CGC Molecular Genetics & Mechanisms of Inheritance 3

Question 1

Heteroplasmy in mitochondrial genetics refers to:

Accepted Answer

A mixture of two or more mitochondrial DNA genotypes within a cell or individual

Answer

The presence of two alleles at a nuclear locus in a diploid cell

Answer

The exclusive presence of mutant mitochondrial DNA throughout all cells

Answer

Recombination between paternal and maternal mitochondrial genomes

Question 2

Germline mosaicism occurs when a mutation is present in some but not all of a parent's germline cells. What is the primary clinical implication for genetic counseling?

Accepted Answer

The recurrence risk for future pregnancies is higher than the population risk but lower than 50%, making standard dominant inheritance risk estimates inaccurate

Answer

The parent will show clinical features of the condition proportional to the level of mosaicism

Answer

The mutation cannot be detected in the parent by standard blood-based genetic testing, so recurrence risk is always 0%

Answer

Germline mosaicism only occurs for autosomal recessive conditions

Question 3

Which term describes the situation where a single gene mutation causes multiple, seemingly unrelated phenotypic effects?

Accepted Answer

Pleiotropy

Answer

Genetic heterogeneity

Answer

Variable expressivity

Answer

Epistasis

Question 4

Locus heterogeneity in genetics means that:

Accepted Answer

The same disease phenotype can result from mutations at different genetic loci

Answer

A single genetic locus produces multiple different disease phenotypes

Answer

Two alleles at the same locus produce different disease severity

Answer

Different mutations at the same locus produce different diseases

Question 5

The protein product of a dominant negative mutation typically:

Accepted Answer

Interferes with the function of the normal protein produced by the wild-type allele

Answer

Is completely absent, causing haploinsufficiency

Answer

Gains a completely new function unrelated to the wild-type protein

Answer

Is expressed at twice the normal level due to gene duplication

Question 6

A point mutation at the first position of a codon changes AUG (methionine/start) to AUG → AUA. This change would most likely result in:

Accepted Answer

Loss of the start codon, preventing normal translation initiation from that site

Answer

A conservative amino acid change with minimal functional effect

Answer

A frameshift disrupting all downstream codons

Answer

Nonsense-mediated mRNA decay

Question 7

Which of the following best describes the concept of incomplete penetrance versus variable expressivity?

Accepted Answer

Incomplete penetrance refers to whether the genotype produces any phenotype at all; variable expressivity refers to the range of phenotypic severity among those who are affected

Answer

Incomplete penetrance means the trait skips generations; variable expressivity means the trait is always mild

Answer

Incomplete penetrance applies only to recessive conditions; variable expressivity applies only to dominant conditions

Answer

Both terms are interchangeable and describe the same phenomenon