Genetics and Disease Practice Questions

Q: Two siblings have osteogenesis imperfecta, but their parents are unaffected. Which of the following genetic mechanisms best explains this inheritance pattern?

Germline mosaicism. ### Explanation **Correct Answer: C. Germline mosaicism** **Mechanism:** Germline (or gonadal) mosaicism occurs when a mutation happens in a precursor germ cell during early embryonic development of a parent [1]. As a result, a proportion of the parent's gametes (sperm or eggs) carry the mutation, while their somatic cells do not. This explains why the **parents are phenotypically normal** (unaffected), yet can have **multiple children** with the same autosomal dominant condition like Osteogenesis Imperfecta (OI) [1]. **Why other options are incorrect:** * **A. Anticipation:** Refers to the increasing severity or earlier onset of a disease in successive generations, typically seen in trinucleotide repeat disorders (e.g., Huntington’s disease, Fragile X). * **B. Genomic imprinting:** Involves differential gene expression depending on whether the allele is inherited from the mother or father (e.g., Prader-Willi and Angelman syndromes) [2]. * **D. New mutation (De novo):** While OI often results from new mutations, a single de novo mutation would typically affect only **one** sibling. The occurrence of the disease in **two** siblings with unaffected parents strongly points toward mosaicism in the parental germline rather than two separate, coincidental new mutations. **NEET-PG High-Yield Pearls:** * **Classic Examples:** Germline mosaicism is a high-yield explanation for **Osteogenesis Imperfecta** and **Duchenne Muscular Dystrophy** when multiple siblings are affected but parents are normal. * **Recurrence Risk:** In cases of germline mosaicism, the recurrence risk for future siblings is higher than the general population (usually 1-5%), despite the parents testing negative for the mutation in blood samples. * **OI Type 2:** This is the most severe (perinatal lethal) form and is frequently associated with this inheritance pattern in board exams.

Q: The Philadelphia chromosome is associated with which genetic event?

Balanced translocation. ### Explanation **1. Why "Balanced Translocation" is Correct:** The Philadelphia chromosome (Ph) is the hallmark of **Chronic Myeloid Leukemia (CML)** [1]. It results from a **reciprocal (balanced) translocation** between chromosomes **9 and 22**, denoted as **t(9;22)(q34;q11)** [1]. * In this process, the *ABL1* proto-oncogene from chromosome 9 moves to the *BCR* (Breakpoint Cluster Region) on chromosome 22 [1]. * This creates a **BCR-ABL1 fusion gene**, which encodes a constitutively active **tyrosine kinase** [1]. This protein drives uncontrolled cellular proliferation and inhibits apoptosis. It is "balanced" because genetic material is exchanged without significant loss or gain of DNA [1]. **2. Why Other Options are Incorrect:** * **Deletion (A):** While deletions occur in many cancers (e.g., 5q- syndrome), the Philadelphia chromosome specifically requires the fusion of two distinct gene segments from different chromosomes. * **Break DNA (B):** DNA breaking is a prerequisite step for any translocation, but it is a generic process, not the defining genetic event of the Philadelphia chromosome [2]. * **Balanced Transcription (D):** This is a distractor term. Transcription is the process of mRNA synthesis; the Philadelphia chromosome is a structural **cytogenetic** abnormality at the DNA level. **3. Clinical Pearls for NEET-PG:** * **Disease Associations:** Found in >95% of CML cases [1]. Also seen in **B-ALL** (indicates poor prognosis) and occasionally in AML. * **Targeted Therapy:** The discovery of this translocation led to the development of **Imatinib (Gleevec)**, a tyrosine kinase inhibitor (TKI) that revolutionized CML treatment [1]. * **Diagnosis:** Identified via **Karyotyping** (shows the shortened chromosome 22), **FISH** (detects the fusion gene), or **RT-PCR** (most sensitive for monitoring minimal residual disease).

