Genetics and Disease — MCQs

Genetics and Disease Indian Medical PG Practice Questions and MCQs

Question 141: What is the definition of a "proband" in the context of a pedigree analysis?

A. Male child
B. Female child
C. An individual exhibiting the trait of interest (Correct Answer)
D. Pregnancy

Explanation: **Explanation:** In genetics and pedigree analysis, the **Proband** (also known as the *index case* or *propositus*) is the first individual in a family to come to the attention of a geneticist or clinician. This individual exhibits the phenotype or trait of interest (e.g., a specific genetic disorder) and serves as the starting point for tracing the inheritance pattern within the family tree. **Analysis of Options:** * **Option C (Correct):** The proband is the affected individual through whom the family is identified. In a pedigree, the proband is typically indicated by an **arrow (↗)** pointing to their symbol. * **Option A & B (Incorrect):** While a proband can be male or female, these terms do not define the role. In pedigree nomenclature, a square represents a male and a circle represents a female. * **Option D (Incorrect):** Pregnancy is represented in a pedigree by a diamond symbol (if the sex is unknown) or by the letter "P" inside a symbol. **High-Yield Clinical Pearls for NEET-PG:** * **Propositus vs. Proposita:** A male proband is technically called a *propositus*, and a female is a *proposita*. * **Consultand:** This is the person seeking genetic counseling. They may or may not be the proband (e.g., an unaffected parent seeking advice about an affected child). * **Generations:** Roman numerals (I, II, III) denote generations, while Arabic numerals (1, 2, 3) denote individuals within a generation. * **Consanguinity:** Represented by a **double horizontal line** between a male and female, indicating a union between related individuals—a high-yield risk factor for Autosomal Recessive disorders.

Question 142: What is the chance of an offspring being affected when one parent is an autosomal dominant heterozygote?

A. 25%
B. 50% (Correct Answer)
C. 100%
D. None

Explanation: ### Explanation **1. Why 50% is the Correct Answer:** Autosomal dominant (AD) inheritance occurs when a single copy of a mutated gene (on a non-sex chromosome) is sufficient to cause the disease. In this scenario, the affected parent is a **heterozygote (Aa)** and the other parent is typically **unaffected (aa)**. According to Mendelian laws, the offspring possibilities are: * **Aa** (Affected): 50% * **aa** (Unaffected): 50% Because the "A" allele is dominant, every child who inherits it will express the phenotype, regardless of gender. **2. Why the Other Options are Incorrect:** * **A (25%):** This is the recurrence risk for an **Autosomal Recessive** disorder when both parents are asymptomatic carriers (Aa x Aa). * **C (100%):** This would only occur if one parent was an **autosomal dominant homozygote (AA)**, which is rare as many such conditions are lethal in utero or significantly more severe. * **D (None):** This is incorrect because AD traits do not skip generations (vertical transmission). **3. NEET-PG Clinical Pearls & High-Yield Facts:** * **Vertical Transmission:** AD disorders appear in every generation. * **Male = Female:** Both sexes are affected with equal frequency. * **Key Examples:** Huntington’s disease, Marfan syndrome, Neurofibromatosis (NF-1 and NF-2), Achondroplasia, and Familial Hypercholesterolemia. * **Variable Expressivity:** Different patients with the same AD mutation may show varying degrees of clinical severity. * **Reduced Penetrance:** An individual may carry the dominant gene but not show the clinical phenotype (skipping a generation phenotypically, but not genotypically). * **Paternal Age Effect:** Advanced paternal age is associated with new (de novo) autosomal dominant mutations (e.g., Achondroplasia).

Question 143: In a family, the father has widely spaced eyes, increased facial hair, and deafness. One of the three children has deafness with similar facial features. The mother is normal. Which one of the following patterns of inheritance is least likely in this case?

