Mitochondrial inheritance Practice Questions
19 free Mitochondrial inheritance practice questions for the USMLE Step 1. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.
Which inheritance pattern best describes transmission of mutations in mitochondrial DNA (mtDNA) in humans?
- A Autosomal dominant inheritance
- B X-linked recessive inheritance
- C Mitochondrial (maternal) inheritance
- D Autosomal recessive inheritance
Correct answer: Mitochondrial (maternal) inheritance
Human mitochondrial DNA is inherited almost exclusively from the mother — sperm mitochondria are typically destroyed after fertilization — so mtDNA mutations follow a maternal, non-Mendelian inheritance pattern.
In a family pedigree for a mitochondrial disease, which of the following patterns is expected if the disease is due to an mtDNA mutation?
- A Affected fathers transmit the disease to all of their children
- B Only the male children in the family are ever affected
- C Both sexes affected, but only through affected mothers
- D The trait appears only in siblings of the parent's sex
Correct answer: Both sexes affected, but only through affected mothers
Because mtDNA is maternally inherited, both sons and daughters of an affected mother can be affected; affected fathers do not transmit it because paternal mitochondria are excluded.
What is “heteroplasmy” in the context of mitochondrial genetics?
- A Presence of both mtDNA and nuclear DNA in one cell
- B A mix of mutant and wild-type mtDNA in one cell
- C Inheritance of mitochondria from both parents equally
- D Complete deletion of the mitochondrial genome
Correct answer: A mix of mutant and wild-type mtDNA in one cell
Heteroplasmy refers to a mixture of mutated and normal mitochondrial DNA within a cell or organism—because each cell has many mitochondria and many mtDNA copies. This mixture influences disease severity and penetrance.
Which phenomenon explains why siblings of the same mother may have different severity of a mitochondrial disorder despite inheriting the same mutation?
- A Ordinary Mendelian segregation of alleles
- B Paternal leakage of mtDNA into the embryo
- C The mtDNA bottleneck combined with heteroplasmy
- D Somatic recombination between mtDNA molecules
Correct answer: The mtDNA bottleneck combined with heteroplasmy
During oogenesis, only a small subset of mtDNA molecules is transmitted to the egg (the bottleneck), and random sampling can result in variable proportions of mutant vs. wild-type mtDNA in different children — leading to variable expression.
Which structural feature of human mtDNA distinguishes it from nuclear DNA?
- A It is linear and very large
- B It is circular and contains 37 genes
- C It is double-stranded RNA
- D It is identical in size and structure to nuclear chromosomes
Correct answer: It is circular and contains 37 genes
Human mtDNA is a small, circular, double-stranded genome encoding 37 genes (tRNAs, rRNAs, and a few polypeptides) — in contrast to linear nuclear chromosomes with tens of thousands of genes.
Why do mitochondrial diseases often affect tissues with high energy demand (like brain and muscle)?
- A Because mtDNA is expressed only within those particular tissues
- B Because these tissues depend heavily on mitochondrial ATP production
- C Because nuclear DNA compensates fully in low-energy tissues
- D Because mitochondria are entirely absent from other tissues
Correct answer: Because these tissues depend heavily on mitochondrial ATP production
Tissues that demand a lot of energy rely on mitochondria for ATP via oxidative phosphorylation. When mtDNA mutations impair mitochondrial function, high-energy tissues like brain and muscle are most vulnerable.
Which of the following statements about recombination in human mitochondrial DNA is most accurate?
- A mtDNA frequently recombines between maternal and paternal genomes
- B mtDNA recombines only during meiotic division
- C mtDNA does not recombine and passes clonally from mother to child
- D mtDNA recombination gives rise to X-linked traits
Correct answer: mtDNA does not recombine and passes clonally from mother to child
Human mtDNA is typically transmitted maternally without recombination, so it behaves as a single haploid locus — which aids in tracing maternal lineages.
Which of the following is a possible mechanism by which paternal mitochondria are excluded from inheritance in humans?
- A Paternal mtDNA integrates into nuclear DNA right after fertilization
- B Sperm mitochondria are tagged (e.g. by ubiquitin) and destroyed
- C The egg cell selectively rejects all female mitochondria
- D mtDNA recombination erases the paternal sequences
Correct answer: Sperm mitochondria are tagged (e.g. by ubiquitin) and destroyed
After fertilization, paternal mitochondria are typically tagged (e.g. via ubiquitin) and eliminated, ensuring that mtDNA comes almost exclusively from the mother.
If a woman carries a homoplasmic harmful mtDNA mutation, what is the risk that her child (regardless of sex) will inherit the mutation?
- A 0%
- B 25%
- C 50%
- D 100%
Correct answer: 100%
If the mother’s mtDNA is homoplasmic (all mitochondria carry the mutation), then all her children inherit her mitochondria — so there is a 100% chance the mutation is passed to each child.
Which of the following scenarios would argue against a strictly maternal mtDNA inheritance model in humans?
