Y-STR and Mitochondrial DNA — MCQs
Select the FALSE combination of chromosomal pattern and the syndrome:
True regarding mitochondrial DNA is:
Best method for the detection of mutations with low allele frequency is:
An affected male does not have affected children but an affected female always has affected children. Type of inheritance?
Which of the following statements regarding mitochondrial DNA is FALSE?
DNA amplification is done by all, except:
Best sample for DNA profiling in sexual assault after 72 hours?
Best site for DNA extraction from a 2-month-old decomposed body?
DNA fingerprinting can be done with all, except:
Which of the following is an example of an antiapoptotic gene?
Y-STR and Mitochondrial DNA Indian Medical PG Practice Questions and MCQs
Question 1: Select the FALSE combination of chromosomal pattern and the syndrome:
- A. Swyer's syndrome-46XY
- B. Klinefelter's syndrome-47XXY
- C. Turner's syndrome-45XO
- D. Mayer Rokitansky-46XY (Correct Answer)
Explanation: ***Mayer Rokitansky-46XY*** - Mayer-Rokitansky-Küster-Hauser (MRKH) syndrome is characterized by **vaginal agenesis** and **uterine anomalies** in individuals with a normal **46,XX** **karyotype**, making 46,XY an incorrect chromosomal pattern for this syndrome. - Individuals with MRKH have female external genitalia and normal ovarian function, differentiating it from disorders of sexual development involving sex chromosome abnormalities. *Swyer's syndrome-46XY* - This is a **correct** combination; Swyer's syndrome is a form of **XY gonadal dysgenesis** where individuals have a **46,XY karyotype** but develop female external genitalia due a non-functional SRY gene [2]. - These individuals typically present with **primary amenorrhea** and **streak gonads**, requiring hormone replacement therapy and gonadectomy to prevent gonadoblastoma. *Klinefelter's syndrome-47XXY* - This is a **correct** combination; Klinefelter's syndrome is characterized by the presence of an **extra X chromosome** in biological males, resulting in a **47,XXY karyotype** [2]. - Affected individuals typically exhibit **hypogonadism**, **infertility**, **gynecomastia**, and often have a taller stature [3]. *Turner's syndrome-45XO* - This is a **correct** combination; Turner's syndrome is characterized by the **absence or partial absence of one X chromosome** in females, resulting in a **45,XO karyotype** [1]. - Clinical features include **short stature**, **ovarian dysgenesis** (streak gonads), **primary amenorrhea**, and distinctive physical features such as a **webbed neck** and **broad chest** [1].
Question 2: True regarding mitochondrial DNA is:
- A. Linear double stranded
- B. All respiratory proteins are synthesized within mitochondria itself
- C. Inherited from mother (Correct Answer)
- D. Low mutation rate
Explanation: ***Inherited from mother*** - **Mitochondrial DNA (mtDNA)** is exclusively inherited from the mother because the sperm's mitochondria are typically destroyed after fertilization or do not enter the oocyte. - This **maternal inheritance pattern** makes mtDNA useful for tracing lineage and studying human population movements. *Linear double stranded* - **Mitochondrial DNA** is typically **circular**, not linear, and double-stranded, similar to a plasmid in bacteria. - **Linear DNA** is characteristic of nuclear chromosomes in eukaryotes. *All respiratory proteins are synthesized within mitochondria itself* - While mitochondria contain their own ribosomes and synthesize some proteins, the majority of **respiratory chain proteins** are encoded by **nuclear DNA** and imported into the mitochondria. - The **mitochondrial genome** encodes only a small fraction of the proteins necessary for mitochondrial function, primarily components of the electron transport chain. *Low mutation rate* - **Mitochondrial DNA** has a **higher mutation rate** compared to nuclear DNA due to a less robust DNA repair system and exposure to reactive oxygen species generated during oxidative phosphorylation. - The high mutation rate can contribute to mitochondrial diseases and can also be used in evolutionary studies.
