Selected MCQ

Current Question
An allele is:
  • A. A mutated chromosome
  • B. A variant form of a gene at a particular locus
  • C. A gene on the Y chromosome
  • D. A plasmid containing genetic information
Correct Answer: B
Explanation:
An allele is one of the alternative forms of the same gene found at the same chromosomal locus.
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
In genomic imprinting, the expression of a gene depends on its parental origin. For example, only the paternally inherited allele of a certain gene is expressed. This is known as:
  • A. Epigenetic modification
  • B. Loss of heterozygosity
  • C. Imprinting
  • D. Mendelian inheritance
Correct Answer: C
Explanation:
Genomic imprinting is an epigenetic phenomenon where only one allele (maternal or paternal) is expressed, depending on the parent of origin, while the other is silenced.
Related Question 3
If a couple consists of one carrier for an autosomal recessive allele and one homozygous normal, what fraction of their children will be carriers (heterozygotes)?
  • A. 0%
  • B. 25%
  • C. 50%
  • D. 100%
Correct Answer: C
Explanation:
A cross Aa (carrier) × aa (normal) yields 50% Aa (carriers) and 50% aa (unaffected, non-carriers).