Genetics Quiz: Mitochondrial Inheritance

What this quiz covers

This quiz focuses on Mitochondrial Inheritance, giving you a quick way to practice the rules, question types, and explanations that matter most for Genetics.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. Rule out a mitochondrial etiology, as the blood test for a common mutation was negative.
2. Test the father's blood for the mutation to confirm he is not a carrier and rule out paternal transmission.
3. Immediately initiate treatment with a vitamin cocktail, as this is the standard of care regardless of genetic confirmation.
4. Perform a muscle biopsy for mtDNA sequencing, as mutation load can be low or absent in blood but high in affected tissues. (correct answer)

Explanation: When evaluating suspected mitochondrial disorders, you need to understand that mitochondrial DNA (mtDNA) mutations show tissue-specific heteroplasmy - the percentage of mutant versus wild-type mtDNA varies dramatically between different tissues in the same patient. The correct approach is D) Perform a muscle biopsy for mtDNA sequencing. Blood typically has a much lower mutant load than metabolically active tissues like muscle, brain, or heart. A <2% mutant load in blood doesn't rule out Leigh syndrome because the same patient could have 70-90% mutant load in muscle tissue, which would be pathogenic. Muscle biopsy is the gold standard for diagnosing mitochondrial disorders when blood testing is inconclusive. A is wrong because a negative or low-positive blood test cannot rule out mitochondrial disease due to tissue-specific heteroplasmy patterns. Many confirmed cases show minimal blood involvement. B misunderstands mitochondrial inheritance - mtDNA is maternally inherited, so testing the father is irrelevant. Paternal mitochondrial transmission is extremely rare and wouldn't be the next diagnostic step. C puts the cart before the horse. While vitamin cocktails (CoQ10, B vitamins, etc.) are used in mitochondrial disorders, you need genetic confirmation first, especially since the clinical presentation could have other causes. Study tip: Remember that mitochondrial disorders require tissue-specific testing. When blood mtDNA analysis is negative or shows low heteroplasmy, always consider muscle biopsy as the next step, since muscle has high metabolic demand and typically shows higher mutant loads than blood.

Question 2

1. Somatic mutations are acquired during an individual's lifetime and are not passed to offspring, whereas MELAS is caused by a germline mutation from the mother. (correct answer)
2. Age-related mutations only occur in nuclear genes that support mitochondria, while MELAS is a primary mtDNA mutation.
3. Inherited mitochondrial diseases like MELAS are typically homoplasmic, whereas age-related changes result in heteroplasmy.
4. Somatic mutations are efficiently repaired by mitochondrial proofreading, while inherited germline mutations bypass these repair mechanisms.

Explanation: The key distinction is between somatic and germline mutations. Age-related accumulation of mtDNA mutations occurs in somatic cells and is not heritable. In contrast, classic mitochondrial diseases are caused by pathogenic mutations present in the mother's germline (oocytes) and are therefore passed down to all offspring, establishing the mutation in all their cells from the zygote stage.

Question 3

1. Man's children: 100% risk; Woman's children: 50% risk.
2. Man's children: 0% risk; Woman's children: 50% risk. (correct answer)
3. Man's children: 0% risk; Woman's children: 100% risk.
4. Man's children: 50% risk; Woman's children: 50% risk.

Explanation: The inheritance pattern is dictated by the location of the gene. The man's mutation is in MT-ND1, a mitochondrial gene. Men do not pass mitochondria to their children, so his risk of transmission is 0%. The woman's mutation is in NDUFS1, a nuclear gene. The mutation is dominant, meaning she is heterozygous. She will pass the mutant allele to her children with a 50% probability, following standard Mendelian autosomal dominant inheritance.

Question 4

1. Meiotic drive
2. Somatic mosaicism
3. Replicative segregation (correct answer)
4. Gene conversion

Explanation: Replicative segregation is the specific term for the process where mitochondria, and the mtDNA molecules within them, are stochastically segregated into daughter cells during mitosis. Because there is no tight control over this distribution, it can lead to shifts in heteroplasmy levels in different cell lineages as development proceeds. Somatic mosaicism typically refers to post-zygotic mutations in nuclear DNA, while meiotic drive and gene conversion are processes related to nuclear genetics in meiosis.

