DNA Evidence Analysis

1. Trace DNA vs Touch DNA

Trace DNA refers to small amounts of DNA found at a scene, typically not directly linked to a specific body fluid. It represents minute biological material left behind unintentionally.

Touch DNA is a subset of trace DNA, specifically DNA obtained from skin cells or cell-free DNA (cfDNA) deposited by physical contact. It often comes from fluids or secretions containing cells and/or cfDNA.

Trace DNA includes all small DNA deposits, while Touch DNA specifically involves DNA left by skin contact or cell-free DNA.

1. Touch DNA

• Touch DNA: DNA obtained specifically from skin cells left by physical contact.
• Amount and quality of DNA depend on:
  • Surface type: porous vs non-porous surfaces affect DNA retention.
  • Individual variability: some people are better DNA shedders than others.
  • Environmental and personal variables influence DNA transfer.

2. Epithelial Cells and DNA Sensitivity

• DNA mainly comes from epithelial cells in touch DNA samples.
• Current technology requires 0.5 ng of DNA for a full genetic profile.
• Molecular weights:
  • One genome: 0.00346 ng
  • One cell: 0.00692 ng (diploid)
• Number of cells needed for a full profile: approximately 70 cells.
• Human skin sheds about 30,000–40,000 cells per hour, influencing DNA availability.

3. Factors Affecting DNA Recovery

Key point: Touch DNA varies widely due to surface type, individual shedding rates, and environmental factors, requiring careful collection and interpretation.

1. Trace DNA and Touch DNA Fundamentals

Trace DNA refers to minute quantities of DNA left behind from skin cells or other biological material after contact with an object or surface. Touch DNA is a subset of trace DNA specifically deposited by skin contact.

2. Factors Affecting DNA Deposition

DNA deposition is highly variable and influenced by multiple factors:

3. Sampling Methods for Trace DNA

Sampling techniques depend on the surface type:

4. Importance of Swab and Extraction Pairing

• The choice of swab and DNA extraction method significantly impacts DNA recovery efficiency.
• Optimizing this pairing enhances the yield and quality of trace DNA for analysis.

5. DNA Shedders

• Individuals vary in their ability to deposit DNA, categorized as high shedders or low shedders.
• Shedder status affects the amount of DNA left on touched objects, influencing forensic interpretation.

Key point: DNA deposition and recovery depend on a complex interplay of surface type, environmental factors, individual shedding status, and sampling methods.

1. DNA Sensitivity and Evidence Screening

Shedder Status

• Good Shedder: Leaves enough epithelial material to generate a full DNA profile from mere touch.
• Bad Shedder: Leaves only partial DNA profiles.
• High Shedding increases susceptibility to secondary transfer, which can be used as a defense argument in forensic cases.

2. Factors Affecting Shedding of Epithelial Cells

To retain: High shedder status is variable and influenced by physiological and behavioral factors, impacting DNA evidence sensitivity and interpretation.

3. Implications for Evidence Screening

• Understanding shedder variability is crucial for evaluating DNA evidence reliability.
• Screening must consider the likelihood of full vs partial profiles based on shedder status and contact conditions.
• Secondary transfer risk is higher with high shedders, affecting the weight of DNA evidence in court.

1. Hair Structure and DNA Exception

• Hair vs Fibres: Hair cannot be individualized like fibres, except through DNA analysis.
• DNA Exception: Hair DNA originates from the follicle/root, enabling individual identification.
• Hair Growth: Hair grows out of the follicle; the root or bulb extends from it.

2. Hair Anatomy

3. Cuticle Details

• Provides resistance to hair damage.
• Scale patterns are species-specific.
• Scales overlap and point toward the hair tip.
• Composed of specialized keratinized cells.
• Analysis methods: Scanning Electron Microscopy (SEM) and casting.

4. Cortex Details

• Contains regularly arranged cortical cells.
• Houses pigment granules responsible for hair color.

5. Medulla Details

• Appears as a central canal within the hair.
• The medullary index is a crucial measurement: $$\text{Medullary Index} = \frac{\text{Medulla Diameter}}{\text{Total Hair Diameter}}$$
• In humans, this index is typically less than 1/3.
• Medulla patterns vary between individuals and hair origins.
• Patterns include continuous, interrupted, fragmented, or absent.
• Used for species identification based on pattern type.

Hair analysis combines structural features and DNA from the follicle for individualization, unlike fibres.

1. DNA Shedders and Individual Variation

DNA shedding refers to the release of DNA from an individual into the environment, primarily through skin cells, hair, saliva, sweat, or other biological materials. The amount and quality of shed DNA vary significantly between individuals, influencing forensic DNA recovery and analysis.

