Genetic Factors in Developmental Anatomy — MCQs
All of the following are true about Down syndrome except for one.
Gene not involved in SCID:
Prader-Willi syndrome and Angelman syndrome are examples of what genetic phenomenon?
Differential expression of the same gene depending on parent of origin is referred to as
What is the pattern of inheritance in neural tube defects?
In Marfan syndrome, the defect is in the gene encoding which protein?
Syndrome which is characterized by 2X chromosomes and 1Y chromosome is:
An intrauterine scan at the 13th week of pregnancy showed a fetus with multiple long bone fractures. What is commonly associated with this finding?
The labia majora develop from which embryological structure?
All of the following are neural tube defects except:
Genetic Factors in Developmental Anatomy Indian Medical PG Practice Questions and MCQs
Question 1: All of the following are true about Down syndrome except for one.
- A. Incidence of Robertsonian translocation is 1:1000 (Correct Answer)
- B. Most common cause is trisomy 21
- C. Mosaicism 21 has no association with maternal age
- D. Extra chromosome is of maternal origin
- E. Incidence increases with advanced maternal age
Explanation: **Incidence of Robertsonian translocation is 1:1000** - This statement is **not accurate** for Down syndrome. Robertsonian translocation accounts for only about **3-4% of Down syndrome cases**, not a general population incidence of 1:1000. - The vast majority of Down syndrome cases (~95%) are due to trisomy 21 from nondisjunction, not translocation. - This is the **correct answer** as it is the FALSE statement. *Extra chromosome is of maternal origin* - In approximately **90-95% of Down syndrome cases**, the extra copy of chromosome 21 originates from the mother due to **nondisjunction** during meiosis. - This maternal origin is strongly correlated with **advanced maternal age**. *Most common cause is trisomy 21* - **Trisomy 21** (due to meiotic nondisjunction) accounts for about **95% of all Down syndrome cases**, making it the most common genetic mechanism. - This results in three separate copies of chromosome 21 in all body cells. *Mosaicism 21 has no association with maternal age* - **Mosaic Down syndrome** occurs when nondisjunction happens *after fertilization* in early embryonic development, leading to a mixture of cells with normal and trisomic cells. - Because it is a **post-zygotic event**, its incidence is independent of **maternal age**, unlike full trisomy 21. *Incidence increases with advanced maternal age* - **TRUE statement** - The risk of Down syndrome (particularly trisomy 21) increases significantly with **maternal age**. - Risk is approximately 1:1500 at age 20, 1:1000 at age 30, 1:400 at age 35, and 1:100 at age 40. - This is due to increased risk of meiotic nondisjunction in older oocytes.
Question 2: Gene not involved in SCID:
- A. BTK (Correct Answer)
- B. ZAP70
- C. IL2RG
- D. JAK3
Explanation: ***BTK*** - **Bruton's tyrosine kinase (BTK)** is associated with **X-linked agammaglobulinemia (XLA)**, a primary immunodeficiency characterized by the absence of mature B cells and significantly reduced antibody production. While it causes severe immune deficiency, it is not a direct cause of **SCID**. - XLA results in recurrent bacterial infections due to an inability to produce antibodies, rather than the severe combined T and B cell dysfunction seen in SCID. *ZAP70* - **ZAP70** deficiency is a cause of **SCID**. It leads to impaired T-cell receptor signaling, resulting in profound functional T-cell lymphopenia. - Patients with ZAP70 deficiency have normal numbers of CD4 T cells but very low or absent CD8 T cells, and their T cells are functionally impaired, leading to severe immunodeficiency. *IL2RG* - The **IL2RG** gene encodes the common gamma chain (γc), a crucial component of several **interleukin receptors (IL-2, IL-4, IL-7, IL-9, IL-15, IL-21)**. [1] - Mutations in IL2RG cause **X-linked SCID (X-SCID)**, the most common form of SCID, leading to a block in T-cell and NK-cell development due to defective cytokine signaling. [1] *JAK3* - **Janus kinase 3 (JAK3)** is a tyrosine kinase that associates with the **common gamma chain (γc)** and is essential for cytokine signaling downstream of the γc-containing receptors. [1] - **JAK3 deficiency** results in an **autosomal recessive form of SCID**, clinically indistinguishable from X-SCID, with impaired T-cell and NK-cell development due to defective cytokine signaling. [1] **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 247-248.
Question 3: Prader-Willi syndrome and Angelman syndrome are examples of what genetic phenomenon?