Question 5

1. The son inherited his father's (I-1) mitochondria, which were healthy and compensated for the defect.
2. A spontaneous reversion mutation occurred in the mtDNA that was passed to the son, correcting the defect.
3. The disorder is actually X-linked recessive, and the son inherited his mother's healthy X chromosome.
4. The son inherited a low proportion of mutant mitochondria from his mother, placing him below the phenotypic threshold. (correct answer)

Explanation: When analyzing pedigrees for mitochondrial disorders, remember that mitochondrial inheritance follows maternal lineage and involves the concept of heteroplasmy—cells containing both normal and mutant mitochondria in varying proportions. The correct answer is D because mitochondrial disorders exhibit a threshold effect. Even though the son (II-3) inherited mitochondria from his affected mother, he likely received a lower proportion of mutant mitochondria compared to his sister. Since cells contain hundreds to thousands of mitochondria, the ratio of normal to mutant mitochondria determines whether symptoms appear. If the proportion of defective mitochondria falls below the critical threshold needed for normal cellular function, the individual remains asymptomatic. Option A is incorrect because humans inherit mitochondria exclusively from their mothers through the egg cytoplasm—fathers contribute essentially no mitochondria to offspring. Option B is wrong because spontaneous reversion mutations in mtDNA are extremely rare events and wouldn't explain the typical inheritance pattern seen in mitochondrial disorders. Option C misidentifies the inheritance pattern entirely; the fact that an affected grandmother has an affected daughter who then has an affected child clearly indicates maternal inheritance, not X-linked inheritance. The key study point for mitochondrial genetics is understanding heteroplasmy and threshold effects. Unlike nuclear genes where you're either homozygous or heterozygous, mitochondrial inheritance involves variable proportions of normal versus mutant organelles. This explains why severity can vary dramatically within families and why some individuals escape symptoms entirely despite having an affected mother.

Question 6

1. The blood sample is the most reliable indicator of overall mutation load and predicts that the disease will be mild.
2. A new somatic mutation must have occurred in the patient's muscle tissue, as the level is much higher than in blood.
3. The difference in heteroplasmy levels between tissues likely reflects replicative segregation and tissue-specific selective pressures. (correct answer)
4. The mother is not the source of the mutation because her heteroplasmy level is well below the pathogenic threshold.

Explanation: Heteroplasmy levels can vary significantly between different tissues in the same individual. This is a result of random replicative segregation of mitochondria during cell division throughout development. Tissues with high energy requirements, like muscle, may also have selective pressures that favor the replication of mitochondria with certain mutations. Therefore, testing the affected tissue is critical, and the discrepancy between blood and muscle is an expected finding, not evidence of a new mutation.

Question 7

1. It ensures that all offspring of a heteroplasmic mother inherit an identical percentage of mutant mtDNA.
2. It leads to a significant amplification or reduction of mutant mtDNA proportion in the oocyte relative to the mother's somatic cells. (correct answer)
3. It facilitates the selective elimination of paternal mitochondria immediately following fertilization.
4. It causes all oocytes with any level of mutant mtDNA to be targeted for apoptosis, preventing transmission.

Explanation: The mitochondrial bottleneck involves a reduction in the number of mtDNA molecules that are segregated into primordial germ cells and later into the mature oocytes. This small sample of mitochondria then replicates to populate the oocyte. Due to this sampling effect, the proportion of mutant mtDNA in an oocyte can, by chance, be very different from the average proportion in the mother's somatic tissues, leading to variable outcomes in the offspring.

Question 8

1. 16%
2. 32%
3. 50% (correct answer)
4. 84%

Explanation: The child will be unaffected if the heteroplasmy level is less than or equal to 80%. The distribution of heteroplasmy in the oocytes has a mean of 80%. For a normal distribution, the mean is equal to the median. Therefore, 50% of the distribution lies at or below the mean, and 50% lies above the mean. The probability of having a heteroplasmy level of 80% or less is 50%.

Question 9

1. 0%, because the hearing loss was an acquired condition caused by an environmental drug exposure.
2. 50%, because the drug unmasked a dominant genetic trait that follows Mendelian inheritance.
3. The risk cannot be determined, as the drug may have induced a new somatic mutation not present in her germline.
4. 100%, because she is homoplasmic for the mtDNA mutation and will pass it to all her offspring. (correct answer)

