Biological Evidence

1. Forensic Serology and ABO & Rhesus Blood Grouping Systems [40 marks]

Forensic serology is a branch of forensic science used to identify and analyze biological fluids, particularly blood, to establish potential links between a suspect and a crime scene. It played a critical role in forensic investigations prior to the advent of DNA profiling. Blood grouping in forensic serology involves the identification of antigens present on red blood cells (RBCs). The ABO blood grouping system and Rhesus (Rh) system are the most significant in identifying blood types.

ABO Blood Grouping System:

• Principle: The ABO system identifies the presence or absence of two antigens, A and B, on the surface of red blood cells.

  • Type A: Red blood cells have A antigens, and plasma contains anti-B antibodies.

  • Type B: Red blood cells have B antigens, and plasma contains anti-A antibodies.

  • Type AB: Red blood cells have both A and B antigens, and plasma contains no antibodies against A or B.

  • Type O: Red blood cells have neither A nor B antigens, and plasma contains both anti-A and anti-B antibodies.

• Identification Process: The ABO type is identified by performing an agglutination test:

  1. The blood sample is mixed with antibodies (anti-A and anti-B).

  2. If the blood contains the corresponding antigen, agglutination (clumping) will occur.

  3. The presence or absence of agglutination determines the blood group.

Rhesus Blood Grouping System:

• Principle: The Rhesus (Rh) system is based on the presence or absence of the Rh antigen (also called the Rh factor) on the surface of red blood cells.

  • Rh-positive (Rh+): The red blood cells have the Rh antigen.

  • Rh-negative (Rh-): The red blood cells lack the Rh antigen.

• Identification Process: The Rh factor is identified by adding anti-Rh antibodies to the blood sample. If agglutination occurs, the blood is Rh-positive; if no agglutination occurs, the blood is Rh-negative.

By combining the ABO and Rh blood group results, a blood type can be fully identified (e.g., A+, O-, AB+, etc.), which can provide valuable information during forensic investigations.

2. Morphology of the Hair Shaft [10 marks]

The hair shaft consists of three main layers:

1. Cuticle:

   • The outermost layer of the hair shaft.

   • Composed of overlapping, keratinized cells that point towards the tip of the hair.

   • Provides structural integrity and makes the hair resistant to chemical decomposition.

   • The cuticle's scale patterns can be observed under scanning electron microscopy (SEM) and are useful in species identification.

2. Cortex:

   • The middle layer, containing pigment granules responsible for hair color.

   • The distribution, shape, and color intensity of these granules are used to compare hair samples from different individuals.

3. Medulla:

   • A central canal-like structure running through the hair shaft.

   • In humans, the medulla is fragmented or absent, but in animals, it can occupy more than half the diameter of the hair.

   • The presence, absence, or structure of the medulla can assist in identifying the species of origin.

The combination of these three layers contributes to the overall physical characteristics of the hair, which can be used in forensic investigations to help link a suspect to a crime scene.

3. Identification and Comparison of Hair Based on Morphology [20 marks]

Hair analysis in forensic science often involves the use of a comparison microscope, which allows the examination of two hair samples side-by-side for comparison. The following morphological features are considered during hair comparison:

1. Cuticle Characteristics:

   • Observing the pattern of the cuticle scales, which vary between species. This can help determine the origin of the hair (human vs. animal).

   • The scale pattern may be continuous, fragmented, or absent, depending on the species.

2. Cortex Pigmentation:

   • The presence, shape, and distribution of pigment granules in the cortex provide information about the hair color and can be compared across individuals.

   • The intensity and type of pigment (e.g., eumelanin for brown/black hair, pheomelanin for red hair) can help in the identification of individuals.

3. Medulla Structure:

   • The medulla’s presence or absence is a key distinguishing factor in species identification. Human hair often has a fragmented medulla, while animal hair may have a continuous or larger medulla.

4. Diameter and Shape:

   • The diameter and overall shape of the hair are unique to individuals and can provide insights into the origin of the sample (head, body, pubic, etc.).

5. Root Characteristics:

   • Hair roots in different growth phases (Anagen, Catagen, Telogen) can provide insights into the time of collection and the condition of the hair.

   • A follicular tag (found in anagen-phase hair) can be crucial for DNA analysis.

By analyzing these morphological features, forensic scientists can compare hair samples found at crime scenes with samples from suspects or victims, providing valuable investigative leads.

4. Limitations of Hair as Crime Scene Evidence [10 marks]

While hair can be a valuable source of forensic evidence, several limitations must be considered:

1. Lack of Nuclear DNA:

   • Hair found at a crime scene may not always yield nuclear DNA, especially if the root is absent or in the telogen phase (which is often shed naturally).

2. Individualization Challenges:

   • Hair comparison can indicate whether two hairs share similar features, but it often cannot definitively individualize a suspect's hair unless a follicular tag with DNA is present.

3. Contamination:

   • Hair samples may be contaminated with DNA from various sources (e.g., victim’s or suspect's skin cells), complicating analysis and interpretation.

4. Human vs. Animal Hair:

   • Distinguishing between human and animal hair is possible, but it can be difficult to tell the specific individual or species, especially if the hair has been altered or degraded.

5. Environmental Factors:

   • Hair is subject to environmental conditions, such as humidity and temperature, which can cause degradation or loss of identifying features (e.g., pigment or cuticle structure).

Thus, while hair can provide useful information, its limitations should be carefully considered in forensic investigations.

5. Presumptive and Confirmative Tests for Semen and Blood & Blood Evidence Collection [50 marks]

Presumptive and Confirmative Tests for Semen:

• Presumptive Test for Semen:

  • Phadebas® Amylase Test: This test detects the presence of alpha-amylase, an enzyme in high concentrations in human saliva and semen. A blue dye is released when alpha-amylase is present.

• Confirmative Test for Semen:

  • Christmas Tree Stain: A staining technique that specifically identifies sperm cells by coloring the sperm heads red and tails green.

  • Microscopic Examination: Observing the sample under a microscope can confirm the presence of spermatozoa.

Presumptive and Confirmative Tests for Blood:

• Presumptive Test for Blood:

  • Kastle-Meyer Test: This test detects the presence of hemoglobin in blood by reacting with a chemical solution, producing a pink color if blood is present.

• Confirmative Test for Blood:

  • Takayama Test: This test detects hemoglobin crystals in blood, providing a definitive confirmation.

  • Blood Typing (ABO & Rh): Identifying blood group and Rh factor can confirm blood presence and match it to potential suspects or victims.

Methods for Collecting Blood Evidence:

1. Swabbing: Using a sterile swab to collect blood from surfaces or objects at a crime scene.

2. Absorbent Material: Blood can be absorbed onto gauze or cotton pads for collection.

3. Scraping: If blood is dried, scraping off the residue from surfaces may be necessary.

4. Blood Samples from Individuals: Blood can be collected directly from victims or suspects for DNA profiling or blood typing.

These tests and methods are crucial for establishing the presence of biological evidence, which can help identify perpetrators or victims in forensic investigations.