A 5-year-old boy is brought to the clinic by his mother for an annual check-up. The family recently moved from Nebraska and is hoping to establish care. The patient is home schooled and mom is concerned about her son’s development. He is only able to say 2 to 3 word sentences and has been “behind on his alphabet." He always seems to be disinterested and "just seems to be behind.” The patient is observed to be focused on playing with his cars during the interview. Physical examination demonstrate a well-nourished child with poor eye contact, a prominent jaw, a single palmar crease, and bilaterally enlarged testicles. What is the most likely mechanism of this patient’s findings?
Q42
Two healthy adults have only one child. He has Friedreich ataxia (FA). They are considering having more children, but are uncertain of their risk of having another child with the condition. What should they do?
Q43
A 64-year-old woman presents to an urgent care clinic with edema of her lips and difficulty breathing. She reports that she had multiple root canals performed earlier today, and she started to notice swelling of her lips 2 hours ago. The symptoms have now progressed to where she is having trouble breathing. She notes similar episodes in the past after minor procedures such as this. The blood pressure is 118/76 mm Hg, the heart rate is 84/min, and the respiratory rate is 16/min. Physical examination is remarkable for edema of her lips and mild inspiratory stridor. The laboratory results are remarkable for a low level of C1 esterase inhibitor. Of the following options, which is the most likely diagnosis?
Q44
A 5-year-old boy is brought to the emergency room by his parents after slipping on a rug at home and experiencing exquisite pain and swelling of his arms. Radiographs reveal a new supracondylar fracture of the humerus, as well as indications of multiple, old fractures that have healed. His parents note that an inherited disorder is present in their family history. A comprehensive physical exam also reveals blue-tinted sclera and yellow-brown, discolored teeth. What is the etiology of the patient’s disorder?
Q45
A 5-year-old boy is brought to the physician because of behavioral problems. His mother says that he has frequent angry outbursts and gets into fights with his classmates. He constantly complains of feeling hungry, even after eating a full meal. He has no siblings, and both of his parents are healthy. He is at the 25th percentile for height and is above the 95th percentile for weight. Physical examination shows central obesity, undescended testes, almond-shaped eyes, and a thin upper lip. Which of the following genetic changes is most likely associated with this patient's condition?
Q46
A 29-year-old woman presents to her gynecologist for a routine check-up. She is sexually active with multiple partners and intermittently uses condoms for contraception. She denies vaginal discharge, burning, itching, or rashes in her inguinal region. Pelvic examination is normal. Results from a routine pap smear are shown. The cellular changes seen are attributable to which of the following factors?
Q47
A researcher is investigating compounds that modulate the cell cycle as possible chemotherapeutic agents against peripheral T-cell lymphoma. The researcher discovers a group of natural compounds with inhibitory activity against histone deacetylases, a class of enzymes that remove acetyl groups from the lysine residues of histones. A histone deacetylase inhibitor most likely causes which of the following?
Q48
A genetic counselor sees a family for the first time for genetic assessment. The 24-year-old businessman and his 19-year-old sister are concerned about having a mutant allele and have decided to get tested. Their grandfather and great aunt both have Huntington’s disease which became apparent when they turned 52. Their father who is 47 years old appears healthy. The geneticist discusses both the benefits and risks of getting tested and orders some tests. Which of the following tests would best provide evidence for whether the siblings are carriers or not?
Q49
A mother brings her 3-year-old daughter to the pediatrician because she is concerned about her development. She states that her daughter seemed to regress in her motor development. Furthermore, she states she has been having brief episodes of uncontrollable shaking, which has been very distressing to the family. During the subsequent work-up, a muscle biopsy is obtained which demonstrates red ragged fibers and a presumptive diagnosis of a genetic disease is made. The mother states that she has another 6-year-old son who does not seem to be affected or have any similar symptoms. What genetic term explains this phenomenon?
Q50
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?
Molecular Genetics US Medical PG Practice Questions and MCQs
Question 41: A 5-year-old boy is brought to the clinic by his mother for an annual check-up. The family recently moved from Nebraska and is hoping to establish care. The patient is home schooled and mom is concerned about her son’s development. He is only able to say 2 to 3 word sentences and has been “behind on his alphabet." He always seems to be disinterested and "just seems to be behind.” The patient is observed to be focused on playing with his cars during the interview. Physical examination demonstrate a well-nourished child with poor eye contact, a prominent jaw, a single palmar crease, and bilaterally enlarged testicles. What is the most likely mechanism of this patient’s findings?