A. Autosomal dominant (Correct Answer)
B. Autosomal recessive
C. X-linked dominant
D. X-linked recessive

Explanation: **Explanation:** The clinical presentation described—widely spaced eyes (hypertelorism), increased facial hair (synophrys), and deafness—is highly suggestive of **Waardenburg Syndrome**, which typically follows an **Autosomal Dominant (AD)** inheritance pattern. However, the question asks which inheritance pattern is **least likely** given the specific scenario: a father is affected, the mother is normal, and only **one out of three** children is affected. 1. **Why Autosomal Dominant is the "Least Likely" (in the context of this specific question logic):** While Waardenburg is classically AD, the question is a logic-based trap. In AD inheritance, if one parent is affected (Aa) and the other is normal (aa), there is a **50% probability** for each child to be affected. Statistically, seeing only 1 out of 3 children affected is common. However, in many NEET-PG style "least likely" questions regarding pedigree, if a father passes a trait to a son, **X-linked Dominant** is often scrutinized because an affected father *must* pass the trait to *all* daughters and *no* sons. * *Note:* There is a known discrepancy in this specific recalled question; usually, if a father and son are both affected, **X-linked Dominant** is the least likely because fathers cannot pass X-chromosomes to sons. If the child in the prompt is a son, X-linked patterns are ruled out. 2. **Analysis of Other Options:** * **Autosomal Recessive (B):** Possible if the mother is a carrier (Aa), though less likely for rare syndromes. * **X-linked Dominant (C):** Impossible if the affected child is a male (fathers give Y to sons). * **X-linked Recessive (D):** Possible if the mother is a carrier. **Clinical Pearls for NEET-PG:** * **Waardenburg Syndrome:** Features include sensorineural deafness, *dystopia canthorum* (lateral displacement of inner canthi), heterochromia iridis, and a white forelock (poliosis). * **Inheritance Rule:** If a father passes a trait to his son, **X-linked inheritance (Dominant or Recessive) is eliminated.** * **Most Common Pattern:** Most skeletal and structural dysmorphology syndromes are Autosomal Dominant. (No suitable references were found in the provided fragments to support the specific genetics of Waardenburg Syndrome or the pedigree analysis required for this question).

Question 144: What is the chance of having cystic fibrosis if only one parent is affected and the other parent is a carrier?

A. 25%
B. 50% (Correct Answer)
C. 75%
D. 100%

Explanation: Cystic Fibrosis (CF) is an **Autosomal Recessive (AR)** disorder caused by mutations in the *CFTR* gene on chromosome 7 [1]. To manifest the disease, an individual must inherit two defective alleles (homozygous). In this scenario: * **Affected Parent:** Genotype is **aa** (homozygous recessive). * **Carrier Parent:** Genotype is **Aa** (heterozygous). Using a Punnett Square (aa × Aa): * 50% of offspring will be **Aa** (Carriers) * 50% of offspring will be **aa** (Affected) Thus, there is a **50% chance** of the child having cystic fibrosis [1]. This specific inheritance pattern (Affected × Carrier) mimics a pseudo-dominant pattern. **Analysis of Incorrect Options:** * **A (25%):** This is the risk when **both parents are carriers** (Aa × Aa). * **C (75%):** This is the probability of an offspring being "unaffected" (either carrier or healthy) when both parents are carriers. * **D (100%):** This occurs only if **both parents are affected** (aa × aa). **NEET-PG High-Yield Pearls:** * **Most common mutation:** ΔF508 (Class II mutation – protein misfolding and degradation). * **Diagnosis:** Sweat Chloride Test (>60 mEq/L) is the gold standard. * **Clinical Triad:** Chronic sinopulmonary disease, exocrine pancreatic insufficiency, and meconium ileus (in neonates) [2]. * **Infertility:** 95% of males are infertile due to Congenital Bilateral Absence of Vas Deferens (CBAVD). * **Organism:** *Pseudomonas aeruginosa* is the most common cause of pulmonary infections in older CF patients.

Question 145: Which type of cancer is NOT associated with BRCA2 mutation?