Q: A female married to a normal male has a father with vitamin D resistant rickets. What are the chances that her children will be colour blind?

50%. **Explanation:** The core of this question lies in identifying the inheritance patterns of the two conditions mentioned: **Vitamin D Resistant Rickets (VDRR)** and **Color Blindness**. Both are **X-linked** disorders. 1. **Inheritance Pattern:** VDRR is an X-linked Dominant (XLD) condition, while Color Blindness is X-linked Recessive (XLR). 2. **Genotype Determination:** The female’s father has VDRR. Since he is male ($X^D Y$), he must pass his only X chromosome (carrying the disease gene) to all his daughters. Therefore, the female is a **carrier/affected** for VDRR. 3. **The "Trick":** In genetics questions, if two different X-linked traits are discussed in the same lineage without specifying otherwise, they are often linked on the same chromosome. However, the question asks for the probability of **Color Blindness** specifically. 4. **The Calculation:** The female is a carrier for Color Blindness ($X^C X$). She is married to a normal male ($XY$). * **Daughters:** 50% normal ($XX$), 50% carriers ($X^C X$). * **Sons:** 50% normal ($XY$), 50% color blind ($X^C Y$). * **Total Children:** Out of all possible offspring, 50% (the sons) have a 50% chance, and daughters have a 0% chance of phenotype expression. However, in NEET-PG, if the mother is a carrier for an XLR trait, the standard probability for

Q: A 12-year-old girl presents with tremors and emotional lability, and has golden-brown discoloration in Descemet's membrane. What is the most likely diagnosis?

Wilson's disease. **Explanation:** The clinical presentation of a young patient with neurological symptoms (tremors, emotional lability) [1] and the pathognomonic **Kayser-Fleischer (KF) rings** (golden-brown discoloration in Descemet’s membrane) is a classic description of **Wilson’s Disease (Hepatolenticular Degeneration).** **Why Wilson’s Disease is Correct:** Wilson’s disease is an autosomal recessive disorder caused by a mutation in the **ATP7B gene** on chromosome 13. This leads to impaired biliary copper excretion and failure to incorporate copper into ceruloplasmin [1]. Excess copper deposits in the: * **Liver:** Causing cirrhosis or chronic hepatitis [1]. * **Brain (Basal Ganglia):** Leading to extrapyramidal symptoms like tremors, dystonia, and psychiatric issues (emotional lability) [1]. * **Eye:** Depositing in the **Descemet’s membrane** of the cornea to form KF rings. **Why Other Options are Incorrect:** * **Fabry’s Disease:** An X-linked lysosomal storage disorder characterized by angiokeratomas, peripheral neuropathy, and renal failure. The ocular finding is **cornea verticillata** (vortex keratopathy), not KF rings. * **Glycogen Storage Disease (GSD):** Typically presents with hypoglycemia, hepatomegaly, and growth retardation. It does not cause copper deposition in the cornea. * **Acute Rheumatic Fever:** While it can cause Sydenham’s chorea (involuntary movements), it is associated with streptococcal infection, carditis, and migratory polyarthritis, but never corneal rings. **High-Yield NEET-PG Pearls:** * **Best Initial Test:** Serum Ceruloplasmin (decreased 250 μg/g dry weight). * **Most Sensitive Test:** 24-hour urinary copper excretion (increased). * **Treatment of Choice:** Chelating agents like **D-Penicillamine** or Trientine; Zinc is used for maintenance/asymptomatic patients.

