Genetic Disorders and Biochemical Pathology Practice Questions

Q: What is the underlying defect in Ataxia telangiectasia?

Double-strand DNA break repair. ***ds DNA break repair*** - Ataxia telangiectasia is primarily caused by a defect in the **ATM gene**, which plays a critical role in **double-strand DNA break repair** [1]. - Patients often present with **ataxia**, **telangiectasia**, and an increased risk of **cancers** due to impaired DNA repair mechanisms [1]. *Nucleotide Excision repair* - This pathway is responsible for repairing **DNA damage** caused by UV light and chemical exposure, which is not the defect in ataxia telangiectasia. - Disorders like **xeroderma pigmentosum** stem from defects in this pathway [1], not ataxia telangiectasia. *Mismatch repair* - Mismatch repair is important for correcting errors that occur during **DNA replication**, leading to conditions like **Lynch syndrome** when defective. - Ataxia telangiectasia is not associated with errors in this process, thus making it an incorrect option. *Base Excision Repair* - This repair mechanism handles small-scale DNA damage, especially from oxidative stress, which does not relate to ataxia telangiectasia pathology. - Conditions like **familial adenomatous polyposis** are associated with base excision repair defects, highlighting the difference. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Neoplasia, pp. 322-323.

Q: Acute intermittent porphyria is associated with which type of inheritance?

Autosomal dominant. ***Autosomal dominant*** - **Acute intermittent porphyria (AIP)** is inherited in an **autosomal dominant** pattern, meaning only one copy of the mutated gene (HMBS gene) is sufficient to cause the disorder. - This inheritance pattern leads to a 50% chance of passing the condition to each offspring from an affected parent. - **Important clinical feature:** AIP demonstrates **incomplete penetrance** (10-20%), meaning many individuals with the mutation never develop clinical symptoms, which is why family history may appear negative despite autosomal dominant inheritance. *Autosomal recessive* - **Autosomal recessive** disorders require two copies of the mutated gene (one from each parent) for the disease to manifest. - This pattern is characteristic of conditions like cystic fibrosis or sickle cell anemia, not acute intermittent porphyria. *X-linked dominant* - **X-linked dominant** inheritance involves a gene located on the X chromosome, where a single copy of the mutated gene is sufficient for disease expression in both males and females. - This pattern is typically seen in conditions such as fragile X syndrome or Rett syndrome, with distinct inheritance patterns differing from AIP. *X-linked recessive* - **X-linked recessive** disorders, such as hemophilia or Duchenne muscular dystrophy, primarily affect males as they only have one X chromosome. - Females are typically carriers but can be affected if they inherit two mutated X chromosomes, a pattern not seen in acute intermittent porphyria.

Q: Inheritance associated with fragile X syndrome is-

X-linked dominant. ***X-linked dominant*** - Fragile X syndrome is caused by a **trinucleotide repeat expansion** (CGG) in the **FMR1 gene** located on the X chromosome. - The inheritance pattern is **X-linked dominant with reduced penetrance** and variable expressivity. - Males are typically more severely affected due to having only one X chromosome (hemizygous state). - **Importantly, carrier females are often affected** - approximately 50% show mild to moderate intellectual disability, anxiety, or social difficulties, which distinguishes it from X-linked recessive inheritance. - The condition shows **genetic anticipation** - repeat expansions increase in successive generations, particularly through maternal transmission. *X-linked recessive inheritance* - In true X-linked recessive disorders (like hemophilia A), carrier females are typically asymptomatic. - Fragile X syndrome does not fit this pattern because **carrier females frequently manifest symptoms**, making it dominant rather than recessive. - If it were recessive, heterozygous females would be unaffected carriers. *Autosomal dominant inheritance* - This pattern involves genes on autosomes (non-sex chromosomes) affecting males and females equally. - Fragile X syndrome is **X-linked**, not autosomal - the FMR1 gene is located on the X chromosome. - The sex-linked nature explains the higher prevalence and severity in males. *Autosomal recessive inheritance* - This pattern requires two copies of a mutated gene on autosomes, with both parents typically being carriers. - Fragile X syndrome is **X-linked**, not autosomal, and shows sex-specific inheritance patterns. - The pattern of inheritance through the X chromosome rules out any autosomal pattern.

Q: Trinucleotide sequence associated with spinocerebellar ataxia is?

