Biological Evidence

Any material of biological (life) origin that can be gathered from a crime scene to support a forensic inquiry is referred to as biological evidence.
Cells, DNA, proteins, or biological remnants that can identify people, establish events, and connect suspects, victims, and crime scenes are examples of this type of evidence.

Sources of Biological Evidence 

Blood

• One of the most common biological evidence types.

• Can identify:

  • Whose blood is present (using DNA)

  • Whether the blood is human or animal

  • The blood group (ABO, Rh)

• Bloodstain Pattern Analysis (BPA) can reveal:

  • Point of origin

  • Type of weapon

  • Number of blows

  • Position of attacker/victim

  • Direction of travel

• Used in homicide, assault, and accident cases.

Semen

• Crucial in sexual assault cases.

• Contains:

  • Sperm cells (nuclear DNA — high value)

  • Seminal fluid proteins (useful even when sperm is absent)

• Can determine:

  • DNA profile

  • Sexual activity evidence

  • Presence of multiple contributors

Saliva

Present on:

• Bite marks

• Food items

• Cigarette butts

• Bottles

• Envelopes
  Contains:

• Buccal epithelial cells → good DNA source

• Enzymes like amylase used for identification

Hair

Hair provides valuable forensic data:

• Root present? → nuclear DNA

• No root? → mitochondrial DNA (mtDNA)
  Hair analysis reveals:

• Species

• Drug use patterns

• Environmental exposures (gunshot residue, cosmetics)

Sweat and Skin Cells

Also known as Touch DNA.
Skin cells are left when someone touches surfaces like:

• Weapons

• Clothing

• Door handles

Even a few cells can generate a DNA profile using advanced techniques (e.g., Low Copy Number DNA).

Bones and Teeth

Extremely important when:

• Bodies are decomposed

• Burned

• Buried

• Skeletonized

Bones and teeth are highly durable and preserve:

• DNA (nuclear and mitochondrial)

• Proteins (forensic proteomics)

• Age estimation clues

• Trauma evidence (fractures, sharp force marks)

Nails

Found in cases involving struggles.
Under nails, investigators may find:

• Skin cells

• Blood

• Hair fragments

• DNA from the attacker
  Can help reconstruct violent interactions.

Tissues and Organs

Used in:

• Autopsies

• Poisoning cases

• Disease or pathology evaluation

• DNA extraction
  Can detect drugs, alcohol, and toxins via toxicological analysis.

What Biological Evidence Can Reveal

 Personal Identification

• DNA profiling identifies individuals with extreme accuracy.

• Even degraded samples can work using mtDNA or SNP analysis.

 Reconstruction of Crime Events

BPA, saliva location, hair distribution, etc., help reconstruct:

• Sequence of events

• Type of assault

• Location of victim before and after attack

 Relationship Testing

Used for:

• Paternity/maternity

• Missing persons cases

• Child trafficking investigations

 Time Since Death (PMI)

Biological changes in tissue, bones, or proteins help estimate PMI.

 Sexual Crime Analysis

Presence of semen, epithelial cells, or lubricants helps confirm sexual contact.

HOW TO EXAMINE & COLLECT BIOLOGICAL EVIDENCE

Biological evidence is highly valuable but extremely fragile, so proper examination and collection are critical to prevent contamination, degradation, or loss.

Crime Scene Examination for Biological Evidence

Biological evidence may be visible or invisible.

A. Visual Examination

Look for:

• Blood stains/splashes

• Hair

• Saliva (on bite marks, cigarettes)

• Semen stains

• Urine/feces

• Nails

• Sweat stains

• Tissue fragments

• Clothing with bodily fluids

Use LED flashlights for small or dried stains.

B. Use of Alternative Light Sources (ALS)

ALS helps locate invisible biological traces.

• Semen fluoresces under 450–495 nm blue light

• Saliva and sweat reflect under UV

• Blood absorbs light, appears dark under ALS

ALS avoids unnecessary destructive testing.

C. Presumptive Tests (On-site Screening)

Used to confirm suspicion before collection.

Blood

• Kastle–Meyer (pink color)

• Luminol (blue glow in dark)

• Hemastix

Semen

• Acid phosphatase test

• p30/PSA test

Saliva

• Amylase test

These tests guide collection but are not confirmatory.

Methods of Collecting Biological Evidence

Each biological sample type has a specific collection method.

1. Collection of Blood Evidence

 Wet blood

• Use a sterile cotton swab

• Allow to air-dry

• Pack in paper envelope/bag

 Dry blood stains

Two options:

(a) Swabbing Method

• Moisten sterile swab with distilled water

• Rub gently over stain

• Air-dry

• Package

(b) Cutting Method

• Cut stained portion of fabric

• Store in paper bag

 Blood pools

• Use sterile gauze to absorb

• Let it dry

• Pack in paper

2. Semen Collection

Found in sexual assault cases.

 On clothing/bedding

• Air-dry

• Cut stained area

• Package separately

 From body (victim/suspect)

• Use sterile swabs

• Collect vaginal, anal, or oral swabs

• Air-dry

• Pack individually

 Confirmatory test later:

• Presence of spermatozoa

• PSA / p30 protein

• Semenogelin

3. Saliva Collection

Found on:

• Cigarette butts

• Bite marks

• Cups

• Envelopes

Swabbing method

• Moisten swab

• Swab the suspected area

• Air-dry, pack in paper swab boxes

 Whole items

• Small items like butts or straws can be collected directly in a paper envelope.

4. Hair Collection

 Pick up visible hairs using forceps

 Collect reference hair samples

• 20–25 pulled hairs from suspect/victim

• From different scalp regions

 Package in folded paper (pharmacist fold)

Hair must remain dry.

5. Skin Cells / Touch DNA

Often very small quantities.

 Use double-swab technique

1. Moist swab — collect cells

2. Dry swab — absorb leftover material

Best for:

• Weapons

• Door handles

• Clothing

• Mobile phones

6. Bone & Teeth Collection

Used in decomposed or burned cases.

 Use clean gloved hands or forceps

 Store in paper bags or boxes

 Do not wash or scrub — it destroys DNA

 For large bones, wrap in clean paper

7. Nail Scrapings & Clippings

Important in assault cases.

 Scrapings

• Use sterile toothpick/scalpel

• Collect debris under nails

• Store in small envelopes

 Clippings

• Cut nails carefully

• Place in labeled envelope

8. Tissue Samples

Collected during autopsies or major accidents.

 Use scalpel to remove tissue

 Store in airtight sterile containers

(NOT in formalin if used for DNA — formalin damages DNA)

9. Biological Fluids (Urine, Feces, Vomit)

 Collect in sterile leak-proof containers

 Refrigerate immediately

Useful for:

• Toxicology

• Drug analysis

 Packaging & Preservation Rules

 Use paper bags → allow drying

 Do NOT use plastic → causes mold

 Air-dry all wet samples before packing

 Store DNA samples in a cool place

 Seal with evidence tape

 Label properly:

• Case number

• Sample type

• Date, time

• Collector's name

• Location from crime scene

 Chain of Custody

A legal record that tracks:

• who collected

• who handled

• where it was stored

• how it was transported

This ensures evidence is legally admissible in court.

Transport & Storage

 Refrigerate:

• Blood

• Urine

• Tissue

 Freeze (–20°C):

• Semen samples

• DNA samples

• Organs

 Room temperature:

• Hair

• Bones

• Teeth

• Dried stains

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