2. Key Concepts

• Shedding propensity: Individuals differ in how much DNA they shed. Some are "good shedders" (high DNA depositors), others are "poor shedders".
• Factors influencing shedding:
  • Biological: skin type, sweat rate, hygiene habits.
  • Environmental: surface type, time since deposition.
  • Activity: physical contact intensity and duration.

3. Individual Variation in DNA Shedding

4. Practical Implications

• DNA evidence interpretation must consider individual shedding variability to avoid false assumptions about contact or involvement.
• Low DNA recovery does not necessarily imply absence of contact; the individual may be a poor shedder.
• High DNA recovery could result from a good shedder or repeated contact.
• Contamination risk increases with high sensitivity methods detecting low-level DNA deposits, including from secondary transfer.

To retain: Individual variation in DNA shedding significantly affects forensic DNA evidence; understanding shedding propensity is crucial for accurate interpretation of DNA profiles.

1. Hair as Biological Evidence

• DNA profiling from hair evidence can link suspects to crimes even when the hair is found on unrelated items, such as clothing not connected to the crime scene.

• Casework Example A:

  • In a high-profile murder investigation of a young girl, DNA profiling of the victim's clothing (unrelated to the crime scene) produced a full male DNA profile.
  • This profile was cross-referenced with the National DNA Database (NDNADB), revealing connections to two unsolved crimes: a burglary and a drug-related offense in different regions.
  • This demonstrates how hair evidence can provide intelligence leads beyond the immediate case.

• Casework Example B (partial):

  • A burnt female murder victim was found in a public park in London.
  • A toothbrush from the suspected victim’s home was submitted for analysis, implying the use of personal items for DNA comparison in cases where hair evidence may be compromised or limited.

Hair evidence, when combined with DNA profiling and database searches, can link suspects across multiple crimes and provide crucial investigative leads.

1. Contamination at Crime Scenes

Contamination occurs when DNA from an unrelated source is unintentionally introduced into evidence, leading to mixed or misleading DNA profiles.

a) Key Points on Contamination

• Mixed DNA profiles can result from contamination, showing DNA from the victim plus minor components from unknown individuals.
• A male DNA profile found at a crime scene may not always match suspects or individuals connected to the case, raising suspicion of contamination.
• Contamination can cause false links between different crimes or locations, as seen when DNA profiles from one crime scene match profiles from unrelated offences hundreds of miles away.
• Such discrepancies often trigger internal inquiries within forensic services and police investigations to identify contamination sources.

b) Implications

Contamination at crime scenes can compromise the integrity of DNA evidence, making strict procedural controls essential.

1. Laboratory Contamination

Contamination in DNA analysis labs can occur despite strict protocols, affecting negative controls and case samples.

• Negative control batches have shown contamination, indicating lab-origin DNA presence.
• Case studies:
  • Case A: DNA matched 2 negative controls collected 4 months apart.
  • Case B: DNA matched 9 negative controls over 3 years and matched a QC check of a returned batch of tubes.
• Over 1 million samples tested in 3 years revealed persistent contamination issues (Howitt et al., 2003).

2. Contamination Sources and Challenges

• Contamination can arise from lab staff, equipment, or consumables.
• Cross-contamination between exhibits and scenes is a risk, e.g., DNA transfer from Exhibit 1 Scene A to Exhibit 2 Scene A or to Exhibit 1 Scene B.
• Unexpected contamination events ("WTF" moments) highlight the complexity of contamination control.

3. Strategies to Overcome Contamination

At the Crime Scene and Laboratory:

Key point: Rigorous PPE protocols and strict scene-to-exhibit handling procedures are essential to minimize laboratory contamination and ensure DNA evidence integrity.

1. Training and Awareness

• Training is essential for all personnel to understand and implement contamination prevention.
• Maintain awareness of contamination risks at every stage: from scene to court.

2. Proper Procedures and PPE

• Use Personal Protective Equipment (PPE) consistently to minimize contamination.
• Follow strict proper procedures for handling evidence, including packaging and tools.

3. Evidence Handling and Storage

4. Scene to Laboratory Transitions

• Track samples meticulously during transfers (e.g., Exhibit 1 Scene A to Scene B).
• Avoid using the same tools or vehicles without decontamination.
• Maintain clean examination rooms and benches to prevent background DNA buildup.

5. Decontamination Protocols

• Implement decontamination procedures for all workspaces regularly.
• Perform deep cleaning every 3 months to reduce rising background DNA.
• Use bleach for disinfecting surfaces and tools:
  • Discard disposable items after use.
  • Soak reusable items before re-bleaching.
• Re-bleach surfaces as necessary to maintain cleanliness.

> Preventing cross-contamination requires rigorous training, strict procedural adherence, dedicated equipment, and regular decontamination to ensure DNA evidence integrity.