Selected MCQ

Current Question
A mother carries a mutated X-linked recessive gene but is unaffected. What is the probability that a son of hers will express the trait?
  • A. 0%
  • B. 25%
  • C. 50%
  • D. 100%
Correct Answer: C
Explanation:
A carrier mother (X^mX) has a 50% chance of passing the mutant X to a son. If he receives it, he will express the recessive trait (since sons inherit only one X chromosome).
Related Question 1
Phenylketonuria (PKU) is inherited as an autosomal recessive trait. If two carriers have a child, what is the probability the child will be unaffected (no mutant alleles)?
  • A. 100%
  • B. 75%
  • C. 50%
  • D. 25%
Correct Answer: B
Explanation:
For two carriers (Aa × Aa), the offspring genotype ratio is 1 AA: 2 Aa: 1 aa. 75% (AA + Aa) are unaffected (though 50% are carriers).
Related Question 2
A man has an X-linked recessive disease and marries an unaffected woman who is not a carrier. What is the outcome for their children?
  • A. All daughters will be carriers; none of the sons will be affected
  • B. All daughters will be affected, sons unaffected
  • C. All sons will be affected; no daughters will be carriers
  • D. None of their children will inherit the disease allele
Correct Answer: A
Explanation:
The father passes his mutant X to all daughters, making them carriers (but not affected). He passes his Y to his sons, so no son is affected or a carrier.
Related Question 3
A female exhibits an X-linked recessive trait. What must be true of her parents’ genotypes?
  • A. Her father must have the trait, and her mother must be at least a carrier
  • B. Both parents must be affected
  • C. Only her mother is affected
  • D. Either parent could have the trait independently
Correct Answer: A
Explanation:
For a daughter to be homozygous for an X-linked recessive allele, her father must have the allele (so he is affected) and her mother must carry at least one mutant X.
Related Question 4
Which viral vector carries a high risk of insertional mutagenesis due to genome integration?
  • A. Adenoviral vector
  • B. Retroviral vector
  • C. Adeno-associated viral vector
  • D. Liposome-based vector
Correct Answer: B
Explanation:
Retroviruses integrate their genome into the host DNA. This random insertion can disrupt host genes, increasing the risk of mutagenesis (used with caution in gene therapy).
Related Question 5
Alternative splicing occurs during gene expression at the level of:
  • A. DNA replication
  • B. Transcription initiation
  • C. Pre-mRNA processing (RNA splicing)
  • D. Protein folding
Correct Answer: C
Explanation:
Alternative splicing of pre-mRNA allows different exons to be joined or skipped, producing multiple mRNA variants from one gene during RNA processing.