Mendelian inheritance patterns US Medical PG Practice Questions and MCQs
Practice US Medical PG questions for Mendelian inheritance patterns. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Mendelian inheritance patterns US Medical PG Question 1: A 25-year-old man with a genetic disorder presents for genetic counseling because he is concerned about the risk that any children he has will have the same disease as himself. Specifically, since childhood he has had difficulty breathing requiring bronchodilators, inhaled corticosteroids, and chest physiotherapy. He has also had diarrhea and malabsorption requiring enzyme replacement therapy. If his wife comes from a population where 1 in 10,000 people are affected by this same disorder, which of the following best represents the likelihood a child would be affected as well?
- A. 0.01%
- B. 2%
- C. 0.5%
- D. 1% (Correct Answer)
- E. 50%
Mendelian inheritance patterns Explanation: ***Correct Option: 1%***
- The patient's symptoms (difficulty breathing requiring bronchodilators, inhaled corticosteroids, and chest physiotherapy; diarrhea and malabsorption requiring enzyme replacement therapy) are classic for **cystic fibrosis (CF)**, an **autosomal recessive disorder**.
- For an autosomal recessive disorder with a prevalence of 1 in 10,000 in the general population, **q² = 1/10,000**, so **q = 1/100 = 0.01**. The carrier frequency **(2pq)** is approximately **2q = 2 × (1/100) = 1/50 = 0.02**.
- The affected man is **homozygous recessive (aa)** and will always pass on the recessive allele. His wife has a **1/50 chance of being a carrier (Aa)**. If she is a carrier, she has a **1/2 chance of passing on the recessive allele**.
- Therefore, the probability of an affected child = **(Probability wife is a carrier) × (Probability wife passes recessive allele) = 1/50 × 1/2 = 1/100 = 1%**.
*Incorrect Option: 0.01%*
- This percentage is too low and does not correctly account for the carrier frequency in the population and the probability of transmission from a carrier mother.
*Incorrect Option: 2%*
- This represents approximately the carrier frequency (1/50 ≈ 2%), but does not account for the additional 1/2 probability that a carrier mother would pass on the recessive allele.
*Incorrect Option: 0.5%*
- This value would be correct if the carrier frequency were 1/100 instead of 1/50, which does not match the given population prevalence.
*Incorrect Option: 50%*
- **50%** would be the risk if both parents were carriers of an autosomal recessive disorder (1/4 chance = 25% for affected, but if we know one parent passes the allele, conditional probability changes). More accurately, 50% would apply if the disorder were **autosomal dominant** with one affected parent, which is not the case here.
Mendelian inheritance patterns US Medical PG Question 2: A healthy 30-year-old woman comes to the physician with her husband for preconception counseling. Her husband is healthy but she is concerned because her brother was recently diagnosed with a genetic liver condition for which he takes penicillamine. Her father-in-law has liver cirrhosis and a tremor. The results of genetic testing show that both the patient and her husband are carriers of a mutation in the ATP7B gene. Which of the following is the chance that this patient’s offspring will eventually develop the hereditary condition?
- A. 0%
- B. 25% (Correct Answer)
- C. 100%
- D. 50%
- E. 75%
Mendelian inheritance patterns Explanation: ***25%***
- The familial history (brother with a genetic liver condition, father-in-law with cirrhosis and tremor) and the **ATP7B gene mutation** indicate **Wilson's disease**, which is typically inherited in an **autosomal recessive** pattern.
- If both parents are carriers (heterozygous for the mutation), the probability that their offspring will inherit two copies of the mutated gene (one from each parent) and, therefore, develop the condition is **25%** as per Mendelian inheritance.
*0%*
- This is incorrect because both parents are identified as carriers, meaning there is a definite risk of passing on the mutated genes to their offspring.
- For the risk to be 0%, at least one parent would need to be completely free of the mutation or the inheritance pattern would need to be dominant with no penetrance.
*100%*
- This would only be the case if both parents had the disease (were homozygous for the mutation) or if the condition were dominant and at least one parent had the disease and passed on the dominant allele.
