CGC Molecular Genetics & Mechanisms of Inheritance 1 — Questions and Answers
Question 1: Which type of mutation involves the insertion of one or two nucleotides, causing a shift in the reading frame of downstream codons?
- Missense mutation
- Nonsense mutation
- Frameshift mutation (Correct answer)
- Silent mutation
Correct answer: Frameshift mutation
A frameshift mutation results from insertion or deletion of nucleotides in a number not divisible by three, altering every codon downstream of the mutation.
Question 2: Which DNA repair mechanism corrects single-strand mismatches introduced during replication and is defective in Lynch syndrome?
- Nucleotide excision repair
- Mismatch repair (Correct answer)
- Base excision repair
- Homologous recombination
Correct answer: Mismatch repair
Mismatch repair (MMR) corrects replication errors; germline mutations in MMR genes (MLH1, MSH2, MSH6, PMS2) cause Lynch syndrome.
Question 3: What term describes the phenomenon where the expression of a gene depends on whether it was inherited from the mother or the father?
- Variable expressivity
- Anticipation
- Genomic imprinting (Correct answer)
- Incomplete penetrance
Correct answer: Genomic imprinting
Genomic imprinting is an epigenetic process by which gene expression is determined by the parental origin of the allele.
Question 4: During meiosis I, homologous chromosomes exchange segments in a process called:
- Non-disjunction
- Crossing over (recombination) (Correct answer)
- Independent assortment
- Sister chromatid exchange
Correct answer: Crossing over (recombination)
Crossing over (recombination) occurs during prophase I of meiosis when homologous chromosomes exchange corresponding segments, generating genetic diversity.
Question 5: Which of the following best describes a de novo mutation?
- A mutation inherited from a carrier parent
- A mutation present in all somatic cells but absent from germline
- A mutation that arises newly in the affected individual and is not present in either parent (Correct answer)
- A mutation that reverts to the wild-type sequence
Correct answer: A mutation that arises newly in the affected individual and is not present in either parent
A de novo mutation arises spontaneously in the proband and is not inherited from either parent, often explaining isolated cases of dominant disorders.
Question 6: In the context of trinucleotide repeat expansion disorders, which mechanism best explains the phenomenon of anticipation?
- Increased penetrance in successive generations due to environmental factors
- Repeat sequences become unstable and tend to expand further with each successive transmission (Correct answer)
- Epigenetic silencing accumulates over generations reducing gene expression
- Somatic mosaicism increases in severity with each generation
Correct answer: Repeat sequences become unstable and tend to expand further with each successive transmission
Trinucleotide repeat tracts are meiotically unstable and tend to expand during transmission, leading to progressively larger repeats and earlier/more severe disease in successive generations.
Question 7: Which of the following epigenetic modifications most directly contributes to gene silencing by adding a chemical group to cytosine residues in CpG islands?
- Histone acetylation
- DNA methylation (Correct answer)
- Histone phosphorylation
- Chromatin remodeling via SWI/SNF
Correct answer: DNA methylation
DNA methylation at CpG islands, catalyzed by DNA methyltransferases, is strongly associated with transcriptional silencing of the associated gene.
CGC Molecular Genetics & Mechanisms of Inheritance 1
Which type of mutation involves the insertion of one or two nucleotides, causing a shift in the reading frame of downstream codons?