A. Ovarian cancer
B. Prostate cancer
C. Bile duct cancer
D. Liposarcoma (Correct Answer)

Explanation: The **BRCA2 gene** (located on chromosome 13q) is a tumor suppressor gene involved in the repair of double-stranded DNA breaks via homologous recombination. While BRCA1 is primarily associated with breast and ovarian cancers, **BRCA2** has a broader spectrum of associated malignancies involving various epithelial tissues. **Why Liposarcoma is the correct answer:** Liposarcoma is a mesenchymal tumor (soft tissue sarcoma). There is no established clinical or genetic association between BRCA2 mutations and the development of liposarcoma. BRCA mutations typically predispose individuals to carcinomas (epithelial origin) rather than sarcomas. **Analysis of Incorrect Options:** * **Ovarian Cancer:** BRCA2 mutations significantly increase the lifetime risk of ovarian cancer (approximately 10-20%), typically presenting as high-grade serous carcinoma [1]. * **Prostate Cancer:** BRCA2 is the most important genetic risk factor for aggressive, high-grade prostate cancer in men, often occurring at an earlier age. * **Bile Duct Cancer (Cholangiocarcinoma):** BRCA2 mutations are associated with an increased risk of various gastrointestinal malignancies, including pancreatic, gallbladder, and bile duct cancers. **NEET-PG High-Yield Pearls:** * **BRCA1 vs. BRCA2:** BRCA1 has a higher risk for breast/ovarian cancer; BRCA2 has a higher risk for **male breast cancer**, pancreatic cancer, and prostate cancer [2]. * **Fanconi Anemia:** Biallelic mutation (homozygous) of BRCA2 (also known as FANCD1) leads to Fanconi Anemia Type D1. * **Associated Cancers (BRCA2):** Breast (male and female), Ovary, Pancreas, Prostate, Melanoma, and Bile duct. * **Treatment:** Cancers with BRCA mutations are particularly sensitive to **PARP inhibitors** (e.g., Olaparib) due to the concept of "synthetic lethality."

Question 146: Which of the following is NOT a common tumor associated with neurofibromatosis?

A. Gliomas
B. Retinoblastoma (Correct Answer)
C. Schwannomas
D. Meningiomas

Explanation: **Explanation:** **Neurofibromatosis (NF)** is a group of autosomal dominant neurocutaneous syndromes (phakomatoses) characterized by a high predisposition to tumors of the central and peripheral nervous systems. **Why Retinoblastoma is the Correct Answer:** Retinoblastoma is caused by a mutation in the **RB1 gene** on chromosome 13. It is not associated with Neurofibromatosis Type 1 (NF1) or Type 2 (NF2). While NF1 is associated with ocular findings like **Lisch nodules** and **Optic Nerve Gliomas**, it does not increase the risk for retinoblastoma. **Analysis of Incorrect Options:** * **A. Gliomas:** These are highly characteristic of **NF1**. Specifically, **Optic Pathway Gliomas** (pilocytic astrocytomas) occur in up to 15% of NF1 patients. * **C. Schwannomas:** These are the hallmark of **NF2**. The classic presentation is **bilateral vestibular schwannomas** (acoustic neuromas) [1]. NF2 is often remembered by the mnemonic "MISME" (Multiple Inherited Schwannomas, Meningiomas, and Ependymomas). * **D. Meningiomas:** These are common intracranial tumors associated with **NF2**. Patients with NF2 often develop multiple meningiomas at a younger age than the general population. **High-Yield Clinical Pearls for NEET-PG:** * **NF1 (von Recklinghausen Disease):** Chromosome **17** (Protein: Neurofibromin). Key features: Café-au-lait spots, Lisch nodules (iris hamartomas), Axillary freckling (Crowe sign), and Neurofibromas. * **NF2:** Chromosome **22** (Protein: Merlin/Schwannomin). Key features: Bilateral acoustic neuromas [1], cataracts (juvenile posterior subcapsular lenticular opacities), and meningiomas. * **Rule of 2s for NF2:** Chromosome **22**, Bilateral (2) Eighth (2x4) nerve tumors.

Question 147: Which of the following conditions is known to have multifactorial inheritance?

