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DNA Typing and Forensics Overview

Overview of DNA Typing in Forensics

  • Applications of DNA Testing:
    • Crime solving: Matching suspects with evidence.
    • Paternity testing: Determining biological relationships.
    • Missing person investigations: Identifying remains.
    • Immigration testing: Establishing relationships.
    • Disaster victims identification and military personnel identification.
  • Important Concepts:
    • DNA Structure: Large molecule made of nucleotides (sugar, phosphate, base).
    • Bases: Adenine (A), Thymine (T), Cytosine (C), Guanine (G).
    • Base pairing rules: A=T, G=C.

Detailed Structure of DNA

  • A nucleotide consists of:
    • Sugar: Deoxyribose.
    • Phosphate group.
    • Nitrogenous base (A, T, C, G).
  • Double Helix: Strands coil around each other forming a double helix structure, stabilized by base pairs.

DNA Typing and Its Importance

  • Tandem Repeats:
    • DNA regions with repeating sequences, critical for distinguishing individuals.
    • Variability among individuals at specific loci (positions in the genome).
  • These repeats are crucial for creating DNA profiles used in forensic investigations.

History of DNA Fingerprinting

  • Key Milestones:
    • 1985: Alec Jeffreys develops the DNA profiling technique.
    • 1986: The Pitchfork murders exonerate a falsely accused suspect and identify the real perpetrator.

Steps in Forensic DNA Analysis

  1. Collection: Gathering samples from crime scenes.
  2. Storage: Properly storing specimens until analysis.
  3. Extraction: Isolating DNA from biological samples.
  4. Quantification: Measuring the amount of DNA available.
  5. PCR Amplification: Replicating small quantities of DNA.
  6. STR Typing: Analyzing specific regions of DNA for variations.
  7. Data Interpretation:
    • Matching DNA profiles with known references.
  8. Database Storage & Searches.

Polymerase Chain Reaction (PCR)

  • Technique used to amplify DNA.
  • Steps:
    1. Denaturation: Heating DNA to separate strands.
    2. Primer Annealing: Adding primers to target DNA sequences.
    3. Extension: DNA polymerase adds nucleotides to replicate DNA strands.

Capillary Electrophoresis

  • Method for separating DNA fragments by size using an electric current in a gel medium.
  • Produces an electropherogram to visualize the results.

Interpretation of DNA Evidence

  • DNA profiles must be compared:
    • Evidence from a crime scene to suspect profiles.
    • Mother’s and father’s genetic contributions for paternity tests.
  • Possible Outcomes of DNA Analysis:
    • Exclusion (no match), Non-exclusion (match), and inconclusive results.

Reference Databases and Systems

  • CODIS (Combined DNA Index System): Main source of DNA profiles in the U.S.
  • Canadian National DNA Data Bank: Composed of two indices:
    • Convicted Offender Index
    • Crime Scene Index

Ethical and Privacy Concerns

  • Potential misuse of DNA information.
  • Sample retention issues after exoneration.

Witness Cases and Applications in Wildlife Forensics

  • Wildlife DNA forensics apply the same principles but differ in species identification.
  • Markers used in wildlife include blood, tissue, and other biological samples.

Advancements in Genetic Genealogy

  • Recent cases demonstrate the power of DNA analysis in solving cold cases through advanced genetic testing.