Molecular Genetics Practice Questions

Q: An adult tall male presents with a long arm span, pectus excavatum, and cardiac abnormalities. What is the most likely defective protein?

Fibrillin. ***Fibrillin*** - The constellation of **tall stature**, **long arm span**, **pectus excavatum**, and **cardiac abnormalities** (e.g., aortic dissection or mitral valve prolapse) is classic for **Marfan syndrome**. - Marfan syndrome is caused by a defect in the *FBN1* gene, which codes for **fibrillin-1**, a glycoprotein essential for the formation of elastic fibers in connective tissue. *Elastin* - Defects in **elastin** are associated with conditions like **supravalvular aortic stenosis** (due to **Williams syndrome**) or cutis laxa, which have different phenotypic presentations. - While both elastin and fibrillin are components of elastic fibers, the specific features of Marfan syndrome point to fibrillin as the primary defect. *Collagen* - Defects in **collagen** (especially type I, III, or V) are associated with conditions such as **osteogenesis imperfecta** (brittle bones) or **Ehlers-Danlos syndrome** (hypermobility, skin hyperextensibility). - These conditions typically present with different clinical manifestations, lacking the specific combination of features seen in this patient. *Myosin* - **Myosin** is a fibrous protein primarily involved in **muscle contraction** and is not directly implicated in widespread connective tissue disorders affecting skeletal and cardiovascular systems in this manner. - Defects in myosin are more commonly associated with various forms of **cardiomyopathy** or skeletal muscle myopathies, not Marfan-like features. *Laminin* - **Laminin** is a major component of the **basement membrane** and plays a role in cell adhesion, migration, and tissue architecture. - Laminin defects are associated with certain forms of **muscular dystrophy** (e.g., congenital muscular dystrophy) and **epidermolysis bullosa**, not the specific skeletal and cardiovascular features of Marfan syndrome.

Q: A 4-year-old male presents with a 1-year history of swaying while walking and recent episodes of tripping when ambulating. He has trouble trying to sit and get up from chairs, as well as walking up the stairs to his bedroom. On physical exam, the pediatrician notices nystagmus, absent deep tendon reflexes, significant loss of vibratory and proprioceptive sensation in his extremities, pes cavus, and slight kyphoscoliosis. A blood sample is sent for DNA sequencing and the results show a significant expansion of the trinucleotide GAA on chromosome 9. Which of the following diseases displays a similar mode of inheritance as the disease affecting this patient?

Von Gierke's disease. ***Von Gierke's disease*** - The patient's symptoms (ataxia, nystagmus, absent deep tendon reflexes, sensory loss, pes cavus, kyphoscoliosis) and genetic finding (GAA trinucleotide expansion on chromosome 9) are characteristic of **Friedreich's ataxia**. - Friedreich's ataxia is inherited in an **autosomal recessive** pattern, similar to Von Gierke's disease. *Osteogenesis imperfecta* - This condition is primarily inherited in an **autosomal dominant** pattern, although some severe forms can be recessive. - It is caused by defects in **collagen genes** and is characterized by brittle bones, blue sclera, and hearing loss, which are not present here. *Fragile X syndrome* - This is an **X-linked dominant** disorder characterized by a trinucleotide (CGG) repeat expansion on the X chromosome. - It presents with intellectual disability, distinctive facial features, and macroorchidism, which differ from the patient's presentation. *Leber hereditary optic neuropathy* - This condition is inherited via **mitochondrial inheritance**, meaning it is passed down exclusively from the mother. - It primarily causes bilateral vision loss due to optic nerve degeneration, which is not the patient's main complaint. *Menkes disease* - This is an **X-linked recessive** disorder, caused by a mutation in the ATP7A gene leading to copper deficiency. - It is characterized by sparse, kinky hair, developmental delay, and neurological degeneration, a different clinical picture than described.

Q: A 1-year-old child who was born outside of the United States is brought to a pediatrician for the first time because she is not gaining weight. Upon questioning, the pediatrician learns that the child has had frequent pulmonary infections since birth, and on exam the pediatrician appreciates several nasal polyps. Genetic testing is subsequently ordered to confirm the suspected diagnosis. Testing is most likely to show deletion of which of the following amino acids from the protein involved in this child's condition?