Question 3: Best method for the detection of mutations with low allele frequency is:
- A. FISH
- B. Droplet digital PCR (Correct Answer)
- C. Sanger sequencing
- D. Nested PCR
Explanation: ***Droplet digital PCR*** - **Droplet digital PCR (ddPCR)** offers superior sensitivity for detecting **low allele frequency mutations** by partitioning the sample into thousands of individual reactions. - This compartmentalization allows for the direct quantification of target DNA molecules without relying on a standard curve, making it highly accurate for rare mutation detection. *FISH* - **Fluorescence in situ hybridization (FISH)** primarily detects **chromosomal abnormalities** like translocations, deletions, or amplifications, rather than single-nucleotide variants or small indels with low allele frequencies [2]. - It visualizes genetic changes at a **cytogenetic level** on an intracellular basis, not typically for quantifying rare DNA mutations in a heterogeneous sample. *Sanger sequencing* - **Sanger sequencing** is the gold standard for **sequencing individual DNA fragments** but has a detection limit of around 15-20% for allele frequency, making it unsuitable for very low allele frequency mutations [1]. - It struggles to reliably detect minor alleles when they are present in a small proportion of the total DNA pool. *Nested PCR* - **Nested PCR** increases the sensitivity and specificity of amplification by using two sets of primers in a sequential manner but does not inherently provide the **quantification capability** or the same level of **low allele frequency detection** as ddPCR processes. - While sensitive for detecting target sequences, it is not designed for precise quantification of rare mutations in a background of wild-type sequences. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Genetic Disorders, p. 185. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Genetic Disorders, pp. 185-186.
Question 4: An affected male does not have affected children but an affected female always has affected children. Type of inheritance?
- A. Autosomal recessive
- B. Mitochondrial (Correct Answer)
- C. X linked recessive
- D. X linked dominant
Explanation: ***Correct Option: Mitochondrial*** - This pattern describes **mitochondrial inheritance**, where all children of an **affected mother** inherit the condition because mitochondria are exclusively inherited from the ovum (maternal inheritance). - An **affected father** cannot pass on the condition to his children, as sperm contribute only nuclear DNA and essentially no mitochondria. - This is the **only inheritance pattern** where an affected male has no affected children while an affected female has all children affected. *Incorrect Option: Autosomal recessive* - This pattern would typically show affected individuals having unaffected parents (who are carriers) and both males and females being affected in equal proportions. - It does not explain the complete absence of transmission from an affected father or universal transmission from an affected mother. - An affected individual could have unaffected children if their partner is not a carrier. *Incorrect Option: X linked recessive* - In **X-linked recessive inheritance**, affected males cannot pass the trait to their sons, but all their daughters would be carriers (not affected). - An affected mother would pass the trait to all her sons (affected) and make all her daughters carriers (not affected), which does not match the described pattern of all children being affected. *Incorrect Option: X linked dominant* - In **X-linked dominant inheritance**, an affected father passes the trait to all his daughters but none of his sons (contradicts "no affected children"). - An affected mother has a 50% chance of passing the trait to **each child**, which is inconsistent with all children of an affected female being affected.
Question 5: Which of the following statements regarding mitochondrial DNA is FALSE?
- A. Double stranded
- B. Inherited from mother
- C. High mutation rate
- D. All respiratory proteins are synthesized within the mitochondria (Correct Answer)
Explanation: ***All respiratory proteins are synthesized within the mitochondria.*** - While mitochondrial DNA (mtDNA) encodes some proteins essential for the **electron transport chain** (respiratory proteins), not all respiratory proteins are synthesized within the mitochondria. - Many crucial respiratory proteins are encoded by **nuclear DNA** and imported into the mitochondria from the cytoplasm. *Double stranded* - **Mitochondrial DNA (mtDNA)** is a **double-stranded**, circular molecule, similar to bacterial chromosomes. - This structure provides stability and allows for efficient replication within the organelle. *Inherited from mother* - Mitochondria and their DNA are exclusively inherited from the **mother** during fertilization, as sperm primarily contributes nuclear DNA. - This **maternal inheritance pattern** is a key feature of mtDNA and is used in tracing ancestry. *High mutation rate* - mtDNA has a significantly **higher mutation rate** compared to nuclear DNA due to several factors, including lack of robust repair mechanisms and exposure to reactive oxygen species. - This contributes to the rapid evolution of mtDNA and its use in **population genetics** studies.