A. CGG trinucleotide repeat expansion (Correct Answer)
B. Microdeletion of the long arm of chromosome 7
C. Meiotic nondisjunction of chromosome 21
D. CTG trinucleotide repeat expansion
E. Microdeletion of the short arm of chromosome 5
Explanation: ***CGG trinucleotide repeat expansion***
- The patient presents with key features of **Fragile X Syndrome**: developmental delay ("behind on his alphabet," 2-3 word sentences), poor eye contact, prominent jaw, and **bilaterally enlarged testicles (macroorchidism)**.
- Fragile X Syndrome is caused by an expansion of a **CGG trinucleotide repeat** in the *FMR1* gene on the X chromosome.
*Microdeletion of the long arm of chromosome 7*
- This mechanism is associated with **Williams Syndrome**, characterized by elfin facies, an outgoing personality, and cardiovascular anomalies (especially supravalvular aortic stenosis).
- These features differ significantly from the patient's presentation; specifically, macroorchidism and poor eye contact are not typical of Williams Syndrome.
*Meiotic nondisjunction of chromosome 21*
- This leads to **Down Syndrome**, which presents with intellectual disability, distinctive facial features such as epicanthal folds and a flat nasal bridge, a single palmar crease, and hypotonia.
- While a single palmar crease is present in this patient, the prominent jaw, poor eye contact, and macroorchidism are not characteristic of Down Syndrome.
*CTG trinucleotide repeat expansion*
- A **CTG trinucleotide repeat expansion** is associated with **Myotonic Dystrophy**, a multisystem disorder characterized by myotonia, muscle wasting, cataracts, and frontal baldness.
- This condition typically presents with muscle weakness and myotonia, which are not described in the patient's symptoms, and does not cause macroorchidism or prominent jaw.
*Microdeletion of the short arm of chromosome 5*
- This microdeletion causes **Cri-du-chat syndrome**, characterized by a distinctive high-pitched cry resembling a cat's meow, microcephaly, intellectual disability, and widely spaced eyes.
- The patient's symptoms (e.g., prominent jaw, macroorchidism, no mention of a characteristic cry) do not align with the typical presentation of Cri-du-chat syndrome.
Question 42: Two healthy adults have only one child. He has Friedreich ataxia (FA). They are considering having more children, but are uncertain of their risk of having another child with the condition. What should they do?
A. Proceed with conception; risk of having another child with FA is unpredictable
B. See a genetic counselor; risk of having another child with FA is 66%
C. See a genetic counselor; risk of having another child with FA is 25% (Correct Answer)
D. Proceed with conception; risk of having another child with FA is 0%
E. See a genetic counselor; risk of having another child with FA is 50%
Explanation: ***See a genetic counselor; risk of having another child with FA is 25%***
- **Friedreich ataxia (FA)** is an **autosomal recessive** disorder. For a child to inherit an autosomal recessive disorder, both parents must be carriers of the recessive allele, and the child must inherit one copy from each parent.
- Since their first child has FA, both parents must be **heterozygous carriers**. In this scenario, the probability of each subsequent child inheriting two recessive alleles (one from each carrier parent) is **25%** (1 in 4). A genetic counselor can provide precise risk assessment and discuss options.
*Proceed with conception; risk of having another child with FA is unpredictable*
- The risk is **predictable** as FA follows an autosomal recessive inheritance pattern.
- Parents who are both carriers have a consistent 25% chance of having an affected child with each pregnancy.
*See a genetic counselor; risk of having another child with FA is 66%*
- A 66% risk is not associated with an autosomal recessive inheritance pattern for subsequent children once carrier status is established for both parents.
- This percentage might refer to the probability of an unaffected sibling being a carrier (2/3 chance), but not the risk of having an affected child.
*Proceed with conception; risk of having another child with FA is 0%*
- This statement is incorrect because both parents are confirmed carriers, meaning there is always a **25% chance** of having another child with FA.
- The disease has already manifested in a previous child, confirming the genetic risk.
*See a genetic counselor; risk of having another child with FA is 50%*
- A **50% risk** is characteristic of an autosomal dominant inheritance pattern, where only one affected parent passes on the gene, or if one parent is a carrier for a recessive disease and the other parent is affected.
- This scenario does not apply to two healthy parents who are both carriers for an **autosomal recessive** condition.