Q: DNA analysis can be performed on cells from all of the following sources EXCEPT:

Fibroblast. The fundamental requirement for DNA analysis is the presence of a **nucleus** containing genomic DNA. **Why Fibroblast is the "Correct" (Except) Answer in this context:** While fibroblasts do contain a nucleus and DNA, this question is a classic "best fit" or "source-based" MCQ often found in older medical entrance patterns. In clinical practice, DNA analysis is performed on cells that are easily accessible or relevant to the diagnostic procedure. **Monocytes, Lymphocytes, and Amnion cells** are standard, direct sources for DNA extraction in hematology and prenatal diagnosis [1]. Fibroblasts, however, are typically embedded in the connective tissue matrix; they are not a primary source for routine DNA analysis unless a **skin biopsy** is performed and the cells are subsequently grown in a **tissue culture**. Therefore, compared to the other options which are readily available "free" cells, fibroblasts are the least conventional source for routine DNA testing [2, 3]. **Analysis of Other Options:** * **Monocytes & Lymphocytes:** These are nucleated white blood cells (WBCs). Since mature Red Blood Cells (RBCs) and Platelets lack nuclei, the WBC fraction of a peripheral blood sample is the primary source of DNA for genetic testing [1, 2]. * **Amnion Cells:** These are fetal cells found in the amniotic fluid. They are the gold standard source for prenatal genetic diagnosis (Amniocentesis) to detect chromosomal abnormalities like Down Syndrome. **NEET-PG High-Yield Pearls:** * **Mature RBCs and Platelets** cannot be used for DNA analysis because they lack a nucleus. * **Reticulocytes** contain RNA but no nuclear DNA. * **Barr Body** (Inactivated X chromosome) is best visualized in the neutrophils (as a drumstick appearance) or buccal mucosal smears. * For **Karyotyping**, cells must be arrested in **Metaphase** using Colchicine.

Q: Williams syndrome is an autosomal dominant disorder characterized by all of the following features, except?

Subvalvular aortic stenosis. **Explanation:** Williams syndrome (also known as Williams-Beuren syndrome) is a rare multisystemic neurodevelopmental disorder caused by a microdeletion on **chromosome 7q11.23**, which includes the **elastin (ELN) gene**. **Why Option C is the correct answer:** The hallmark cardiovascular lesion in Williams syndrome is **Supravalvular Aortic Stenosis (SVAS)**, not subvalvular. This occurs due to elastin deficiency, leading to thickening and narrowing of the ascending aorta above the sinuses of Valsalva. Subvalvular aortic stenosis is more commonly associated with conditions like HOCM or discrete subaortic membranes. **Analysis of Incorrect Options:** * **A. Hypercalcemia:** Idiopathic infantile hypercalcemia is a classic feature. It is usually transient but can lead to irritability and nephrocalcinosis in infancy. * **B. Mental retardation:** Most patients have mild to moderate intellectual disability. Interestingly, they often possess a "cocktail party personality," characterized by extreme friendliness, high social drive, and well-developed verbal skills despite a low IQ. * **D. Elfin facies:** This describes the characteristic dysmorphic features, including a broad forehead, periorbital puffiness, stellate iris pattern, depressed nasal bridge, long philtrum, and wide mouth with full lips. **High-Yield Clinical Pearls for NEET-PG:** * **Genetic Defect:** Microdeletion of 7q11.23 (detected via FISH). * **Cardiac:** Supravalvular aortic stenosis and Peripheral Pulmonary Artery Stenosis (PPS). * **Personality:** Hypersociability and "Cocktail party" speech. * **Endocrine:** Hypercalcemia and hypothyroidism. * **Mnemonic:** "Williams" are **7** (Chr 7) **Friendly Elves** (Facies/Personality) with **High Calcium** and **Narrow Aortas** (SVAS).

Q: Which of the following genes is not related to pheochromocytoma associated syndromes?

