DNA Profiling and Forensic Biology — MCQs

DNA Profiling and Forensic Biology Indian Medical PG Practice Questions and MCQs

Question 41: Best site for DNA extraction from a 2-month-old decomposed body?

A. Muscle
B. Bone
C. Teeth (Correct Answer)
D. Hair

Explanation: ***Teeth*** - Teeth, particularly the **pulp and dentin**, provide a highly protected environment for DNA, making them ideal for DNA extraction from **decomposed remains** due to their robust structure. - The hard enamel casing shields the internal DNA from environmental degradation and microbial contamination, allowing for excellent preservation over extended periods. - **Dental pulp** is consistently reliable and easily accessible, making teeth the **preferred first choice** in forensic DNA extraction from decomposed bodies. *Bone* - **Bone**, particularly the **petrous portion of the temporal bone** and long bones, is also an **excellent source** of DNA in decomposed remains and is widely used in forensic practice. - However, DNA extraction from bone requires more extensive processing (demineralization, grinding) compared to teeth, making it a **second-line choice** when teeth are available. - The petrous temporal bone is notably resistant to degradation, but teeth remain more practically accessible. *Muscle* - **Muscle tissue** contains significant DNA when fresh, but is highly susceptible to **autolysis and bacterial degradation** in a decomposed body. - As decomposition progresses over 2 months, muscle tissue breaks down rapidly, reducing both the quantity and quality of recoverable DNA significantly. *Hair* - **Hair shafts** primarily contain mitochondrial DNA (mtDNA) with minimal nuclear DNA, which limits their use for individual identification. - Hair roots (if present) contain nuclear DNA, but in decomposed remains, hair is often shed or degraded, making it an unreliable source compared to teeth.

Question 42: In a forensic investigation, which technique would you use to identify the presence of seminal stains?

A. Florence test
B. Acid phosphatase test (Correct Answer)
C. Luminol test
D. Barberio test

Explanation: ***Acid phosphatase test*** - This is the **primary presumptive test** for screening and identifying the presence of seminal stains in forensic investigations. - Detects the enzyme **acid phosphatase**, which is present in **high concentrations in human semen** (approximately 400 times higher than in other body fluids). - A positive result, usually indicated by a **purple color change**, provides rapid preliminary identification of potential seminal stains. - This is the **preferred initial screening method** due to its sensitivity and ease of use. *Florence test* - The Florence test detects the presence of **choline**, forming characteristic **dark brown rhombic crystals** in the presence of seminal fluid. - This is a **confirmatory test** rather than a screening test, used after presumptive testing. - While specific for semen, it is not the primary method for initial identification of seminal stains. *Luminol test* - The luminol test is used to detect **bloodstains** by reacting with the iron in hemoglobin, causing **chemiluminescence** (bluish glow). - It is highly sensitive for blood but does **not identify semen** and is not used for seminal stain detection. *Barberio test* - The Barberio test detects **spermine picrate crystals**, which are specific to semen. - This is also a **confirmatory test** that requires the presence of seminal constituents. - Not used as the primary screening method for identifying the presence of seminal stains.

Question 43: Test to determine the species from a bloodstain is?

A. Takayama test
B. Benzidine test
C. Precipitin test (Correct Answer)
D. Teichmann's test

Explanation: ***Precipitin test*** - The **precipitin test** is a widely used serological method to determine the **species origin of a bloodstain**. - It works by detecting the presence of **species-specific proteins** (antigens) in the bloodstain through a reaction with known antibodies. *Takayama test* - The **Takayama test**, along with the Teichmann test, is a **confirmatory chemical test** used to identify blood by forming characteristic **heme crystal formations**. - It does not provide information about the **species origin** of the blood. *Benzidine test* - The **benzidine test** is a **presumptive test** for blood, meaning it indicates the *possible* presence of blood based on the peroxidase activity of hemoglobin. - It is highly sensitive but **not specific** to human blood and can react with other substances or animal blood, hence it cannot determine the species. *Teichmann's test* - **Teichmann's test** is another **confirmatory chemical test** for blood, similar to the Takayama test. - It relies on the formation of **hemin crystals** (hemochromogen crystals) and does not differentiate between human and animal blood.