A. Neurofibroma (Correct Answer)
B. Hemophilia
C. Cardiac septal defects
D. Hypophosphatemic rickets

Explanation: **Explanation:** **Correct Answer: A. Neurofibroma** Neurofibromatosis (Type 1 and 2) follows an **Autosomal Dominant** pattern of inheritance [1]. However, the question asks for "multifactorial inheritance," and in many standard medical examinations, including NEET-PG, there is often a debate regarding the classification of complex traits. While Neurofibromatosis is classically Mendelian, **Cardiac Septal Defects (Option C)** are the classic textbook example of **Multifactorial Inheritance**. *Note: If the provided key marks Neurofibroma as correct, it may be referring to the "Two-Hit Hypothesis" (Knudson's hypothesis) where a germline mutation is combined with an acquired somatic mutation, or it may be a controversial key. Classically, Cardiac Septal Defects are the most accurate representation of multifactorial inheritance.* **Analysis of Options:** * **Cardiac Septal Defects (Option C):** These are the prototype for multifactorial inheritance, resulting from an interaction between multiple genes and environmental triggers (e.g., maternal diabetes, alcohol). * **Hemophilia (Option B):** This is a classic **X-linked Recessive** disorder (Hemophilia A and B). * **Hypophosphatemic Rickets (Option D):** This is the classic example of **X-linked Dominant** inheritance (Vitamin D-resistant rickets). **High-Yield Clinical Pearls for NEET-PG:** * **Multifactorial Inheritance Examples:** Cleft lip/palate, Pyloric stenosis, Neural tube defects, Hypertension, and Diabetes Mellitus. * **Neurofibromatosis Type 1 (NF1):** Located on Chromosome 17 (17 letters in Neurofibromatosis). Features include Café-au-lait spots, Lisch nodules, and optic gliomas. * **Neurofibromatosis Type 2 (NF2):** Located on Chromosome 22. Characterized by bilateral acoustic neuromas. * **Rule of Thumb:** If a disease involves a single gene mutation, it is Mendelian. If it involves "threshold traits" and environmental factors, it is multifactorial.

Question 148: Multifactorial inheritance is seen in which of the following conditions?

A. Neurofibroma
B. Hemophilia
C. Cardiac septal defects (Correct Answer)
D. Hypophosphataemic rickets

Explanation: **Explanation:** **Multifactorial inheritance** refers to conditions caused by the complex interaction of multiple genes (polygenic) and environmental factors [2]. These diseases do not follow classic Mendelian patterns but often show a "threshold effect" and tend to cluster in families. **Why Cardiac Septal Defects are the Correct Answer:** Congenital heart diseases, including **Atrial Septal Defects (ASD)** and **Ventricular Septal Defects (VSD)**, are classic examples of multifactorial inheritance. While some cases are associated with chromosomal anomalies (like Down syndrome), the majority of isolated septal defects result from the cumulative effect of several low-penetrance genes combined with maternal environmental triggers (e.g., folate deficiency, viral infections, or teratogen exposure) [1]. **Analysis of Incorrect Options:** * **A. Neurofibroma (Neurofibromatosis Type 1):** This is an **Autosomal Dominant** condition caused by a mutation in the *NF1* gene on chromosome 17. * **B. Hemophilia:** This follows an **X-linked Recessive** inheritance pattern (Hemophilia A: Factor VIII deficiency; Hemophilia B: Factor IX deficiency). * **D. Hypophosphataemic Rickets:** This is the classic example of **X-linked Dominant** inheritance (Vitamin D-resistant rickets). **High-Yield Clinical Pearls for NEET-PG:** * **Common Multifactorial Conditions:** Cleft lip/palate, Neural tube defects, Pyloric stenosis, Hypertension, Type 2 Diabetes Mellitus, and Hirschsprung disease. * **Recurrence Risk:** In multifactorial inheritance, the risk of recurrence in first-degree relatives is approximately the square root of the population prevalence ($\sqrt{p}$). * **Threshold Model:** The disease manifests only when the combined genetic and environmental liability exceeds a specific threshold [2].

Question 149: Which of the following inheritable orofacial deformities is a recessive trait?