Question 43: A 64-year-old woman presents to an urgent care clinic with edema of her lips and difficulty breathing. She reports that she had multiple root canals performed earlier today, and she started to notice swelling of her lips 2 hours ago. The symptoms have now progressed to where she is having trouble breathing. She notes similar episodes in the past after minor procedures such as this. The blood pressure is 118/76 mm Hg, the heart rate is 84/min, and the respiratory rate is 16/min. Physical examination is remarkable for edema of her lips and mild inspiratory stridor. The laboratory results are remarkable for a low level of C1 esterase inhibitor. Of the following options, which is the most likely diagnosis?
A. Allergic reaction
B. Drug-induced angioedema
C. Hypothyroidism
D. Contact dermatitis
E. Hereditary angioedema (Correct Answer)
Explanation: ***Hereditary angioedema***
- The patient's history of **recurrent angioedema** following minor procedures, coupled with a **low C1 esterase inhibitor level**, is highly characteristic of hereditary angioedema. Swelling can affect the **lips, airways, and gastrointestinal tract**.
- **Triggers** such as dental procedures, trauma, and stress are common, and the condition is not mediated by histamine, explaining the lack of prominent urticaria.
*Allergic reaction*
- While allergic reactions can cause angioedema, they are typically accompanied by **urticaria (hives)** and often respond to antihistamines and epinephrine. The patient's symptoms **lack urticaria** and are linked to C1 inhibitor deficiency.
- The onset and progression, as well as the specific laboratory finding of **low C1 inhibitor**, rule out a typical IgE-mediated allergic reaction.
*Drug-induced angioedema*
- Drug-induced angioedema, most commonly associated with **ACE inhibitors**, can present similarly. However, the explanation for this angioedema, a low C1 esterase inhibitor level, points to an underlying genetic disorder.
- While drug-induced angioedema can cause isolated swelling without urticaria, the **recurrent nature** and **specific C1 inhibitor deficiency** are more consistent with hereditary angioedema.
*Hypothyroidism*
- Severe hypothyroidism can cause **myxedema**, leading to facial swelling and macroglossia, but this swelling is **non-pitting**, generalized, and develops gradually.
- It would not typically present as acute, localized angioedema with inspiratory stridor after a dental procedure, nor would it involve a **low C1 esterase inhibitor level**.
*Contact dermatitis*
- Contact dermatitis is an **inflammatory skin reaction** caused by direct contact with an allergen or irritant, typically manifesting as a **rash with erythema, pruritus, and vesicles**.
- It would not cause rapid onset lip edema and airway compromise without prominent skin lesions, nor is it associated with a **low C1 esterase inhibitor level**.
Question 44: A 5-year-old boy is brought to the emergency room by his parents after slipping on a rug at home and experiencing exquisite pain and swelling of his arms. Radiographs reveal a new supracondylar fracture of the humerus, as well as indications of multiple, old fractures that have healed. His parents note that an inherited disorder is present in their family history. A comprehensive physical exam also reveals blue-tinted sclera and yellow-brown, discolored teeth. What is the etiology of the patient’s disorder?
A. Defect in the glycoprotein that forms a sheath around elastin
B. Deficiency of type 3 procollagen
C. Defect in the hydroxylation step of collagen synthesis
D. Deficiency of type 1 collagen (Correct Answer)
E. Deficiency of type 5 collagen
Explanation: ***Deficiency of type 1 collagen***
- This patient presents with a history of **multiple fractures**, **blue sclera**, and **discolored teeth**, which are classic signs of **osteogenesis imperfecta (OI)**.
- OI is most commonly caused by **mutations in COL1A1 or COL1A2 genes**, leading to **quantitative or qualitative defects in type I collagen synthesis**, which is a major structural component of bone, sclera, and dentin.
- This represents the primary molecular defect in the majority of OI cases.
*Defect in the glycoprotein that forms a sheath around elastin*
- This defect is characteristic of **Marfan syndrome**, which typically presents with **tall stature**, **arachnodactyly**, and **cardiovascular abnormalities** (e.g., aortic root dilation).
- The clinical presentation with multiple fractures and blue sclera does not align with Marfan syndrome.
*Deficiency of type 3 procollagen*
- A deficiency in type III procollagen is associated with **Ehlers-Danlos syndrome, vascular type**, which primarily involves **fragile blood vessels**, **rupture of internal organs**, and **easy bruising**.
- This does not explain the patient's severe bone fragility or blue sclera.
*Defect in the hydroxylation step of collagen synthesis*
- Defects in collagen hydroxylation can occur due to **vitamin C deficiency (scurvy)** or mutations in hydroxylase enzymes (e.g., **LEPRE1** in some rare OI types).