- Since both are carriers, the chance of inheriting two mutated alleles is not 100%.
*50%*
- A 50% chance would apply if one parent had the disease (homozygous recessive) and the other was a carrier, or if it were an autosomal dominant condition with one affected heterozygous parent.
- This does not reflect the inheritance pattern for two carrier parents in an autosomal recessive condition.
*75%*
- A 75% chance is not typical for a single genetic outcome in standard Mendelian inheritance patterns from carrier parents.
- In the context of two carriers for an autosomal recessive trait, 75% represents the chance of the offspring either being a carrier (50%) or being completely unaffected (25%), but not the chance of developing the condition.
Mendelian inheritance patterns US Medical PG Question 3: A 16-year-old boy is brought to the emergency department after losing consciousness. He had no preceding chest pain or palpitations. His father has cataracts and had frontal balding in his twenties but has no history of cardiac disease. His paternal grandfather also had early-onset balding. His pulse is 43/min. Physical examination shows frontal hair loss, temporal muscle wasting, and testicular atrophy. Neurologic examination shows bilateral foot drop and weakness of the intrinsic hand muscles. An ECG shows bradycardia with third-degree atrioventricular block. The severity of this patient's symptoms compared to that of his father is most likely due to which of the following genetic properties?
- A. Penetrance
- B. Codominance
- C. Anticipation (Correct Answer)
- D. Loss of heterozygosity
- E. Pleiotropy
Mendelian inheritance patterns Explanation: ***Anticipation***
- **Anticipation** describes a genetic phenomenon where the severity of a genetic disorder increases and/or the age of onset decreases in successive generations. This is typical of disorders caused by **trinucleotide repeat expansions**, such as myotonic dystrophy.
- The patient's severe symptoms (third-degree AV block, foot drop, muscle wasting) occurring at a young age, compared to his father's milder symptoms (cataracts, early balding) without cardiac disease, are a classic presentation of anticipation in **myotonic dystrophy type 1 (DM1)**.
*Penetrance*
- **Penetrance** refers to the proportion of individuals with a particular genotype that express the associated phenotype.
- While it explains whether or not a trait is expressed, it does not explain the increasing severity or earlier onset across generations.
*Codominance*
- **Codominance** occurs when two different alleles for a gene are both expressed, and both phenotypes are observable (e.g., AB blood type).
- This concept does not apply to the increasing severity or earlier onset of symptoms observed in this family.
*Loss of heterozygosity*
- **Loss of heterozygosity** is a genetic event where an individual inheriting one mutated allele for a tumor suppressor gene loses the normal allele, leading to disease (e.g., retinoblastoma).
- This mechanism primarily relates to tumor formation and is not relevant to the progressive worsening of symptoms across generations in the context of myotonic dystrophy.
*Pleiotropy*
- **Pleiotropy** refers to a single gene affecting multiple phenotypic traits (e.g., the FBN1 gene in Marfan syndrome affecting skeletal, ocular, and cardiovascular systems).
- While myotonic dystrophy exhibits pleiotropy (affecting multiple systems), pleiotropy itself does not explain the intergenerational increase in severity or decrease in age of onset, which is specifically attributed to anticipation.
Mendelian inheritance patterns US Medical PG Question 4: A 34-year-old woman, gravida 1, para 0, at 18 weeks' gestation, comes to the physician for a prenatal visit. She recently read about a genetic disorder that manifests with gait ataxia, kyphoscoliosis, and arrhythmia and is concerned about the possibility of her child inheriting the disease. There is no personal or family history of this disorder. The frequency of unaffected carriers in the general population is 1/100. Assuming the population is in a steady state without selection, what is the probability that her child will develop this disease?
- A. 1/10,000
- B. 1/20,000
- C. 1/40,000 (Correct Answer)
- D. 1/200
- E. 1/400
Mendelian inheritance patterns Explanation: ***1/40,000***
- This disorder (Friedreich's ataxia) follows **autosomal recessive** inheritance, meaning both parents must be carriers for the child to be affected.
