Genetics and Disease Indian Medical PG Practice Questions and MCQs
Practice Indian Medical PG questions for Genetics and Disease. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Genetics and Disease Indian Medical PG Question 1: Gene not involved in SCID:
- A. BTK (Correct Answer)
- B. ZAP70
- C. IL2RG
- D. JAK3
Genetics and Disease Explanation: ***BTK***
- **Bruton's tyrosine kinase (BTK)** is associated with **X-linked agammaglobulinemia (XLA)**, a primary immunodeficiency characterized by the absence of mature B cells and significantly reduced antibody production. While it causes severe immune deficiency, it is not a direct cause of **SCID**.
- XLA results in recurrent bacterial infections due to an inability to produce antibodies, rather than the severe combined T and B cell dysfunction seen in SCID.
*ZAP70*
- **ZAP70** deficiency is a cause of **SCID**. It leads to impaired T-cell receptor signaling, resulting in profound functional T-cell lymphopenia.
- Patients with ZAP70 deficiency have normal numbers of CD4 T cells but very low or absent CD8 T cells, and their T cells are functionally impaired, leading to severe immunodeficiency.
*IL2RG*
- The **IL2RG** gene encodes the common gamma chain (γc), a crucial component of several **interleukin receptors (IL-2, IL-4, IL-7, IL-9, IL-15, IL-21)**. [1]
- Mutations in IL2RG cause **X-linked SCID (X-SCID)**, the most common form of SCID, leading to a block in T-cell and NK-cell development due to defective cytokine signaling. [1]
*JAK3*
- **Janus kinase 3 (JAK3)** is a tyrosine kinase that associates with the **common gamma chain (γc)** and is essential for cytokine signaling downstream of the γc-containing receptors. [1]
- **JAK3 deficiency** results in an **autosomal recessive form of SCID**, clinically indistinguishable from X-SCID, with impaired T-cell and NK-cell development due to defective cytokine signaling. [1]
**References:**
[1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 247-248.
Genetics and Disease Indian Medical PG Question 2: 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
Genetics and Disease 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.
Genetics and Disease Indian Medical PG Question 3: Genetic disorder predisposing patients to develop Berry aneurysm includes all EXCEPT:
- A. Marfan’s syndrome
- B. Adult polycystic kidney
- C. Neurofibromatosis Type II (Correct Answer)
- D. Fibromuscular dysplasia
Genetics and Disease Explanation: ***Neurofibrofomatosis Type II***
- This condition is primarily associated with **central nervous system tumors** like **vestibular schwannomas** and **meningiomas**, not Berry aneurysms [2].
- While it affects the nervous system, its vascular manifestations are typically different from those predisposing to aneurysms.
*Marfan’s syndrome*
- Patients with Marfan's syndrome have **fragile connective tissue** due to a defect in **fibrillin-1**, which can weaken arterial walls.
- This weakness increases the risk of **aortic aneurysms** and dissections, and can also predispose to intracranial aneurysms like Berry aneurysms.
*Adult polycystic kidney*
- This **autosomal dominant** disorder is characterized by the formation of **cysts in the kidneys**, but also has systemic manifestations [1].
- There is a well-established association between **autosomal dominant polycystic kidney disease (ADPKD)** and an increased incidence of **Berry aneurysms**.
*Fibromuscular dysplasia*
- This condition involves **abnormal cellular development** in the **arterial walls**, leading to areas of narrowing and enlargement.
- It commonly affects the **renal arteries** and **carotid arteries**, and is also a known risk factor for the development of **intracranial aneurysms**, including Berry aneurysms.
Genetics and Disease Indian Medical PG Question 4: What is meant by the melting of double-stranded DNA?
- A. Splitting of double strands into single strands (Correct Answer)
- B. Splitting of DNA into fragments
- C. Formation of triple helix
- D. Separation of double-stranded bases
Genetics and Disease Explanation: ***Splitting of double strands into single strands***
* **DNA melting**, also known as **DNA denaturation**, refers to the process where the two complementary strands of a **double-stranded DNA** molecule separate to form two individual single strands.
* This process involves the breaking of the **hydrogen bonds** between the paired bases (**A-T and G-C**) due to increased temperature or changes in pH.
* The temperature at which 50% of the DNA is denatured is called the **melting temperature (Tm)**, which depends on GC content (higher GC = higher Tm due to three hydrogen bonds vs. two in AT pairs).
*Splitting of DNA into fragments*
* The splitting of DNA into fragments is referred to as **DNA fragmentation**, which typically occurs due to processes like **restriction enzyme digestion**, mechanical shearing, or programmed cell death (apoptosis).
* This process involves the breaking of the **phosphodiester bonds** within the DNA backbone, not just the hydrogen bonds between strands.
*Formation of triple helix*
* The formation of a **triple helix** (triplex DNA) is a less common DNA structure where a third oligonucleotide strand binds into the major groove of a **B-form DNA duplex**.
* This process is distinct from DNA melting, which involves the *separation* of existing double strands rather than the *addition* of a third strand.
*Separation of double-stranded bases*
* The term "double-stranded bases" is imprecise terminology; bases are paired (e.g., A with T, G with C) within the double helix structure.
* While the separation of base pairs does occur during melting, the more accurate description is the **separation of the entire double helix into two single strands**, not just the individual bases.
Genetics and Disease Indian Medical PG Question 5: Mutations are due to changes in:
- A. DNA nucleotide sequence (Correct Answer)
- B. RNA nucleotide sequence
- C. Amino acid sequence of ribonuclease
- D. Cell membrane
Genetics and Disease Explanation: ***DNA nucleotide sequence***
- **Mutations** are defined as changes in the **genetic material**, which is primarily composed of **DNA**.
- These changes in the **nucleotide sequence** of DNA can alter the genetic code, leading to changes in **protein structure and function**.
*RNA nucleotide sequence*
- While RNA can have its nucleotide sequence altered, these changes are generally not considered true **mutations** in the heritable sense for most organisms.
- RNA is typically a temporary molecule, and changes to its sequence are usually not passed down to subsequent generations.
*Amino acid sequence of ribonuclease*
- An altered **amino acid sequence** in a protein like ribonuclease is a consequence of a **mutation in the DNA**, not the mutation itself.
- **Ribonucleases** are enzymes that catalyze the degradation of RNA, and their structure is determined by the **DNA sequence**.
*Cell membrane*
- The cell membrane is a **lipid bilayer** with embedded proteins that regulates cellular transport and communication.
- While its components can be affected by genetic mutations, alterations in the cell membrane itself do not constitute the primary definition of a **mutation**.
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