- While certain rare forms of OI can involve hydroxylation defects, the **classic presentation** with blue sclera and dentinogenesis imperfecta is most characteristic of **primary structural mutations in type I collagen genes** (COL1A1/COL1A2), making "deficiency of type 1 collagen" the more precise answer for this patient's etiology.
*Deficiency of type 5 collagen*
- Deficiency of type V collagen is associated with the **classical type of Ehlers-Danlos syndrome**, characterized primarily by **skin hyperextensibility** and **joint hypermobility**.
- While it is a collagen disorder, it does not typically present with the severe bone fragility and blue sclera seen in this patient.
Question 45: A 5-year-old boy is brought to the physician because of behavioral problems. His mother says that he has frequent angry outbursts and gets into fights with his classmates. He constantly complains of feeling hungry, even after eating a full meal. He has no siblings, and both of his parents are healthy. He is at the 25th percentile for height and is above the 95th percentile for weight. Physical examination shows central obesity, undescended testes, almond-shaped eyes, and a thin upper lip. Which of the following genetic changes is most likely associated with this patient's condition?
A. Mitotic nondisjunction of chromosome 21
B. Mutation of FBN-1 gene on chromosome 15
C. Microdeletion of long arm of chromosome 7
D. Loss of paternal gene expression on chromosome 15 (Correct Answer)
E. Deletion of Phe508 on chromosome 7
Explanation: ***Loss of paternal gene expression on chromosome 15***
- The patient's symptoms, including **hyperphagia**, **obesity**, behavioral issues, short stature, and **hypogonadism** (undescended testes), are characteristic of **Prader-Willi syndrome**.
- Prader-Willi syndrome is most commonly caused by the **loss of paternal gene expression** from the **q11-q13 region of chromosome 15**, either due to a paternal deletion, maternal uniparental disomy, or a defect in the imprinting center.
*Microdeletion of long arm of chromosome 7*
- A microdeletion on the long arm of chromosome 7 (7q11.23) is associated with **Williams syndrome**, characterized by an **elfin facial appearance**, supravalvular aortic stenosis, and intellectual disability.
- This does not match the patient's symptoms of obesity, hyperphagia, or hypogonadism.
*Deletion of Phe508 on chromosome 7*
- A deletion of phenylalanine at position 508 (**ΔF508**) on chromosome 7 is the most common mutation in the **cystic fibrosis transmembrane conductance regulator (CFTR)** gene, causing **cystic fibrosis**.
- Cystic fibrosis is an **autosomal recessive disorder** requiring mutations in both alleles (inherited from both parents), and primarily affects the exocrine glands, leading to lung disease, pancreatic insufficiency, and infertility, which are unrelated to the patient's presentation.
*Mutation of FBN-1 gene on chromosome 15*
- A mutation in the **FBN1 gene** on chromosome 15 (15q21.1) causes **Marfan syndrome**, which is a connective tissue disorder.
- Marfan syndrome presents with tall stature, long limbs (**arachnodactyly**), lens dislocation, and aortic root dilation, none of which are described in this patient.
*Mitotic nondisjunction of chromosome 21*
- Mitotic nondisjunction of chromosome 21 can lead to **mosaic Down syndrome**, but **trisomy 21** (due to meiotic nondisjunction) is the most common cause of Down syndrome.
- Down syndrome is associated with characteristic facial features, intellectual disability, and congenital heart defects, which are distinct from the symptoms presented.
Question 46: A 29-year-old woman presents to her gynecologist for a routine check-up. She is sexually active with multiple partners and intermittently uses condoms for contraception. She denies vaginal discharge, burning, itching, or rashes in her inguinal region. Pelvic examination is normal. Results from a routine pap smear are shown. The cellular changes seen are attributable to which of the following factors?
A. Activation of p53
B. Activation of K-Ras
C. Inhibition of p16
D. Inhibition of p53 (Correct Answer)
E. Activation of Rb
Explanation: ***Inhibition of p53***
- The sexually active lifestyle and routine Pap smear results suggest **human papillomavirus (HPV) infection**, which commonly causes cellular changes in cervical epithelium.
- High-risk HPV types produce **E6 oncoproteins** that bind to and promote the degradation of **p53 tumor suppressor protein**, leading to uncontrolled cell proliferation and thus the cellular changes seen in a Pap smear.