- A. Gene Knockout
- B. Impaired DNA repair
- C. Genomic Imprinting (Correct Answer)
- D. RNA interference
Explanation: ***Genomic Imprinting*** - **Genomic imprinting** is an epigenetic phenomenon where certain genes are expressed in a **parent-of-origin-specific manner**. - In Prader-Willi syndrome, the disease results from the loss of function of specific genes on chromosome 15 (15q11-q13) inherited from the father, while Angelman syndrome results from the loss of function of a different gene (UBE3A) in the same region, but inherited from the mother. *RNA interference* - **RNA interference** is a biological process in which RNA molecules inhibit gene expression or translation, by neutralizing targeted mRNA molecules. - This process is not directly responsible for the parent-of-origin-specific expression patterns observed in these syndromes. *Gene Knockout* - A **gene knockout** is a genetic technique in which an organism's genes are made inoperative. - While it involves modifying gene function, it does not explain the differential expression based on parental origin. *Impaired DNA repair* - **Impaired DNA repair** refers to defects in the mechanisms that correct DNA damage. - This can lead to increased mutations and conditions like cancer, but it is not the underlying mechanism for Prader-Willi or Angelman syndromes.
Question 4: Differential expression of the same gene depending on parent of origin is referred to as
- A. Mosaicism
- B. Nonpenetrance
- C. Anticipation
- D. Genomic imprinting (Correct Answer)
Explanation: ***Genomic imprinting*** - This phenomenon describes the differential expression of a gene based on its **parent of origin**, meaning that the gene is expressed only from the allele inherited from a specific parent (either maternal or paternal). - This differential expression occurs without altering the underlying DNA sequence and is often mediated by **epigenetic mechanisms** such as DNA methylation. *Mosaicism* - This refers to the presence of **two or more populations of cells** with different genotypes within a single individual who has developed from a single fertilized egg. - It does not involve differential expression of the same gene based on parental origin, but rather **genetic differences arising after fertilization**. *Nonpenetrance* - **Nonpenetrance** describes a situation where an individual carries a disease-causing gene mutation but **does not express the associated phenotype** or clinical symptoms. - This concept explains variability in disease manifestation, not differential gene expression based on parental origin. *Anticipation* - **Anticipation** is a phenomenon in genetic disorders where the symptoms become **more severe** and/or appear at an **earlier age** in successive generations. - This is often seen in disorders caused by expansion of trinucleotide repeats, such as Huntington's disease, and is not related to parent-of-origin gene expression.
Question 5: What is the pattern of inheritance in neural tube defects?
- A. Multifactorial inheritance (Correct Answer)
- B. Autosomal recessive
- C. X-linked dominant
- D. Autosomal dominant
- E. X-linked recessive
Explanation: ***Multifactorial inheritance*** - Neural tube defects (NTDs) are a classic example of **multifactorial inheritance**, meaning they result from a combination of **genetic predispositions** and **environmental factors**. - Risk is influenced by multiple genes, and environmental factors like **folate deficiency** play a significant role. *Autosomal recessive* - This pattern involves two copies of an altered gene to cause disease, typically resulting in a **25% recurrence risk** for siblings. - While some rare isolated NTDs might have an autosomal recessive component, the general presentation of NTDs does not fit this classic mendelian pattern. *X-linked dominant* - Involves genes on the **X chromosome** where one altered copy is sufficient to cause disease; affected fathers pass it to all daughters, but no sons. - This inheritance pattern is very rare for NTDs and would present with a distinct sex-linked pattern of affected individuals. *Autosomal dominant* - Requires only one copy of an altered gene to cause disease, leading to a **50% recurrence risk** for offspring. - While some syndromes associated with NTDs can be autosomal dominant, the primary mechanism for isolated NTDs is not solely due to a single dominant gene. *X-linked recessive* - Involves genes on the **X chromosome** where two altered copies are needed in females, but only one in males; typically affects males predominantly. - This inheritance pattern does not account for the observed familial clustering and environmental contribution seen in NTDs.
Question 6: In Marfan syndrome, the defect is in the gene encoding which protein?
- A. Fibrillin I (Correct Answer)
- B. Fibrillin II
- C. Elastin
- D. Collagen
Explanation: ***Fibrillin I*** - Marfan syndrome arises from a defect in **fibrillin I**, which is essential for the proper formation of elastic fibers in connective tissue [1]. - This defect leads to symptoms affecting the **musculoskeletal**, **cardiovascular**, and **ocular systems** due to weakened connective tissue structure [1]. *Collagen* - While collagen is an important structural protein, it is not the primary defect in Marfan syndrome. - Disorders affecting collagen include **Ehlers-Danlos syndrome**, which presents with different clinical features such as hypermobility. *Fibrillin II* - Fibrillin II is related to different connective tissue conditions but is not involved in Marfan syndrome. - Its mutations are associated with **other disorders**, not the hallmark manifestations seen in Marfan. *Elastin* - Elastin provides elasticity in vascular and other tissues; however, it is not directly related to Marfan syndrome. - Conditions such as **Williams syndrome** involve elastin, but they present distinct clinical features from Marfan syndrome. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. With Illustrations By, pp. 35-36.
