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From the Crime Scene

to the Laboratory

Dr. Hossein Fallahi

Dep. of Biology,

School of Sceinces,

Razi University

Kermanshah

Iran

Lecture 1:

An overview and introduction to the filed

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Employing Biological Sciences in Forensic Analysis

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Why biological evidence matters

• Biological evidence can connect people, objects, places, and events through measurable biological characteristics.
• DNA-containing material is often invisible, so scientific reasoning is needed before collection and testing.
• Forensic biology combines cell biology, molecular biology, genetics, biochemistry, microbiology, and statistics.
• The evidential value depends not only on the laboratory result but also on how the material was found, collected, and preserved.
• A useful mental model is to treat the case as a continuous chain from scene observation to scientific interpretation.

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The forensic biology workflow

• The workflow begins with scene assessment and continues through documentation, collection, packaging, transport, examination, analysis, and interpretation.
• Each stage can preserve information, lose information, or introduce new information into the evidence.
• Laboratory methods cannot recover information that was destroyed or contaminated before analysis.
• Interpretation therefore requires knowledge of both the biological result and its provenance.
• Quality assurance must operate across the complete workflow rather than only inside the DNA laboratory.

Question

→

Evidence

→

Method

→

Result

→

Interpretation

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Locard's exchange principle

• Locard's exchange principle describes the expectation that contact between people and environments can produce transfers of material.
• Transferred material may include cells, blood, saliva, semen, hair, tissue, microorganisms, or other biological traces.
• Transfer does not automatically establish when, how, or why material was deposited.
• Secondary transfer can occur when material moves through an intermediate person or object.
• Investigators therefore combine biological results with scene context rather than treating a detected trace as self-explanatory.

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Biological evidence versus biological material

• Biological material is the physical substance recovered from a person, object, or environment.
• Biological evidence is material considered relevant to a forensic question because of its location, association, or potential information.
• A DNA profile is an analytical result derived from biological material rather than the material itself.
• The distinction matters because a strong DNA profile can still have limited meaning if the origin or transfer history is uncertain.
• Case interpretation should therefore preserve the link between the item, the biological material, the analytical result, and the question being asked.

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Major biological evidence types

• Common categories include blood, saliva, semen, epithelial cells, hair with tissue, tissue fragments, bone, teeth, and other body fluids.
• Different materials differ in cellular content, DNA quantity, degradation rate, and susceptibility to environmental damage.
• Some biological traces are readily visible while others require chemical, optical, or molecular screening.
• Presumptive tests can help locate or characterize material but generally do not replace DNA analysis.
• Evidence selection should be driven by the case question and expected information value rather than by a single preferred sample type.

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Cells and the origin of forensic DNA

• Most nucleated human cells contain genomic DNA organized into chromosomes inside the nucleus.
• Mature red blood cells lack nuclei, so their cellular contribution differs from that of nucleated leukocytes in blood.
• Epithelial cells and leukocytes are major contributors to forensic DNA profiles from many biological traces.
• Mitochondrial DNA is located in mitochondria and can remain informative when nuclear DNA is limited or degraded.
• Understanding cell biology helps explain why some biological traces yield abundant DNA while others yield very little.

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DNA as forensic information

• DNA contains sequence variation that can be measured at selected genetic markers.
• Forensic profiling usually targets highly variable regions that can discriminate among individuals in a population.
• Short tandem repeats, or STRs, are widely used because they are polymorphic and can be amplified by PCR.
• The laboratory observes alleles at selected loci and converts them into a DNA profile for comparison and statistical evaluation.
• The profile is evidence about genetic similarity, not a direct measurement of identity, behavior, or guilt.

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Presumptive and confirmatory thinking

• Presumptive testing is designed to indicate that a target biological material may be present.
• Confirmatory or highly specific testing seeks stronger evidence that the material is consistent with a particular biological source.
• A positive presumptive reaction can be affected by specificity, sensitivity, substrate, and environmental conditions.
• DNA testing may identify a contributor even when the original body-fluid classification remains uncertain.
• Students should learn to state exactly what each test establishes and what it does not establish.

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Scene documentation before sampling

• The scene should be documented before evidence is disturbed whenever circumstances permit.
• Notes, photographs, sketches, item identifiers, and locations create a record of the original context.
• Biological evidence should be interpreted with knowledge of where it was found and what surfaces or objects were involved.
• Scene documentation also helps distinguish expected background material from potentially probative deposits.
• Poor documentation can weaken an otherwise technically excellent laboratory result.

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Collection strategy

• Collection priorities should consider fragility, environmental exposure, expected DNA yield, and the investigative question.
• Reference samples may be collected from relevant individuals to support comparisons with questioned evidence.
• Control or substrate samples can help evaluate background material and the effect of the surface itself.
• Collection procedures should minimize loss, cross-transfer, and contamination between items.
• Local laboratory standard operating procedures should govern detailed collection methods because requirements vary by jurisdiction.

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Preservation and packaging

• Biological evidence should generally be dried appropriately before packaging when the evidence type and procedure permit.
• Paper packaging is commonly preferred for dried biological evidence because it allows moisture exchange.
• Moisture, heat, microbes, and prolonged environmental exposure can accelerate biological degradation.
• Packaging should protect the evidence from loss, contamination, and physical damage during transport and storage.
• Preservation is part of the scientific process because DNA quality depends strongly on what happens before extraction.

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Contamination control

• Contamination is the introduction of biological material that is not part of the original evidentiary deposit.
• Potential sources include investigators, medical personnel, laboratory personnel, reagents, equipment, packaging, and other evidence items.
• Personal protective equipment and controlled workflows reduce opportunities for transfer.
• Negative controls and contamination monitoring help detect problems during laboratory processing.
• Modern forensic DNA standards emphasize validation and documented procedures for preventing, detecting, and addressing contamination.

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Chain of custody

• Chain of custody documents who collected, handled, transferred, stored, and examined an evidence item.
• The record links the physical item tested in the laboratory to the item documented at the scene.
• Dates, times, identifiers, condition, packaging, transfers, and signatures or electronic records may form part of the chain.
• An analytical result can be scientifically sound while still facing questions if item identity or handling history is poorly documented.
• Students should regard chain of custody as an information-integrity system rather than merely administrative paperwork.

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Laboratory intake and triage

• Laboratory examination begins with evidence receipt, verification, documentation, and assessment of the submitted items.
• Analysts consider the case question, sample condition, packaging, controls, and requested examinations.
• Triage helps prioritize limited biological material and reduces unnecessary consumption of irreplaceable evidence.
• Sampling decisions should be documented because they influence what can later be inferred from the case.
• Good laboratory practice connects the submitted item, sample selection, analytical method, and final report.

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DNA extraction

• DNA extraction separates nucleic acids from cells and other components of a biological sample.
• Common strategies use chemical lysis, enzymatic digestion, binding to a solid phase, magnetic particles, or differential processing.
• The extraction method must balance DNA recovery with removal of inhibitors and unwanted material.
• Sexual-assault samples may require strategies that help separate sperm-rich and non-sperm cellular fractions.
• Extraction controls are important because contamination or process failure can otherwise be difficult to recognize.

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DNA quantification

• Quantification estimates the amount of amplifiable human DNA available for downstream testing.
• Quantification helps the laboratory select an appropriate amount of template for PCR and assess sample quality.
• Too little template can increase stochastic effects, while excessive template can affect assay performance and interpretation.
• Modern assays may also provide information related to human DNA quantity and, in some workflows, degradation or inhibition.
• Quantification is therefore a decision point between extraction and amplification rather than a purely descriptive measurement.

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