Explanation: When you encounter questions about mitochondrial genetics, remember that mitochondrial DNA (mtDNA) follows completely different inheritance rules than nuclear DNA. Mitochondria are inherited exclusively from the mother because sperm contribute virtually no cytoplasm during fertilization. The key insight here is understanding what "homoplasmic" means. This woman has the A1555G mutation in all copies of her mitochondrial 12S rRNA gene across all her mitochondria. Since she will pass her mitochondria to all her children regardless of their sex, and she carries only the mutated version, her son has a 100% chance of inheriting this predisposition. The gentamicin exposure didn't create the genetic risk—it revealed it. The mutation was already present and made her mitochondrial ribosomes more susceptible to aminoglycoside-induced damage. Answer A is wrong because while the hearing loss required environmental exposure, the underlying genetic predisposition is inherited and will be passed on. Answer B incorrectly applies Mendelian inheritance patterns to mitochondrial genetics—mtDNA doesn't follow dominant/recessive rules or show 50% inheritance risks. Answer C misunderstands the situation entirely; this isn't about new somatic mutations but rather about expressing a pre-existing germline mitochondrial mutation. For genetics exams, remember this pattern: maternal inheritance questions involving mitochondrial conditions typically result in either 0% risk (if mother is unaffected) or 100% risk (if mother carries the mutation), never the 25% or 50% risks you see with nuclear inheritance.

Question 10

1. The high mutation rate of mtDNA, which acts as a precise molecular clock for dating evolutionary events.
2. The lack of significant recombination and strict maternal inheritance, which creates a clear, unbroken line of descent. (correct answer)
3. The presence of heteroplasmy in ancient populations, which provides the necessary genetic diversity for tracking.
4. The conservation of gene function across all mitochondria, ensuring that the same markers are always present for analysis.

Explanation: While the mutation rate is important for dating, the very ability to trace a purely maternal lineage back through time depends on two key facts: 1) mtDNA is passed from mother to all offspring without any input from the father, and 2) mtDNA molecules do not undergo significant recombination. This means the mtDNA genome is passed down as a single unit (a haplotype), creating a clear, unblended line of matrilineal inheritance that can be followed back to a common ancestor.

Question 11

1. He will be unaffected because the father's healthy mitochondria will compensate for the mother's mutant mitochondria.
2. He will have the same 70% mutant load as his mother's fibroblasts, resulting in a similar disease presentation.
3. He has a 50% chance of being affected and a 50% chance of being unaffected, consistent with dominant inheritance patterns.
4. He will inherit the mutation, but his disease severity is unpredictable and could range from asymptomatic to severely affected. (correct answer)

Explanation: The mitochondrial bottleneck refers to the reduction in the number of mitochondria that populate the developing oocytes. This, combined with random segregation, means the mutation load (heteroplasmy level) in each oocyte can vary dramatically from the mother's somatic tissues and from oocyte to oocyte. Therefore, while the son will inherit the mutation, his specific heteroplasmy level and resulting phenotype are difficult to predict.

Question 12

1. Mitochondrial inheritance, as the disorder directly impairs mitochondrial function.
2. Autosomal recessive, as the causative mutation is on a nuclear chromosome. (correct answer)
3. Autosomal dominant, because a defect in a critical enzyme has severe consequences.
4. X-linked inheritance, as many metabolic disorders affecting males are linked to the X chromosome.

Explanation: The mode of inheritance is determined by the location of the mutated gene, not the location of its function. The POLG gene is located on a nuclear chromosome (an autosome). The patient has a homozygous mutation, meaning they have two copies of the recessive allele. This is characteristic of an autosomal recessive inheritance pattern. Although the gene product functions in the mitochondrion, the gene itself follows Mendelian inheritance rules.

Question 13

1. A 50/50 mix of haplotype M and haplotype F, creating systemic heteroplasmy.
2. Primarily haplotype M with a very low, potentially undetectable, level of haplotype F. (correct answer)
3. Exclusively haplotype F, as the paternal mtDNA would outcompete the maternal mtDNA.
4. A novel recombinant haplotype R, resulting from the fusion of haplotypes M and F.

Explanation: Paternal leakage, when it occurs, involves the survival of a very small number of the approximately 100 mitochondria from the sperm in the zygote, which contains over 100,000 maternal mitochondria. Therefore, the offspring's cells would be overwhelmingly populated with maternal mtDNA (haplotype M), with only a tiny fraction of paternal mtDNA (haplotype F), resulting in very low-level heteroplasmy.

Question 14

1. The mitochondrial genome is selectively transcribed in different tissues, with only certain genes being active.
2. Different tissues have varying bioenergetic demands and different pathogenic thresholds for dysfunction in specific respiratory chain complexes. (correct answer)
3. Paternal leakage of mitochondria specifically targets certain tissues, causing disease only in those locations.
4. The rate of new somatic mtDNA mutations is much higher in the optic nerve than in muscle, leading to different diseases.