- Since there is no family history, we treat both parents as random individuals from the general population with carrier frequency 1/100.
- **Calculation**: Probability mother is carrier (1/100) × Probability father is carrier (1/100) × Probability child is affected given both parents are carriers (1/4) = **1/40,000**.
- This applies Hardy-Weinberg equilibrium principles for a steady-state population.
*1/10,000*
- This calculation (1/100 × 1/100 = 1/10,000) represents only the probability that both parents are carriers.
- It fails to account for the **1/4 chance** of an affected child when two carriers of an **autosomal recessive** condition conceive.
- This would be the answer if both parents being carriers automatically meant the child would be affected, which is incorrect.
*1/20,000*
- This result would occur if the probability of the child inheriting the disease from carrier parents was 1/2 instead of 1/4 (1/100 × 1/100 × 1/2 = 1/20,000).
- A 1/2 probability would apply to **autosomal dominant** conditions where one affected parent passes the disease, not for **autosomal recessive** inheritance.
- For autosomal recessive disorders, two carrier parents have a 1/4 (not 1/2) chance of an affected child.
*1/200*
- This probability (1/100 × 1/2 = 1/200) would suggest only one parent needed to be a carrier with a 1/2 transmission probability.
- This does not account for the requirement that **both parents must be carriers** for an **autosomal recessive** disorder.
- It represents a fundamental misunderstanding of recessive inheritance patterns.
*1/400*
- This calculation (1/100 × 1/4 = 1/400) incorrectly assumes only one parent needs to be a carrier.
- For **autosomal recessive** inheritance, **both parents must be carriers**, so both their carrier probabilities (1/100 each) must be included in the calculation.
- It omits the second parent's carrier probability entirely.
Mendelian inheritance patterns US Medical PG Question 5: A 23-year-old woman and her husband come to a genetic counselor because she is concerned about the chance of having an inherited defect if they had a child. Family history reveals no significant family history in her husband; however, her sister had a son who has seizures, failure to thrive, and neurodegeneration. She does not remember the name of the disease but remembers that her nephew had sparse, brittle hair that kinked in odd directions. She does not think that any other members of her family including her sister's husband have had this disorder. If this couple had a son, what is the most likely chance that he would have the same disorder that affected the patient's nephew?
- A. 100%
- B. 12.5%
- C. 25% (Correct Answer)
- D. 50%
- E. Close to 0%
Mendelian inheritance patterns Explanation: ***25%***
- The nephew's symptoms of **seizures, failure to thrive, neurodegeneration**, and **sparse, brittle, kinky hair** are highly indicative of **Menkes disease**, an **X-linked recessive** disorder.
- Since the patient's sister had an affected son, the sister is an **obligate carrier** of the mutation.
- The patient and her sister share the same parents, so their mother must be a carrier (or have the mutation).
- The patient herself has a **50% chance of being a carrier**.
- **If the patient is a carrier**, each son has a **50% chance** of being affected.
- **Overall probability**: 0.5 (chance patient is carrier) × 0.5 (chance son inherits mutation) = **0.25 = 25%**.
*Close to 0%*
- This would only be correct if the patient had no chance of being a carrier, which is not the case given her family history.
- Her sister's affected son confirms the mutation is present in the maternal lineage.
*100%*
- This would only occur if the patient were definitely a carrier AND all male offspring inherited the mutation, or if the disorder were autosomal dominant with complete penetrance.
- For **X-linked recessive** disorders, even carrier mothers only pass the mutation to 50% of sons on average.
*12.5%*
- This percentage might represent additional generational steps or compound probabilities not relevant to this direct parent-child scenario.
- The correct calculation for this scenario is 50% × 50% = 25%.
*50%*
- This would be correct if we knew with certainty that the patient is a carrier.
- However, since we only know her sister is a carrier, the patient has a 50% chance of being a carrier herself, making the overall risk 25%.
- This is a common error in genetic counseling calculations—forgetting to account for the uncertain carrier status of the at-risk individual.