*Activation p53*
- **p53 activation** typically occurs in response to DNA damage or cellular stress, leading to cell cycle arrest or apoptosis, which is a protective mechanism against cancer.
- This option would result in the elimination of abnormal cells, not the cellular changes indicative of an HPV infection and potential dysplastic changes.
*Activation of K-Ras*
- **K-Ras activation** is a common oncogenic mutation involved in various cancers like pancreatic, colorectal, and lung cancer, but it is not directly linked to the cellular changes typically observed in a Pap smear due to HPV infection.
- K-Ras is a **proto-oncogene** involved in cell growth and differentiation pathways, and its activation primarily drives cell proliferation through alternative mechanisms not central to HPV pathogenesis.
*Inhibition of p16*
- **p16** is a cyclin-dependent kinase inhibitor that is often **overexpressed** in HPV-related cervical lesions, serving as a biomarker for high-risk HPV infection and dysplasia.
- **Inhibition of p16** would counteract its tumor-suppressing effects, but the hallmark of high-risk HPV is the degradation of p53 and retinoblastoma protein (Rb) by viral oncoproteins, leading to p16 overexpression as a compensatory response.
*Activation of Rb*
- **Activation of Rb** (retinoblastoma protein) would halt the cell cycle by preventing transcription factors from promoting cell division, acting as a **tumor suppressor**.
- High-risk HPV oncoprotein **E7** actually binds to and inactivates **Rb**, thereby promoting uncontrolled cell division and contributing to the development of cervical dysplasia and cancer, making activation an incorrect mechanism.
Question 47: A researcher is investigating compounds that modulate the cell cycle as possible chemotherapeutic agents against peripheral T-cell lymphoma. The researcher discovers a group of natural compounds with inhibitory activity against histone deacetylases, a class of enzymes that remove acetyl groups from the lysine residues of histones. A histone deacetylase inhibitor most likely causes which of the following?
A. Prevention of DNA strand reannealing
B. Increased heterochromatin formation
C. Suppression of gene transcription
D. Relaxation of DNA coiling (Correct Answer)
E. Tighter coiling of DNA
Explanation: ***Relaxation of DNA coiling***
- Histone deacetylase (HDAC) inhibitors block the removal of **acetyl groups** from **histones**, leading to increased histone acetylation.
- Increased acetylation **reduces the positive charge** of histones, loosening their grip on the negatively charged DNA and causing **relaxation of DNA coiling**.
*Prevention of DNA strand reannealing*
- This process is primarily influenced by factors affecting **hydrogen bonding** between DNA strands, such as **temperature** or **DNA denaturing agents**, not directly by histone acetylation.
- DNA reannealing is the reformation of a **double helix** from single strands, a different mechanism than chromatin structure.
*Increased heterochromatin formation*
- **Heterochromatin** is characterized by **tightly coiled DNA** and is associated with **deacetylated histones** and gene silencing.
- Increased acetylation, as caused by HDAC inhibitors, would lead to less heterochromatin and more **euchromatin**.
*Suppression of gene transcription*
- **Relaxation of DNA coiling** makes the DNA more accessible to transcription factors and RNA polymerase, thereby generally **promoting gene transcription**, not suppressing it.
- **HDAC inhibitors** primarily promote gene expression by increasing the accessibility of DNA to the transcriptional machinery.
*Tighter coiling of DNA*
- **Deacetylation of histones** leads to stronger interaction between histones and DNA, resulting in **tighter coiling** and chromatin condensation.
- HDAC inhibitors, by preventing deacetylation, promote the opposite effect: **DNA uncoiling** and relaxation.
Question 48: A genetic counselor sees a family for the first time for genetic assessment. The 24-year-old businessman and his 19-year-old sister are concerned about having a mutant allele and have decided to get tested. Their grandfather and great aunt both have Huntington’s disease which became apparent when they turned 52. Their father who is 47 years old appears healthy. The geneticist discusses both the benefits and risks of getting tested and orders some tests. Which of the following tests would best provide evidence for whether the siblings are carriers or not?
A. Gel electrophoresis
B. Restriction enzyme digestion products
C. Polymerase chain reaction (Correct Answer)
D. Pyrosequencing
E. DNA isolation and purification
Explanation: ***Polymerase chain reaction***
- **PCR is the essential first step** for detecting Huntington's disease carrier status by **amplifying the CAG trinucleotide repeat region** in the *HTT* gene.
- After PCR amplification, **fragment analysis** (capillary electrophoresis or gel electrophoresis) is used to determine the exact number of CAG repeats, which distinguishes carriers (≥36 repeats) from non-carriers (<27 repeats).