A. Dentin Dysplasia
B. Dentinogenesis Imperfecta
C. White Spongy Nevus
D. None of the above (Correct Answer)

Explanation: The correct answer is **None of the above** because all the listed conditions—Dentin Dysplasia, Dentinogenesis Imperfecta, and White Spongy Nevus—are inherited as **Autosomal Dominant (AD)** traits. In medical genetics, most structural protein defects or conditions involving the formation of specialized tissues like enamel, dentin, and keratin follow a dominant inheritance pattern. ### Breakdown of Options: * **Dentin Dysplasia (Type I and II):** Both types are **Autosomal Dominant**. Type I (Radicular) affects root formation ("rootless teeth"), while Type II (Coronal) affects the pulp chamber shape (thistle-tube appearance). * **Dentinogenesis Imperfecta (DI):** This is an **Autosomal Dominant** condition caused by mutations in the *DSPP* gene (Types II and III) or associated with Osteogenesis Imperfecta (Type I). It is characterized by opalescent, brownish-blue teeth and premature enamel loss due to an abnormal dentino-enamel junction. * **White Spongy Nevus (Cannon’s Disease):** This is an **Autosomal Dominant** genodermatosis caused by mutations in Keratin 4 or Keratin 13. It presents as thick, white, velvety plaques on the buccal mucosa that do not disappear upon stretching. ### High-Yield NEET-PG Pearls: * **Rule of Thumb:** Most hereditary orofacial syndromes involving structural defects (e.g., Treacher Collins, Cleidocranial Dysplasia, Peutz-Jeghers) are **Autosomal Dominant**. * **Dentinogenesis Imperfecta vs. Amelogenesis Imperfecta:** While DI is consistently AD, Amelogenesis Imperfecta can be AD, Autosomal Recessive, or X-linked. * **Clinical Distinctions:** In DI, the primary teeth are usually more severely affected than permanent teeth. Radiographically, look for **obliteration of pulp chambers** and "bulbous" crowns.

Question 150: Complications of Wilson's disease include all except?

A. Fanconi syndrome
B. Ataxia
C. Hemolytic anemia
D. Peripheral neuropathy (Correct Answer)

Explanation: Wilson’s Disease (Hepatolenticular degeneration) is an autosomal recessive disorder caused by mutations in the **ATP7B gene**, leading to impaired biliary copper excretion and systemic copper accumulation. **Why Peripheral Neuropathy is the correct answer:** While Wilson’s disease causes significant neurological damage, it primarily affects the **Central Nervous System (CNS)**, specifically the basal ganglia [1]. **Peripheral neuropathy is not a feature** of Wilson’s disease. In fact, if a patient with Wilson’s disease develops peripheral neuropathy, it is usually an adverse effect of treatment with **D-penicillamine** (which can cause Vitamin B6 deficiency). **Analysis of Incorrect Options:** * **Fanconi Syndrome:** Excess copper is toxic to the renal proximal convoluted tubules, leading to Type 2 Renal Tubular Acidosis (Fanconi syndrome), characterized by glucosuria, aminoaciduria, and phosphaturia [1]. * **Ataxia:** Copper deposition in the cerebellum and its pathways leads to cerebellar signs, including ataxia, dysarthria, and coordination deficits [1]. * **Hemolytic Anemia:** Sudden release of free copper into the bloodstream from necrotic hepatocytes causes oxidative stress to RBCs (Coombs-negative hemolytic anemia), often seen during a "Wilsonian crisis" [1]. **High-Yield Clinical Pearls for NEET-PG:** * **Kayser-Fleischer (KF) Rings:** Copper deposition in the **Descemet’s membrane** of the cornea (best seen on slit-lamp exam). * **Diagnosis:** Low serum Ceruloplasmin (<20 mg/dL), increased 24-hour urinary copper excretion, and "Giant Panda" sign on MRI Brain. * **Treatment:** Chelators like **Trientine** (preferred) or D-penicillamine; Zinc is used for maintenance as it inhibits intestinal copper absorption.

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Principles of Medical Genetics

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Genetic Testing and Counseling

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Single Gene Disorders

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Chromosomal Disorders

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Mitochondrial Diseases

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Pharmacogenomics

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Cancer Genetics

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Genetics of Common Diseases

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Epigenetics and Disease

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Genetic Basis of Developmental Disorders

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Ethical Issues in Medical Genetics

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Gene Therapy and Precision Medicine

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