60 Year 2: Molecular Genetics and Cytogenetics exam questions on Genetic Disorders MCQ Examination for medical students. Includes MCQs, answers, explanations an
This MCQ set contains 60 questions on Genetic Disorders MCQ Examination in the Year 2: Molecular Genetics and Cytogenetics unit. Each question includes the correct answer and a detailed explanation for active recall and exam preparation.
Correct answer: B – 25%
In autosomal recessive inheritance, two carrier parents have a 25% chance with each pregnancy of having an affected child (homozygous recessive), 50% chance of a carrier, and 25% chance of being unaffected and not a carrier.
Correct answer: B – Demonstrable chromosomal abnormalities
Approximately 50% of early pregnancy miscarriages are associated with demonstrable chromosomal abnormalities, highlighting the importance of genomic integrity in fetal development.
Correct answer: A – Less than 2%
Less than 2% of human DNA actually codes for proteins. More than half consists of repetitive nucleotide sequences whose functions are not fully understood.
Correct answer: B – Variable expressivity
Variable expressivity occurs when a trait is seen in all individuals with the mutation but is expressed differently in terms of severity or manifestation. Reduced penetrance refers to some individuals not showing the trait at all.
Correct answer: C – Point mutation (non-conservative missens
Sickle cell anemia results from a single nucleotide substitution (point mutation) that causes a non-conservative missense mutation, replacing glutamic acid with valine in the beta-globin chain.
Correct answer: B – Functional cloning
Functional cloning (classic approach) begins with a known affected protein from the clinical phenotype, then isolates and clones the normal gene before determining molecular changes.
Correct answer: C – Nonsense mutation
Nonsense mutations convert an amino acid codon into a stop codon (chain terminator), resulting in premature termination of protein synthesis and usually a non-functional truncated protein.
Correct answer: C – 5-8
It is estimated that every person carries 5 to 8 deleterious genes, most being recessive. About 80% are familial and the remainder are acquired de novo.
Correct answer: C – Not transmit to sons; daughters will be carriers
In X-linked recessive disorders, affected males pass their Y chromosome to sons (who are unaffected) and their X chromosome to daughters (who become carriers).
Correct answer: C – Trinucleotide repeat mutation
Fragile X syndrome results from dynamic amplification of a trinucleotide repeat sequence (typically CGG repeats), with the degree of amplification increasing during gametogenesis.
Correct answer: B – 5%
Approximately 5% of individuals under 25 years develop a serious disease with a significant genetic component, demonstrating the considerable impact of genetic disorders in young populations.
Correct answer: C – Pleiotropy
Pleiotropy occurs when a single mutant gene leads to many different end effects, affecting multiple organ systems or producing various manifestations from one genetic defect.
Correct answer: B – Autosomal recessive
Autosomal recessive disorders constitute the largest category of Mendelian disorders and include almost all inborn errors of metabolism.
Correct answer: C – Binding of transcription factors and gene transcription
Mutations in regulatory sequences (promoters and enhancers) interfere with binding of transcription factors, leading to reduced or absent transcription of the gene, as seen in some hereditary hemolytic anemias.
Correct answer: C – 10 million
In 2020, there were approximately 10 million cancer deaths globally (representing 1 in 6 deaths), up from 6 million in 2000, showing an increasing cancer burden.
Correct answer: C – Frameshift mutation
Insertion or deletion of one or two base pairs (not divisible by 3) causes a frameshift mutation, altering the reading frame and typically producing a non-functional protein.
Correct answer: B – Genetic heterogeneity
Genetic heterogeneity occurs when mutations at different genetic loci can produce the same phenotype or disease trait, explaining why the same disease can result from different genetic defects.
Correct answer: B – Autosomal recessive
Autosomal recessive inheritance typically shows unaffected parents (both carriers) with affected siblings, as the trait requires two copies of the mutant allele to manifest.
Correct answer: B – Autosomal recessive disorders
Complete penetrance is common in autosomal recessive disorders, where expression of the defect tends to be more uniform than in autosomal dominant disorders which often show variable penetrance.
Correct answer: C – 670 per 1000
The lifetime frequency of genetic disease is thought to be approximately 670 per 1000, which is higher than widely appreciated, including cardiovascular diseases, immune disorders, and cancers with genetic components.
Correct answer: C – 99.9%
Humans share 99.9% of their DNA sequence, with only 0.1% (about 3 million base pairs) accounting for genetic diversity among individuals in the human race.
Correct answer: C – 3 base pairs
Insertions or deletions of 3 base pairs (or multiples of 3) do not cause frameshift mutations because they maintain the reading frame, though they result in abnormal proteins missing or gaining amino acids.
Correct answer: B – Older fathers
De novo mutations in autosomal dominant disorders are more frequently associated with relatively older fathers, as mutations accumulate in spermatogonial cells over time.
Correct answer: C – Parents are usually affected
In autosomal recessive disorders, parents are typically NOT affected but are carriers. The trait manifests in children who inherit two mutant alleles.
Correct answer: B – Mapping disease phenotype to chromosome location first
Positional cloning (candidate gene approach) starts by mapping the disease phenotype to a particular chromosome location, then cloning DNA pieces from that region to identify aberrant proteins.