- Among the options listed, **PCR is the critical enabling technology** without which carrier testing cannot proceed.
- The complete diagnostic test is called "CAG repeat analysis" or "trinucleotide repeat analysis," which uses PCR as its foundation.
*Gel electrophoresis*
- **Gel electrophoresis** can be used to visualize the size of PCR-amplified DNA fragments and may help distinguish expanded repeats from normal-sized alleles.
- However, modern laboratories typically use **capillary electrophoresis** (automated fragment analysis) for more precise repeat counting.
- Gel electrophoresis alone, without prior PCR amplification, cannot detect the CAG repeat expansion.
*Restriction enzyme digestion products*
- Huntington's disease is caused by a **CAG trinucleotide repeat expansion**, not a point mutation that creates or abolishes restriction enzyme sites.
- **Restriction fragment length polymorphism (RFLP)** analysis is not the primary method for detecting dynamic repeat expansions.
- This approach was historically used for linkage analysis before direct repeat testing became available.
*Pyrosequencing*
- **Pyrosequencing** is designed for **sequencing short DNA stretches** and detecting single nucleotide polymorphisms (SNPs).
- It is **not suitable for quantifying long trinucleotide repeat expansions** like those in Huntington's disease, where repeat numbers can range from 10 to over 100.
- **Fragment analysis** is the standard method for measuring repeat length, not sequencing.
*DNA isolation and purification*
- **DNA isolation** is a necessary preliminary step for any genetic testing but provides no diagnostic information by itself.
- It simply extracts genomic DNA from blood or tissue samples, which must then be analyzed using specific molecular techniques like PCR and fragment analysis.
Question 49: A mother brings her 3-year-old daughter to the pediatrician because she is concerned about her development. She states that her daughter seemed to regress in her motor development. Furthermore, she states she has been having brief episodes of uncontrollable shaking, which has been very distressing to the family. During the subsequent work-up, a muscle biopsy is obtained which demonstrates red ragged fibers and a presumptive diagnosis of a genetic disease is made. The mother states that she has another 6-year-old son who does not seem to be affected or have any similar symptoms. What genetic term explains this phenomenon?
A. Allelic heterogeneity
B. Heteroplasmy (Correct Answer)
C. Locus heterogeneity
D. Phenotypic heterogeneity
E. Genetic heterogeneity
Explanation: ***Heteroplasmy***
- **Heteroplasmy** refers to the presence of more than one type of mitochondrial DNA (mtDNA) within the same cell or individual. The varying proportions of mutated mtDNA can lead to a wide spectrum of disease severity, explaining why the daughter is severely affected while her brother is unaffected.
- The daughter's symptoms (developmental regression, seizures), combined with the muscle biopsy showing **red ragged fibers**, are characteristic of **mitochondrial disorders**, which are often maternally inherited and can manifest with variable expressivity due to heteroplasmy.
*Allelic heterogeneity*
- **Allelic heterogeneity** occurs when different mutations at the *same gene locus* can cause the same disease.
- This term does not explain the differential severity or presence of disease in siblings with a maternally inherited mitochondrial disorder, as it focuses on different mutations within a single gene rather than varying proportions of mutated mitochondria.
*Locus heterogeneity*
- **Locus heterogeneity** describes situations where mutations in *different genes* can cause the same disease phenotype.
- This concept is not applicable here because the clinical picture and red ragged fibers point towards a specific mitochondrial disorder, and the siblings would typically be expected to share the same genetic locus underlying their condition if it were present.
*Phenotypic heterogeneity*
- **Phenotypic heterogeneity** refers to different clinical manifestations or phenotypes resulting from mutations in the *same gene*.
- While there is varying severity of the disease (different phenotypes) between the siblings, this term doesn't specifically explain the underlying *genetic mechanism* of varying mitochondrial mutation loads, which is crucial for mitochondrial disorders.
*Genetic heterogeneity*
- **Genetic heterogeneity** is a broad term encompassing both allelic and locus heterogeneity, meaning that a single phenotype can be caused by mutations at different loci (locus heterogeneity) or by different mutations within the same gene (allelic heterogeneity).
- While mitochondrial disorders can exhibit genetic heterogeneity, this general term doesn't specifically address the mechanism of variable penetrance and expressivity seen in mitochondrial inheritance due to the varying proportions of mutated mtDNA within cells, which is uniquely explained by heteroplasmy.
Question 50: 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%
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.