Mendelian inheritance patterns US Medical PG Question 6: A 9-month-old female infant is brought in by her mother to the pediatrician because she is concerned that her daughter is not growing normally. On physical exam, the head circumference is 95th percentile and the height is 5th percentile. The child has disproportionate growth such that both the upper and lower extremities show a rhizomelic pattern of shortening, but the axial skeleton appears to be normal. The child appears to have normal intelligence, but has delayed motor milestones; specifically, she is not able to roll or sit up by herself. Which of the following best describes the mode of inheritance for this disorder?
- A. Mitochondrial pattern of inheritance
- B. X-linked recessive
- C. Autosomal recessive
- D. X-linked dominant
- E. Autosomal dominant (Correct Answer)
Mendelian inheritance patterns Explanation: **Autosomal dominant**
- The clinical presentation with **rhizomelic dwarfism**, normal intelligence, and macrocephaly is highly suggestive of **achondroplasia**, which is inherited in an **autosomal dominant** pattern.
- Achondroplasia is caused by a mutation in the **fibroblast growth factor receptor 3 (FGFR3)** gene, which inhibits chondrocyte proliferation in the growth plate, leading to disproportionate short stature.
*Mitochondrial pattern of inheritance*
- This pattern is characterized by transmission only through the **mother** to all offspring, and typically affects tissues with high energy demands like muscle and brain due to impaired oxidative phosphorylation.
- The presented symptoms of disproportional dwarfism are not characteristic of common mitochondrial disorders.
*X-linked recessive*
- This pattern primarily affects **males**, with carrier mothers passing the trait to sons, and affected males not passing it to their sons.
- The described condition affects a female infant and the pattern of inheritance does not align with typical X-linked recessive disorders.
*Autosomal recessive*
- This pattern requires **two copies of the mutated gene** for the disease to manifest, meaning both parents are usually asymptomatic carriers.
- While some forms of dwarfism can be autosomal recessive, the specific clinical features and prevalence of achondroplasia point towards an autosomal dominant inheritance.
*X-linked dominant*
- This pattern affects both males and females, but often more severely in males, and affected fathers pass the trait to **all their daughters** but none of their sons.
- The symptoms described do not fit the typical presentation or inheritance pattern of X-linked dominant disorders, which are generally rarer for forms of skeletal dysplasia.
Mendelian inheritance patterns US Medical PG Question 7: Given the pattern of inheritance shown in the pedigree, where might you find the disease gene in question?
- A. On helical RNA in the cytoplasm
- B. On circular DNA in the mitochondrion (Correct Answer)
- C. On double stranded DNA in the nucleus
- D. On single-stranded DNA in the cytoplasm
- E. On linear DNA in the mitochondrion
Mendelian inheritance patterns Explanation: ***On circular DNA in the mitochondrion***
- The pedigree shows a pattern of **maternal inheritance**, where all offspring of an affected mother are affected, but none of the offspring of an affected father are affected. This is characteristic of **mitochondrial DNA (mtDNA)** inheritance, which is exclusively passed down from the mother.
- Mitochondrial DNA is typically **circular** and found in the mitochondria within the cytoplasm.
*On helical RNA in the cytoplasm*
- **RNA** is not the primary carrier of genetic information for inherited diseases in humans; **DNA** serves this role.
- While some viruses use RNA as their genetic material, human inheritance patterns are based on DNA.
*On double-stranded DNA in the nucleus*
- This describes **nuclear DNA**, which follows Mendelian patterns of inheritance (autosomal dominant, recessive, X-linked) and involves contributions from both parents.
- The observed maternal-only inheritance pattern rules out nuclear DNA as the sole location for this disease gene.
*On single-stranded DNA in the cytoplasm*
- Human genetic material is generally **double-stranded DNA**, except in some viral contexts.
- The cytoplasm contains mitochondria, but the primary form of DNA there is circular and double-stranded, not single-stranded.
*On linear DNA in the mitochondrion*
- **Mitochondrial DNA** is characteristically **circular**, not linear, in most eukaryotic organisms, including humans.