- A Both sexes affected across generations, always through mothers
- B No transmission from affected fathers to their children
- C Father-derived mtDNA variants found in some individuals
- D All children of affected mothers carry the mtDNA variant
Correct answer: Father-derived mtDNA variants found in some individuals
Occasional reports of paternal mtDNA inheritance challenge the dogma of strict maternal transmission, although such cases may reflect nuclear mitochondrial sequences (NUMTs) rather than true paternal inheritance.
Which term describes a condition in which some cells in a person’s body have mutated mtDNA and others have normal mtDNA, leading to variable expression of a mitochondrial disease?
- A Homoplasmy
- B Heterozygosity
- C Heteroplasmy
- D Mosaicism in nuclear DNA
Correct answer: Heteroplasmy
Heteroplasmy refers to the presence of both mutant and wild-type mtDNA within the same individual’s cells, leading to variability in which tissues are affected and disease severity.
Which of the following features is NOT typical of mitochondrial (mtDNA) inheritance of disease?
- A Transmission from affected fathers to children
- B Equal potential to affect sons and daughters
- C Maternal lineage of transmission
- D Possible variability in severity due to heteroplasmy
Correct answer: Transmission from affected fathers to children
Transmission of mtDNA mutations from father to child is generally absent because paternal mitochondria are excluded at fertilization; thus father-to-child transmission is not a feature of mitochondrial inheritance.
Why does mitochondrial inheritance behave as a single locus rather than following Mendelian segregation rules?
- A Because mtDNA recombines before it is inherited
- B Because mtDNA is inherited clonally in high copy number
- C Because mtDNA forms part of the nuclear genome itself
- D Because paternal mtDNA complements the maternal mtDNA
Correct answer: Because mtDNA is inherited clonally in high copy number
Mitochondrial DNA is transmitted in multiple copies from the mother’s oocyte and does not recombine; all copies inherited go together, so it behaves as a single non-Mendelian locus.
Which principle explains why rare deleterious mtDNA mutations may be lost in one generation despite being present in the mother?
- A Recombination within the nuclear genome
- B The bottleneck effect during oogenesis
- C Gene conversion between mtDNA copies
- D A dominant-negative protein effect
Correct answer: The bottleneck effect during oogenesis
During oogenesis, only a small subset of mtDNA molecules is transmitted to each egg; this “bottleneck” may randomly exclude some mutant genomes, so offspring can inherit mostly normal mtDNA despite maternal mutation.
Which of the following is a potential challenge when diagnosing mitochondrial diseases using family history alone?
- A mtDNA always recombines, making lineage tracing hard
- B Variable heteroplasmy and penetrance can mask maternal transmission
- C Both parents transmit mtDNA in equal amounts
- D mtDNA is identical across all individuals
Correct answer: Variable heteroplasmy and penetrance can mask maternal transmission
Because heteroplasmy can vary between individuals and tissues, and disease expression depends on mutant load, an affected mother might appear asymptomatic — obscuring maternal transmission when relying solely on pedigree.
Which of the following statements about nuclear-encoded mitochondrial proteins and their inheritance is correct?
- A They follow maternal inheritance, just like mtDNA does
- B They follow Mendelian, autosomal or X-linked, inheritance
- C They are never implicated in mitochondrial disease
- D They are encoded within the mtDNA genome itself
Correct answer: They follow Mendelian, autosomal or X-linked, inheritance
Most mitochondrial proteins are encoded by nuclear genes and are inherited as autosomal (or occasionally X-linked) traits according to Mendelian genetics — unlike mtDNA-encoded proteins.
Which mitochondrial feature makes mtDNA a useful tool for tracing human maternal ancestry over many generations?
- A High recombination rate that reshuffles alleles each generation
- B Low mutation rate that erases lineage-specific markers over time
- C Maternal inheritance without recombination
- D Biparental inheritance mixing both parental genomes
Correct answer: Maternal inheritance without recombination
Because mtDNA is inherited solely from mothers and does not recombine, it remains relatively stable across generations — making it ideal for tracing maternal lineages and population ancestry.
Which of the following would be strong evidence for a nuclear (rather than mitochondrial) cause of a suspected mitochondrial disease?
- A Trait transmitted only through affected mothers to offspring
- B Affected fathers passing the disease to their children
- C Both sexes affected but no clear maternal lineage pattern
- D Disease severity correlating with mutant mtDNA load in tissues
Correct answer: Affected fathers passing the disease to their children
If affected fathers transmit the disease to their children, this violates maternal-only mtDNA inheritance — suggesting that a nuclear gene (autosomal or X-linked) is responsible instead of mtDNA.
Why might a heteroplasmic mother have both affected and unaffected children when carrying a deleterious mtDNA mutation?
- A Mendelian segregation of mtDNA during cell division
- B Paternal mtDNA contributing in some of the children
- C Random bottleneck sampling giving eggs variable mutant load
- D Recombination of mtDNA during meiotic division
Correct answer: Random bottleneck sampling giving eggs variable mutant load
Random partitioning of mtDNA during egg formation (bottleneck) can result in eggs with varying proportions of mutant vs wild-type mtDNA. Some eggs may carry low mutant load — yielding unaffected children — while others have high load — leading to disease in offspring.