Phenylalanine. ***Phenylalanine*** - The patient's presentation with **failure to thrive**, **recurrent pulmonary infections**, and **nasal polyps** is highly suggestive of **cystic fibrosis (CF)**. - The most common mutation leading to CF is the **delta F508 mutation**, which involves a **deletion of a phenylalanine** residue at position 508 in the **CFTR protein**. *Leucine* - While other rare mutations in the **CFTR gene** can involve various amino acids, **leucine deletion** is not the most common or characteristic mutation associated with the clinical picture of CF. - This option does not represent the primary genetic defect seen in the majority of CF cases. *Histidine* - **Histidine deletion** is not a commonly described or significant mutation in the **CFTR gene** that would explain the overwhelming majority of cystic fibrosis cases. - The typical genetic defect in CF involves the malformation of the CFTR protein due to phenylalanine deletion. *Lysine* - **Lysine deletion** is not the most prevalent type of mutation found in patients with **cystic fibrosis** nor is it associated with the characteristic presentation mentioned. - The CFTR protein dysfunction primarily stems from the common phenylalanine deletion, affecting protein folding and function. *Valine* - Deletion of **valine** is not the most frequently observed mutation in the **CFTR gene** that causes cystic fibrosis. - The classic genetic basis for CF symptoms, particularly severe ones, relates to the absence of phenylalanine.

Question 1

An adult tall male presents with a long arm span, pectus excavatum, and cardiac abnormalities. What is the most likely defective protein?

Accepted Answer

Fibrillin

Answer

Elastin

Answer

Collagen

Answer

Myosin

Answer

Laminin

Question 2

Which disease will show the mode of inheritance depicted in this pedigree?

Accepted Answer

Achondroplasia

Answer

Prader-Willi syndrome

Answer

Wilson disease

Answer

Wiskott-Aldrich syndrome

Answer

Hemophilia A

Question 3

A 12-year-old girl is brought to an oncologist, as she was recently diagnosed with a rare form of cancer. Cytogenetic studies reveal that the tumor is responsive to vinblastine, which is a cell-cycle specific anticancer agent. It acts on the M phase of the cell cycle and inhibits the growth of cells. Which of the following statements best describes the regulation of the cell cycle?

Accepted Answer

Cyclin-dependent activation of CDK1 (CDC2) takes place upon the entry of a cell into M phase of the cell cycle.

Answer

Inhibitors of DNA synthesis act in the M phase of the cell cycle.

Answer

The G0 phase is the checkpoint before G1.

Answer

EGF from a blood clot stimulates the growth and proliferation of cells in the healing process.

Answer

Replication of the genome occurs in the M phase of the cell cycle.

Question 4

A 4-year-old male presents with a 1-year history of swaying while walking and recent episodes of tripping when ambulating. He has trouble trying to sit and get up from chairs, as well as walking up the stairs to his bedroom. On physical exam, the pediatrician notices nystagmus, absent deep tendon reflexes, significant loss of vibratory and proprioceptive sensation in his extremities, pes cavus, and slight kyphoscoliosis. A blood sample is sent for DNA sequencing and the results show a significant expansion of the trinucleotide GAA on chromosome 9. Which of the following diseases displays a similar mode of inheritance as the disease affecting this patient?

Accepted Answer

Von Gierke's disease

Answer

Osteogenesis imperfecta

Answer

Fragile X syndrome

Answer

Leber hereditary optic neuropathy

Answer

Menkes disease

Question 5

A 1-year-old child who was born outside of the United States is brought to a pediatrician for the first time because she is not gaining weight. Upon questioning, the pediatrician learns that the child has had frequent pulmonary infections since birth, and on exam the pediatrician appreciates several nasal polyps. Genetic testing is subsequently ordered to confirm the suspected diagnosis. Testing is most likely to show deletion of which of the following amino acids from the protein involved in this child's condition?

Accepted Answer

Phenylalanine

Answer

Leucine

Answer

Histidine

Answer

Lysine

Answer

Valine

Molecular Genetics — MCQs

Molecular Genetics — MCQs

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