- While located in the mitochondrion, the specific structure (linear vs. circular) is important for its proper function and replication.
Mendelian inheritance patterns US Medical PG Question 8: A 1-year-old male with a history of recurrent pseudomonal respiratory infections and steatorrhea presents to the pediatrician for a sweat test. The results demonstrate a chloride concentration of 70 mEq/L (nl < 40 mEq/L). Which of the following defects has a similar AUTOSOMAL RECESSIVE mode of inheritance as the disorder experienced by this patient?
- A. Abnormal production of type IV collagen
- B. Trinucleotide repeat expansion of CAG on chromosome 4
- C. Mutated gene for mitochondrial-tRNA-Lys
- D. Accumulation of glycogen in the lysosome (Correct Answer)
- E. Inability to convert carbamoyl phosphate and ornithine into citrulline
Mendelian inheritance patterns Explanation: ***Accumulation of glycogen in the lysosome***
- The patient's symptoms (recurrent **pseudomonal respiratory infections**, **steatorrhea**, and elevated sweat chloride) are classic for **cystic fibrosis (CF)**, an **autosomal recessive** disorder.
- Accumulation of glycogen in the lysosome describes **Pompe disease (Type II Glycogen Storage Disease)**, which is also an **autosomal recessive** disorder, making this the correct answer.
*Abnormal production of type IV collagen*
- This defect is characteristic of **Alport syndrome**, which is predominantly **X-linked dominant** (~80-85% of cases), though autosomal recessive forms exist.
- The question context and typical board exam framing classify this as X-linked, not autosomal recessive.
- Alport syndrome primarily affects the kidneys, ears, and eyes, and does not present with recurrent pseudomonal infections or steatorrhea.
*Trinucleotide repeat expansion of CAG on chromosome 4*
- This genetic defect is responsible for **Huntington's disease**, which is an **autosomal dominant** neurodegenerative disorder.
- Huntington's disease presents with chorea, cognitive decline, and psychiatric symptoms, which are distinct from CF.
*Mutated gene for mitochondrial-tRNA-Lys*
- A mutated gene for mitochondrial-tRNA-Lys is associated with **MELAS syndrome (Mitochondrial Encephalomyopathy, Lactic Acidosis, and Stroke-like episodes)**, which is inherited through **maternal (mitochondrial)** inheritance.
- This mode of inheritance is distinct from the autosomal recessive pattern seen in cystic fibrosis.
*Inability to convert carbamoyl phosphate and ornithine into citrulline*
- This describes a defect in **ornithine transcarbamylase (OTC) deficiency**, an **X-linked recessive** disorder, not autosomal recessive.
- OTC deficiency leads to hyperammonemia and metabolic disturbances, without the pulmonary and gastrointestinal symptoms typical of cystic fibrosis.
Mendelian inheritance patterns US Medical PG Question 9: A Caucasian 32-year-old woman has an uncomplicated vaginal delivery, giving birth to male and female fraternal twins at term. At 2 days of life, the twin sister develops abdominal distension without emesis, and the mother states that she has not noticed the passage of stool for this infant. Genetic testing identifies deletion of an amino acid in a membrane channel for the girl. Both parents are healthy. Assuming that twin brother's disease status/symptomatology is unclear, which of the following best approximates the probability that the twin brother is a carrier of the disease allele?
- A. 100%
- B. 67% (Correct Answer)
- C. 0%
- D. 50%
- E. 25%
Mendelian inheritance patterns Explanation: ***67%***
- The sister's symptoms of **abdominal distension** without emesis and lack of stool passage, along with genetic testing identifying a **deletion of an amino acid in a membrane channel**, strongly suggest **Cystic Fibrosis (CF)**. CF is an **autosomal recessive disorder**.
- Since the affected twin sister has CF (genotype **aa**), and both parents are healthy, both parents must be **heterozygous carriers (Aa)**. When two carriers (Aa x Aa) have offspring, the probability of any child being a carrier (Aa) is **2/3** among the unaffected offspring. The twin brother is currently unaffected (phenotypically healthy), so the probability of him being a carrier is 2/3, or approximately 67%.