Question 7: Syndrome which is characterized by 2X chromosomes and 1Y chromosome is:
- A. Turner syndrome
- B. Marfan syndrome
- C. Down syndrome
- D. Klinefelter syndrome (Correct Answer)
Explanation: ***Klinefelter syndrome*** - This syndrome is characterized by a **47, XXY karyotype**, meaning individuals have **two X chromosomes** and one Y chromosome [1]. - It affects males, leading to features such as **small testes**, **infertility**, gynecomastia, and often taller stature [1], [3]. *Turner syndrome* - This syndrome is characterized by a **45, X karyotype**, meaning individuals have only **one X chromosome** and no second sex chromosome [2]. - It affects females, leading to features like **short stature**, a **webbed neck**, and ovarian dysgenesis [2]. *Marfan syndrome* - This is an **autosomal dominant genetic disorder** affecting connective tissue, caused by mutations in the **FBN1 gene**. - It is characterized by **tall stature**, long limbs (arachnodactyly), **cardiovascular abnormalities** (e.g., aortic dissection), and ocular problems (e.g., lens dislocation). *Down syndrome* - This syndrome is caused by **trisomy 21**, meaning individuals have an **extra copy of chromosome 21**. - It is characterized by specific **facial features**, intellectual disability, and an increased risk of certain medical conditions like congenital heart defects.
Question 8: An intrauterine scan at the 13th week of pregnancy showed a fetus with multiple long bone fractures. What is commonly associated with this finding?
- A. Osteogenesis imperfecta (Correct Answer)
- B. Marfan syndrome
- C. Achondroplasia
- D. Cretinism
Explanation: ***Osteogenesis imperfecta*** - **Multiple long bone fractures** detected early in pregnancy are a classic presentation of **osteogenesis imperfecta (OI)**, a genetic disorder characterized by **bone fragility**. - OI is primarily caused by mutations in genes encoding **type I collagen**, leading to defective bone formation. *Achondroplasia* - This condition is a form of **dwarfism** characterized by disproportionately short limbs and a normal-sized trunk, resulting from a mutation in the **FGFR3 gene**. - While it affects bone growth, it typically does not cause **multiple fractures** prenatally. *Marfan syndrome* - This is a connective tissue disorder affecting the skeletal, ocular, and cardiovascular systems, characterized by **tall stature**, **long limbs and fingers**, and **aortic root dilation**. - It results from a mutation in the **fibrillin-1 gene** and is not primarily associated with prenatal long bone fractures. *Cretinism* - This is a historical term for **congenital hypothyroidism**, which results from severely deficient thyroid hormone production in a newborn. - It leads to developmental delays, growth retardation, and intellectual disability, but not to **multiple bone fractures**.
Question 9: The labia majora develop from which embryological structure?
- A. Urogenital folds
- B. Labioscrotal swellings (Correct Answer)
- C. Müllerian ducts
- D. Genital tubercle
Explanation: ***Labioscrotal swellings*** - The **labia majora** develop from the **labioscrotal swellings**, which are paired bilateral structures that appear around week 9-10 of development [1]. - These swellings arise lateral to the urogenital folds and do not fuse in females, forming the labia majora. - In males, these same structures fuse in the midline to form the scrotum. - This is a key example of **sexual differentiation** in embryological development [1]. *Urogenital folds* - The urogenital folds form the **labia minora** in females, not the labia majora. - In males, these folds fuse to form the ventral aspect of the penis and enclose the penile urethra. *Genital tubercle* - The genital tubercle forms the **clitoris** in females and the **glans penis** in males. - It does not contribute to the formation of the labia majora. *Müllerian ducts* - The Müllerian (paramesonephric) ducts form the **upper vagina, uterus, and fallopian tubes** in females. - They are internal structures and do not contribute to external genitalia like the labia majora.
Question 10: All of the following are neural tube defects except:
- A. Holoprosencephaly (Correct Answer)
- B. Encephalocele
- C. Myelomeningocele
- D. Anencephaly
Explanation: ***Holoprosencephaly*** - This condition results from the **failure of the prosencephalon (forebrain)** to properly divide into two hemispheres, often leading to facial abnormalities. - It is considered a **forebrain malformation**, distinct from neural tube closure defects. *Encephalocele* - An encephalocele is a **neural tube defect** characterized by the protrusion of brain tissue and meninges through an opening in the skull. - It results from the **incomplete closure of the neural tube** during embryonic development. *Myelomeningocele* - Myelomeningocele is a severe form of **spina bifida**, a neural tube defect where the spinal cord and meninges protrude through an opening in the spine [2]. - This defect arises from the **failure of the neural tube to close completely** in the caudal region [1]. *Anencephaly* - Anencephaly is a lethal **neural tube defect** characterized by the absence of a major portion of the brain, skull, and scalp [3]. - It occurs due to the **failure of the cranial end of the neural tube to close** [3].
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