purposes John M. Butler (2009) Fundamentals of Forensic DNA Typing, Figure 4.2

  12. Long-Term Storage of Extracted DNA Samples John M. Butler (2009) Fundamentals of Forensic DNA Typing, Figure 4.3

  13. Sample Characterization • Presumptive Tests for Blood, Semen, and Saliva • Blood Stains • Semen Stains • Direct Observation of Sperm • Saliva Stains

  14. RNA for Sample Characterization • Examination of gene expression in tissues permits identification of the tissue from which the specimen originated • Blood, semen, vaginal secretions, and saliva can be differentiated

  15. Determination of Body Fluid Type • Juusola, J. and Ballantyne, J. (2005) Multiplex mRNA profiling for the identification of body fluids. Forensic Sci. Int. 152(1):1-12 • From abstract:We report the development of a multiplex reverse transcription-polymerase chain reaction (RT-PCR) method for the definitive identification of the body fluids that are commonly encountered in forensic casework analysis, namely blood, saliva, semen, and vaginal secretions. Using selected genes that we have identified as being expressed in a tissue-specific manner we have developed a multiplex RT-PCR assay which is composed of eight body fluid-specific genes and that is optimized for the detection of blood, saliva, semen, and vaginal secretions as single or mixed stains. The genes include beta-spectrin (SPTB) and porphobilinogen deaminase (PBGD) for blood, statherin (STATH) and histatin 3 (HTN3) for saliva, protamine 1 (PRM1) and protamine 2 (PRM2) for semen, and human beta-defensin 1 (HBD-1) and mucin 4 (MUC4) for vaginal secretions.

  16. Chapter 4 – Points for Discussion • Why should forensic evidence be retained and stored even after a case has been closed? • What advantages exist for conducting presumptive testing prior to full DNA analysis? • What is the difference between a presumptive test and a confirmatory test? • What advantages might RNA-based assays bring to sample characterization?