CAG. ***CAG*** - The **CAG trinucleotide repeat** is associated with **most forms of spinocerebellar ataxia (SCA)**, including SCA1, SCA2, SCA3 (Machado-Joseph disease), SCA6, SCA7, and SCA17. - An expansion of this repeat leads to an elongated **polyglutamine tract** in the affected protein, causing protein misfolding and neuronal dysfunction. - CAG is the **most common** trinucleotide repeat expansion associated with SCA. *CUG* - **CUG repeats** are primarily associated with **myotonic dystrophy type 1**. - While CUG expansions are found in **SCA8**, CAG repeats are far more commonly associated with the majority of spinocerebellar ataxias. - The expansion of CUG repeats in the **3' untranslated region** causes RNA-mediated toxicity. *GGC* - **GGC repeats** are not a common trinucleotide expansion associated with classic spinocerebellar ataxia. - While other repeat expansions exist in various genetic disorders, GGC is not the primary repeat linked to SCA. *CGG* - **CGG trinucleotide repeat** expansions are characteristic of **Fragile X syndrome**, a common cause of inherited intellectual disability. - This repeat expansion occurs in the FMR1 gene and is not the primary repeat associated with spinocerebellar ataxia.

Q: Which chromosome contains the gene responsible for MEN2 (Multiple Endocrine Neoplasia type 2)?

10q11.2. ***10q11.2*** - The **MEN2 gene** is located on **chromosome 10** at band **q11.2**. - Mutations in this gene, specifically the **RET proto-oncogene**, are responsible for Multiple Endocrine Neoplasia type 2. *11q13* - This chromosomal location is associated with the **MEN1 gene**, which causes Multiple Endocrine Neoplasia type 1. - MEN1 involves tumors of the **parathyroid**, **pancreatic islet cells**, and **anterior pituitary**. *13q11* - This location is not typically associated with the **MEN genes** or other well-known multiple endocrine neoplasia syndromes. - It is more commonly linked to other genetic disorders or specific gene loci unrelated to **endocrine tumor syndromes**. *11q10.2* - This is an incorrect or imprecisely described chromosomal location in the context of Multiple Endocrine Neoplasia syndromes. - While **chromosome 11** is relevant for **MEN1**, the specific band **11q10.2** is not the correct location for either MEN1 or MEN2 genes.

Q: What is the most definitive method for confirming 46,XY disorders of sexual development?

Genetic testing. ***Genetic testing*** - Genetic testing, such as **karyotyping** and **DNA sequencing**, can identify abnormalities in genes responsible for male sexual development, confirming 46,XY DSD. - This method directly pinpoints the underlying genetic cause, providing a **definitive diagnosis** for specific DSDs like **androgen insensitivity syndrome** or **5-alpha-reductase deficiency**. - Molecular analysis can detect mutations in genes like **SRY, AR, SRD5A2**, and other genes involved in sex determination and differentiation. *USG abdomen* - **Ultrasonography** of the abdomen can visualize internal reproductive organs and identify anatomical abnormalities, but it does not reveal the underlying genetic cause. - While useful for assessing morphology, it is an **imaging modality** and not a definitive diagnostic tool for genetic conditions. *Hormonal study* - **Hormonal studies** measure levels of androgens, estrogens, and other related hormones, which can indicate impaired hormone production or action. - While essential for understanding the **physiological impact** of DSDs, hormonal studies provide indirect evidence and do not definitively identify the genetic origin. *Gonadal biopsy* - **Histopathological examination** of gonadal tissue can reveal tissue architecture and cell types, which may support the diagnosis. - However, it is an **invasive procedure** and does not identify the specific genetic mutation responsible for the DSD, making it less definitive than genetic testing.

Q: Gene responsible for Wilson disease is situated on which chromosome?

Chromosome 13. ***Chromosome 13*** - The **ATP7B gene**, responsible for Wilson disease, is located on **chromosome 13** (specifically 13q14.3). - Mutations in this gene lead to impaired copper transport and accumulation in various organs, particularly the liver and brain. *Chromosome 11* - This chromosome is associated with various genetic disorders, such as **sickle cell disease** (HBB gene) and **Beckwith-Wiedemann syndrome**. - Wilson disease is not associated with genes on this chromosome. *Chromosome 12* - **Phenylketonuria (PKU)** (PAH gene) and some forms of **Noonan syndrome** are linked to genes on chromosome 12. - Wilson disease is not associated with genes on this chromosome. *Chromosome 14* - This chromosome contains genes associated with **Alpha-1 antitrypsin deficiency** and some forms of **Alzheimer's disease**. - It does not contain the ATP7B gene responsible for Wilson disease.

Q: Pendred syndrome is caused by a mutation in which gene?

SLC26A4 gene. ***SLC26A4 gene*** - Pendred syndrome is an autosomal recessive disorder characterized by **sensorineural hearing loss** and **goiter**, caused by mutations in the *SLC26A4 gene*. - The *SLC26A4 gene* encodes pendrin, a **chloride-iodide transporter** found in the inner ear and thyroid gland, crucial for endolymph production and iodide transport. *Bartillin* - The *BSND gene* encodes bartillin, which is associated with **Bartter syndrome type IV**, characterized by renal salt wasting, deafness, and metabolic alkalosis. - While it involves deafness, it's distinct from Pendred syndrome as it doesn't primarily involve thyroid dysfunction. *Fibrillin* - **Fibrillin-1** is encoded by the *FBN1 gene* and mutations in this gene cause **Marfan syndrome**, a genetic disorder affecting connective tissue, leading to cardiovascular, skeletal, and ocular abnormalities. - It is not associated with hearing loss or thyroid issues, as seen in Pendred syndrome. *Reticulin* - Reticulin refers to a type of collagen fiber (primarily **collagen type III**) found in connective tissues. - Mutations directly related to the protein "reticulin" are not known to cause specific genetic syndromes like Pendred syndrome.