Explanation: Tissues have different dependencies on oxidative phosphorylation. For example, the optic nerve has extremely high energy demands and may be particularly vulnerable to defects in Complex I (common in LHON). Muscle and brain are vulnerable to defects affecting tRNA synthesis (common in MELAS). The concept is that each tissue has a specific bioenergetic threshold; a certain level of mitochondrial dysfunction must be reached before symptoms appear. This threshold varies from tissue to tissue and depends on which part of the respiratory chain is affected by the mutation.

Question 15

1. Essentially 0%, as she could not have inherited the pathogenic mutation from her father. (correct answer)
2. 50%, because she is an obligate carrier who inherited the mutation but does not show symptoms.
3. 100%, because all offspring inherit mitochondrial DNA from their mother.
4. Approximately 25%, as the mutation follows a recessive inheritance pattern in this pedigree.

Explanation: Mitochondrial DNA (mtDNA) is inherited exclusively through the maternal line. A male does not pass his mitochondria to his offspring. Therefore, the man with LHON cannot pass the mtDNA mutation to his daughter. Because his daughter does not have the mutation, she cannot pass it to her children. The risk is therefore essentially zero.

Question 16

1. The intended mother's pronucleus, the intended father's pronucleus, and an enucleated donor oocyte. (correct answer)
2. The intended mother's oocyte, the intended father's sperm, and injected mitochondria from a donor.
3. A donor's oocyte containing its original nucleus, fertilized by the intended father's sperm.
4. The intended mother's pronucleus and a donor's pronucleus inside the intended mother's enucleated oocyte.

Explanation: In pronuclear transfer, both the intended mother's egg and a donor's egg are fertilized with the father's sperm. Before the pronuclei fuse, the pronuclei from the intended parents' zygote are removed and transferred into the donor zygote, from which the donor's pronuclei had already been removed. The result is an embryo with nuclear DNA from the intended parents and mitochondrial DNA from the donor.

Question 17

1. Essentially 0%, as she could not have inherited the pathogenic mutation from her father. (correct answer)
2. 50%, because she is an obligate carrier who inherited the mutation but does not show symptoms.
3. 100%, because all offspring inherit mitochondrial DNA from their mother.
4. Approximately 25%, as the mutation follows a recessive inheritance pattern in this pedigree.

Explanation: Mitochondrial DNA (mtDNA) is inherited exclusively through the maternal line. A male does not pass his mitochondria to his offspring. Therefore, the man with LHON cannot pass the mtDNA mutation to his daughter. Because his daughter does not have the mutation, she cannot pass it to her children. The risk is therefore essentially zero.

Question 18

1. Mitochondrial inheritance, as the disorder directly impairs mitochondrial function.
2. Autosomal recessive, as the causative mutation is on a nuclear chromosome. (correct answer)
3. Autosomal dominant, because a defect in a critical enzyme has severe consequences.
4. X-linked inheritance, as many metabolic disorders affecting males are linked to the X chromosome.

Explanation: The mode of inheritance is determined by the location of the mutated gene, not the location of its function. The POLG gene is located on a nuclear chromosome (an autosome). The patient has a homozygous mutation, meaning they have two copies of the recessive allele. This is characteristic of an autosomal recessive inheritance pattern. Although the gene product functions in the mitochondrion, the gene itself follows Mendelian inheritance rules.

Question 19

1. The lack of histones and compact chromatin structure makes mtDNA more accessible to mutagens.
2. The mtDNA replication process is carried out by low-fidelity polymerases that lack any proofreading capability.
3. Proximity to the electron transport chain exposes mtDNA to high levels of damaging reactive oxygen species (ROS). (correct answer)
4. Frequent recombination events between mtDNA molecules introduce errors during the repair process.

Explanation: The primary cause for the high mutation rate in mtDNA is its location within the mitochondrial matrix, adjacent to the electron transport chain. Incomplete reduction of oxygen during oxidative phosphorylation generates reactive oxygen species (ROS), which are potent mutagens that can directly damage DNA. While mtDNA repair mechanisms are also less robust than nuclear ones, the constant exposure to a high concentration of endogenous mutagens is the main driver.

Question 20

1. 16%
2. 32%
3. 50% (correct answer)
4. 84%

Explanation: The child will be unaffected if the heteroplasmy level is less than or equal to 80%. The distribution of heteroplasmy in the oocytes has a mean of 80%. For a normal distribution, the mean is equal to the median. Therefore, 50% of the distribution lies at or below the mean, and 50% lies above the mean. The probability of having a heteroplasmy level of 80% or less is 50%.