Question 6: DNA amplification is done by all, except:
- A. DNA sequencing (Correct Answer)
- B. Loop-mediated isothermal amplification (LAMP)
- C. Ligase chain reaction
- D. Polymerase chain reaction
Explanation: ***DNA sequencing*** - **DNA sequencing** determines the **nucleotide base order** in a DNA molecule but does not increase the amount of DNA. - While requiring a DNA template, it is an **analytical technique** rather than an amplification method. *Loop-mediated isothermal amplification (LAMP)* - **LAMP** is an **isothermal DNA amplification** technique that amplifies target DNA sequences at a constant temperature (60-65°C). - It uses a DNA polymerase with strand displacement activity and 4-6 primers to produce large amounts of DNA rapidly. *Ligase chain reaction* - **LCR** is an amplification method that detects specific **DNA sequences** by ligating adjacent probes. - It amplifies the signal from a target DNA sequence rather than the DNA itself by creating many copies of joined probes. *Polymerase chain reaction* - **PCR** is a widely used technique for **amplifying** a specific segment of DNA to produce many copies. - It involves cycles of **denaturation**, **annealing**, and **extension** using a DNA polymerase.
Question 7: Best sample for DNA profiling in sexual assault after 72 hours?
- A. Victim's clothes (Correct Answer)
- B. Cervical swab
- C. Vaginal swab
- D. Fingernail scrapings
Explanation: ***Victim's clothes*** - After 72 hours, **spermatozoa** in or on the skin are often degraded, but foreign DNA (from the perpetrator's skin cells, semen, or other bodily fluids) can persist on clothing, especially in protected areas. - Clothing acts as a **storage medium for trace evidence**, and DNA can remain viable for profiling much longer than on mucous membranes due to drying and lack of enzymatic degradation. *Cervical swab* - **Spermatozoa** typically become undetectable in the cervix within 24-48 hours, though some studies show persistence up to 72 hours. - Beyond 72 hours, the likelihood of obtaining viable **perpetrator DNA** from a cervical swab significantly decreases due to degradation and cellular turnover. *Vaginal swab* - **Spermatozoa** found in the vagina are subject to enzymatic degradation and expulsion, with viability for DNA profiling decreasing significantly after 24-48 hours. - While trace DNA might still be present, the quantity and quality for profiling are usually much lower than on clothing after such an extended period. *Fingernail scrapings* - While fingernail scrapings can yield **perpetrator DNA** if there was physical struggle, this is highly dependent on the act of scraping the assailant's skin. - It is not a guaranteed source in every sexual assault and is less likely to contain strong DNA evidence after 72 hours compared to clothing, which captures shed cells and fluids passively.
Question 8: 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 9: DNA fingerprinting can be done with all, except:
- A. Saliva
- B. WBC
- C. RBC (Correct Answer)
- D. Spermatozoa
Explanation: ***RBC*** - **Mature red blood cells** lack a nucleus and therefore do not contain **DNA**. - DNA fingerprinting relies on analyzing an individual's unique DNA sequence, which is not present in RBCs. *Saliva* - Saliva contains **epithelial cells** from the mouth, which have intact nuclei and thus sufficient DNA for analysis [2]. - It is a common and non-invasive source of DNA for forensic and genetic testing [2]. *WBC* - **White blood cells** (leukocytes) are nucleated cells that contain a full complement of DNA [2]. - They are an excellent source of DNA for genetic analysis, including DNA fingerprinting. *Spermatozoa* - **Sperm cells** are haploid and contain a nucleus with DNA, making them suitable for DNA fingerprinting [1]. - They are frequently used in forensic cases, particularly in sexual assault investigations [1].
Question 10: Which of the following is an example of an antiapoptotic gene?
- A. FLIP (Correct Answer)
- B. P53
- C. BAX
- D. BIM
Explanation: ***FLIP*** - **FLIP** is an **antiapoptotic gene** that inhibits the activation of caspase-8, thereby blocking the extrinsic apoptotic pathway. - It acts as an **FLICE-inhibitory protein**, preventing the formation of the death-inducing signaling complex (DISC) or its downstream activation. *P53* - **P53** is a **tumor suppressor gene** that promotes apoptosis in response to DNA damage or cellular stress. - It is a **pro-apoptotic gene**, orchestrating cell cycle arrest and apoptosis to prevent the propagation of damaged cells. *BAX* - **BAX** is a **pro-apoptotic gene** belonging to the Bcl-2 family, which promotes the release of cytochrome c from mitochondria. - This release initiates the **intrinsic apoptotic pathway**, leading to caspase activation and cell death. *BIM* - **BIM** is a **pro-apoptotic gene** of the Bcl-2 family, acting as a sensitizer for apoptosis by binding to and inhibiting anti-apoptotic Bcl-2 family proteins. - Its activation leads to the **neutralization of survival factors**, thereby promoting mitochondrial outer membrane permeabilization and apoptosis.
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