Q: Which type of genetic disorders are more likely to affect boys?

X-linked Recessive. ***X-linked Recessive*** - X-linked recessive disorders are more likely to affect boys because they have only one X chromosome. If that X chromosome carries the affected gene, they will express the disorder. - Females have two X chromosomes, so if one carries the affected gene, the other typically healthy X chromosome can compensate, making them carriers but often asymptomatic. *Autosomal Dominant* - These disorders affect individuals when they inherit just one copy of a mutated gene on an autosome (non-sex chromosome). - They affect both males and females equally since autosomes are not sex-linked. *Autosomal Recessive* - These disorders require two copies of a mutated gene on an autosome for the disease to manifest. - They affect both males and females equally because the genes are on non-sex chromosomes. *X-linked Dominant* - These disorders are caused by a mutation on an X chromosome, where only one copy of the mutated gene is sufficient to cause the disorder. - While they can affect both males and females, females are usually more commonly affected and often present with milder symptoms, while affected males typically have more severe phenotypes.

Q: Which of the following statements regarding Huntington’s chorea is true?

It is a trinucleotide repeat expansion type of disorder. ***It is a trinucleotide repeat expansion type of disorder*** - Huntington's chorea is caused by an expansion of a **CAG trinucleotide repeat** in the **huntingtin gene (HTT)**. - This expansion leads to a misfolded protein and an **autosomal dominant** neurodegenerative disorder [1]. *There is a loss of function type of mutation.* - Huntington's chorea is primarily a **gain-of-function** mutation, where the expanded polyglutamine tract in the huntingtin protein leads to **toxic protein aggregation** and neuronal dysfunction. - While there might be some aspects of altered protein function, the core pathology is attributed to the **toxic effects** of the abnormal protein rather than a simple loss of its original function [1]. *It is an autosomal recessive disorder.* - Huntington's chorea is an **autosomal dominant** disorder, meaning only one copy of the mutated gene is sufficient to cause the disease. - Each child of an affected parent has a **50% chance** of inheriting the disease. *Increased number of CAA repeats.* - Huntington's chorea is characterized by an increased number of **CAG trinucleotide repeats**, not CAA repeats. - The expansion of these CAG repeats beyond a certain threshold (typically >35-40 repeats) in the huntingtin gene is directly responsible for the disease.

Question 1

What is the underlying defect in Ataxia telangiectasia?

Accepted Answer

Double-strand DNA break repair

Answer

Nucleotide Excision Repair

Answer

Mismatch Repair

Answer

Base Excision Repair

Question 2

Acute intermittent porphyria is associated with which type of inheritance?

Accepted Answer

Autosomal dominant

Answer

Autosomal recessive

Answer

X-linked dominant

Answer

X-linked recessive

Question 3

Inheritance associated with fragile X syndrome is-

Accepted Answer

X-linked dominant

Answer

Autosomal dominant

Answer

Autosomal recessive

Answer

X-linked recessive

Question 4

Trinucleotide sequence associated with spinocerebellar ataxia is?

Accepted Answer

CAG

Answer

CUG

Answer

GGC

Answer

CGG

Question 5

Which chromosome contains the gene responsible for MEN2 (Multiple Endocrine Neoplasia type 2)?

Accepted Answer

10q11.2

Answer

13q11

Answer

11q13

Answer

11q10.2

Question 6

What is the most definitive method for confirming 46,XY disorders of sexual development?

Accepted Answer

Genetic testing

Answer

USG abdomen

Answer

Hormonal study

Answer

Gonadal biopsy

Question 7

Gene responsible for Wilson disease is situated on which chromosome?

Accepted Answer

Chromosome 13

Answer

Chromosome 11

Answer

Chromosome 12

Answer

Chromosome 14

Question 8

Pendred syndrome is caused by a mutation in which gene?

Accepted Answer

SLC26A4 gene

Answer

Fibrillin

Answer

Bartillin

Answer

Reticulin

Question 9

Which type of genetic disorders are more likely to affect boys?

Accepted Answer

X-linked Recessive

Answer

Autosomal Dominant

Answer

Autosomal Recessive

Answer

X-linked Dominant

Question 10

Which of the following statements regarding Huntington’s chorea is true?

Accepted Answer

It is a trinucleotide repeat expansion type of disorder

Answer

There is a loss of function type of mutation.

Answer

It is an autosomal recessive disorder.

Answer

Increased number of CAA repeats.

Genetic Disorders and Biochemical Pathology — MCQs

Genetic Disorders and Biochemical Pathology — MCQs

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