*100%*
- This would only be true if one or both parents were **homozygous affected (aa)**, or if the disease inheritance was **dominant** and the parents were carriers, which is not the case for this autosomal recessive disorder where the parents are healthy carriers and the brother is phenotypically unaffected.
- While both parents *are* carriers, the brother, being unaffected, has a chance of being **homozygous dominant (AA)**, meaning he is not a carrier.
*0%*
- This is incorrect because we know both parents are **obligate carriers** (heterozygous, Aa) for the recessive allele, given their affected child (aa). Therefore, their children have a 75% chance of inheriting at least one disease allele (50% carrier, 25% affected).
- The twin brother being unaffected means he has a 2/3 chance of being a carrier, not 0%.
*50%*
- This probability (1/2) is the chance of a child inheriting a specific allele from one parent, or the chance of being a carrier if one parent is affected and the other is homozygous dominant.
- In an **autosomal recessive** inheritance pattern where both parents are carriers (Aa x Aa) and the offspring is unaffected, the probability of being a carrier is **2/3**, not 1/2.
*25%*
- This is the probability of a child being **homozygous dominant (AA)** from two carrier parents (Aa x Aa), meaning they would neither have the disease nor be carriers.
- It is also the probability of a child being affected (aa) if both parents are carriers. Neither of these scenarios matches the question asking for the probability of the *unaffected* twin brother being a carrier.
Mendelian inheritance patterns US Medical PG Question 10: An 11-year-old male with light purple eyes presents with gradual loss of bilateral visual acuity. Over the past several years, vision has worsened from 20/20 to 20/100 in both eyes. He also has mild nystagmus when focusing on objects such as when he is trying to do his homework. He is diagnosed with a disease affecting melanin production in the iris. If both of his parents are unaffected, which of the following represents the most likely probabilities that another male or female child from this family would be affected by this disorder?
- A. Male: 25% Female: 25%
- B. Same as general population
- C. Male: 50% Female: 50%
- D. Male: 50% Female: 0% (Correct Answer)
- E. Male: 100% Female: 0%
Mendelian inheritance patterns Explanation: ***Male: 50% Female: 0%***
- The symptoms (light purple eyes, gradual vision loss, nystagmus, defective melanin production) are characteristic of **ocular albinism**. This condition is typically **X-linked recessive**.
- If the patient's mother is a **carrier** (XAXa) and the father is unaffected (XAY), there is a **50% chance** that a male child will inherit the affected X chromosome (XaY) and thus be affected, and a **0% chance** for a female child to be affected if the father is unaffected (all female children would either be carriers XAXa or unaffected XAXA).
*Male: 25% Female: 25%*
- This probability pattern would typically suggest an **autosomal recessive** inheritance pattern, where both parents are carriers (Aa x Aa), and there is a 25% chance for any child to be affected regardless of sex.
- However, ocular albinism most commonly follows an X-linked recessive pattern, and the described clinical features (e.g., light purple eyes due to melanin defect in the iris) are highly suggestive of ocular albinism.
*Same as general population*
- This would only be true if the disorder was not genetic or if the parents' carrier status did not increase the risk for subsequent children.
- Given the heritable nature of albinism and the specific family history (parents unaffected, one affected child), the risk for subsequent children is significantly higher than the general population.
*Male: 50% Female: 50%*
- This pattern would occur in an **autosomal dominant** disorder with 100% penetrance, where one parent is affected (Aa x aa), or in some specific scenarios of X-linked inheritance if the father was affected and the mother was a carrier.
- Ocular albinism is X-linked recessive, not autosomal dominant, and the father is stated to be unaffected.
*Male: 100% Female: 0%*
- This genetic pattern is highly unlikely unless the mother was fully mosaic for the condition or an extremely rare and specialized inheritance pattern was at play.
- In a typical X-linked recessive inheritance with an unaffected father and a carrier mother, there is always a 50% chance for a male child to be unaffected.
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