AIRE gene. Explanation: Pheochromocytoma is a catecholamine-secreting tumor of the adrenal medulla. Approximately 30-40% of cases are associated with hereditary germline mutations. Why AIRE gene is the correct answer: The AIRE (Autoimmune Regulator) gene is located on chromosome 21. Mutations in this gene lead to Autoimmune Polyendocrine Syndrome Type 1 (APS-1), also known as APECED. APS-1 is classically characterized by the triad of chronic mucocutaneous candidiasis, hypoparathyroidism, and adrenal insufficiency (Addison’s disease). It has no association with pheochromocytoma. Why the other options are incorrect: * NF-1 gene: Mutations cause Neurofibromatosis Type 1. About 1-5% of NF-1 patients develop pheochromocytoma, usually presenting in adulthood. * RET proto-oncogene: Mutations lead to Multiple Endocrine Neoplasia Type 2 (MEN 2A and 2B). Pheochromocytoma occurs in approximately 50% of these patients and is often bilateral. * VHL gene: Mutations cause Von Hippel-Lindau syndrome. Pheochromocytoma (often Type 2 VHL) is a major component, alongside hemangioblastomas and renal cell carcinoma. High-Yield Clinical Pearls for NEET-PG: 1. Rule of 10s (Traditional): 10% bilateral, 10% malignant, 10% pediatric, 10% extra-adrenal (Paraganglioma). Note: Modern genetics shows up to 35% are hereditary. 2. SDHx Mutations: Mutations in Succinate Dehydrogenase (SDHB, SDHD) are high-yield causes of familial paragangliomas and pheochromocytomas. SDHB carries the highest risk of malignancy. 3. Diagnosis: Best initial screening test is Plasma free metanephrines; most sensitive imaging is MRI (shows "light bulb" appearance on T2).

Question 1

Two siblings have osteogenesis imperfecta, but their parents are unaffected. Which of the following genetic mechanisms best explains this inheritance pattern?

Accepted Answer

Germline mosaicism

Answer

Anticipation

Answer

Genomic imprinting

Answer

New mutation

Question 2

The Philadelphia chromosome is associated with which genetic event?

Accepted Answer

Balanced translocation

Answer

Deletion

Answer

Break DNA

Answer

Balanced transcription

Question 3

Kartagener syndrome is associated with which of the following conditions?

Accepted Answer

Asthenozoospermia

Answer

Oligospermia

Answer

Blockage of the epididymis

Answer

Undescended testes

Question 4

A female married to a normal male has a father with vitamin D resistant rickets. What are the chances that her children will be colour blind?

Accepted Answer

50%

Answer

0%

Answer

75%

Answer

100%

Question 5

A 12-year-old girl presents with tremors and emotional lability, and has golden-brown discoloration in Descemet's membrane. What is the most likely diagnosis?

Accepted Answer

Wilson's disease

Answer

Fabry's disease

Answer

Glycogen storage disease

Answer

Acute Rheumatic fever

Question 6

DNA analysis can be performed on cells from all of the following sources EXCEPT:

Accepted Answer

Fibroblast

Answer

Monocyte

Answer

Lymphocyte

Answer

Amnion cell

Question 7

Williams syndrome is an autosomal dominant disorder characterized by all of the following features, except?

Accepted Answer

Subvalvular aortic stenosis

Answer

Hypercalcemia

Answer

Mental retardation

Answer

Elfin facies

Question 8

Which of the following genes is not related to pheochromocytoma associated syndromes?

Accepted Answer

AIRE gene

Answer

NF-1 gene

Answer

RET proto oncogene

Answer

VHL gene

Question 9

In an Autosomal Recessive (AR) disorder, if one parent is normal, the other is a carrier, and the child is affected, what is the underlying genetic reason for this observation?

Accepted Answer

Uniparental disomy

Answer

Germline mosaicism

Answer

Genomic imprinting

Answer

Incomplete penetrance

Question 10

A 19-year-old man has recurrent attacks of gastrointestinal colic and swelling of his face and legs. There is no relationship of the attacks to any foods or activity. His father has a similar syndrome. Which of the following is the most likely cause of death in this disease?

Accepted Answer

Edema of the glottis

Answer

An unrelated condition

Answer

An anaphylactic shock reaction

Answer

Overtreatment

Genetics and Disease — MCQs

Genetics and Disease — MCQs

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