Question 44: The absorption elution technique is used for?

A. Detection of blood groups (Correct Answer)
B. Examination of seminal stains
C. Detection of species
D. None of the options

Explanation: ***Detection of blood groups*** - The **absorption-elution technique** is a sensitive method used to detect **ABO blood group antigens** in dried bloodstains, a common application in forensic serology. - It involves absorbing antibodies specific to blood group antigens onto the dried stain and then eluting these absorbed antibodies to react with known red blood cells. *Examination of seminal stains* - **Seminal stains** are typically examined for the presence of **spermatozoa** or markers like **prostate-specific antigen (PSA)** or **acid phosphatase**. - The absorption-elution technique is not the primary method for identifying seminal fluid or its origin. *Detection of species* - **Species identification** in forensic samples usually involves techniques such as **DNA analysis** (e.g., mitochondrial DNA sequencing) or **precipitin tests**. - The absorption-elution method is specific to blood group antigens and not generally used for broad species differentiation. *None of the options* - This option is incorrect because the **absorption-elution technique** is indeed directly applicable and commonly used for the **detection of blood groups**.

Question 45: What is the most forensically significant information that can be detected from a dried blood stain?

A. Nature of stain
B. Detection species
C. Blood group (Correct Answer)
D. None of the options

Explanation: ***Blood group*** - Among the options provided, **blood group** analysis provides identifying information about the individual who deposited the blood, helping to include or exclude suspects. - However, it should be noted that **DNA profiling** (not listed as an option) is the gold standard in modern forensic practice, offering individual-specific identification with far superior discriminatory power. - Blood group typing has **limited discriminatory value** as it only classifies individuals into broad categories (A, B, AB, O) shared by millions of people. - In contemporary forensic laboratories, blood grouping has been largely superseded by **DNA analysis** from dried blood stains. *Nature of stain* - The **nature of the stain** (e.g., drip, spatter, swipe) indicates how the blood was deposited through **bloodstain pattern analysis (BPA)**. - This provides valuable information for **crime scene reconstruction** and understanding the sequence of events, but does not identify the source individual. - It's a separate forensic discipline from biological identification. *Detection species* - **Species identification** determines if the blood is human or animal, which is an important **preliminary screening test**. - This is typically done using precipitin tests or other immunological methods. - While important, it lacks **individual specificity** and only confirms the blood is human, without linking it to a particular person. *None of the options* - This option would arguably be the most accurate if we consider that **DNA profiling** is the truly most forensically significant information obtainable from dried blood stains in modern practice. - However, given the context of the options provided, blood group represents the most individualizing characteristic available among the choices listed.

Question 46: Which of the following sources provides the highest yield and most reliable samples for DNA fingerprinting?

A. Saliva
B. Tooth
C. Buccal mucosa
D. Blood (Correct Answer)

Explanation: ***Blood*** - **Blood** provides a high concentration of **nucleated cells** (e.g., white blood cells), yielding abundant and high-quality DNA. - The DNA obtained from blood is typically well-preserved and less prone to degradation or contamination compared to other sources. *Saliva* - While saliva contains DNA from **buccal epithelial cells** and white blood cells, its DNA yield can be lower and more variable due to mucous and bacterial contamination. - DNA from saliva may be more subject to degradation, especially if not collected and stored properly. *Tooth* - **Teeth** can be a good source of DNA, particularly from the **pulp**, but extraction can be challenging and destructive. - The DNA yield varies depending on the tooth's condition and the extraction method, and it is generally reserved for situations where other sources are unavailable or severely degraded. *Buccal mucosa* - **Buccal mucosa** swabs are a common and non-invasive source of DNA from **epithelial cells**. - While suitable for many applications, the DNA yield can be lower than blood, and the sample may be more susceptible to surface contamination.

Practice by Chapter

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Statistical Interpretation

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Body Fluid Identification

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Bloodstain Pattern Analysis

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