USMLE Step 1 Quiz
Practice Mitochondrial And Non Mendelian Inheritance in USMLE Step 1 with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.
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A 10-year-old boy presents with ptosis, ophthalmoplegia, and muscle weakness. A muscle biopsy shows ragged-red fibers. His mother has mild hearing loss and exercise intolerance, but his father is asymptomatic. Molecular testing confirms a large deletion in the mitochondrial DNA.
If this boy grows up and has children with an unaffected woman, what is the likelihood he will pass the mitochondrial DNA deletion to them?
This quiz focuses on Mitochondrial And Non Mendelian Inheritance, giving you a quick way to practice the rules, question types, and explanations that matter most for USMLE Step 1.
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.
A 10-year-old boy presents with ptosis, ophthalmoplegia, and muscle weakness. A muscle biopsy shows ragged-red fibers. His mother has mild hearing loss and exercise intolerance, but his father is asymptomatic. Molecular testing confirms a large deletion in the mitochondrial DNA.
If this boy grows up and has children with an unaffected woman, what is the likelihood he will pass the mitochondrial DNA deletion to them?
Explanation: Mitochondrial DNA is inherited exclusively from the mother via the cytoplasm of the egg cell. A male's mitochondria, located in the midpiece of the sperm, do not enter the ovum during fertilization or are actively degraded. Consequently, a male cannot transmit his mitochondrial DNA to any of his offspring. The risk of transmission from an affected father is 0%.
A 19-year-old woman is evaluated for a multisystem disorder including myopathy, seizures, ataxia, and dementia. A diagnosis of a mitochondrial disease is made. Her physician explains that the presence of both normal and mutated mitochondrial DNA within her cells contributes to her clinical picture.
What is the term for the co-existence of two or more mitochondrial DNA populations within a single individual?
Explanation: Heteroplasmy is the term used to describe the presence of a mixture of more than one type of mitochondrial DNA (mtDNA) within a cell or individual. In the context of mitochondrial disease, this usually refers to a mix of wild-type (normal) and mutant mtDNA. The ratio of mutant to normal mtDNA can vary among tissues and determines the severity of the disease, a key feature of mitochondrial inheritance.
A research study is investigating the molecular basis of genomic imprinting. The researchers are focusing on the imprinting control region (ICR) for the gene cluster associated with Prader-Willi and Angelman syndromes on chromosome 15.
Which of the following molecular modifications is the primary mechanism responsible for establishing and maintaining the parent-specific imprinting of these genes?
Explanation: Genomic imprinting is an epigenetic process that involves the differential marking of genes based on their parental origin. The primary molecular mechanism for establishing and maintaining these imprints is DNA methylation. Specific CpG islands within imprinting control regions (ICRs) are methylated in a sex-specific manner during gametogenesis. This methylation pattern is then maintained in somatic cells after fertilization, leading to the silencing of either the maternal or paternal allele.
A 3-year-old is diagnosed with Angelman syndrome due to a mutation in the UBE3A gene that renders it non-functional. The mutation was inherited from his mother, who is asymptomatic. His father has a normal UBE3A gene. The mother's father (maternal grandfather) was also affected with the same symptoms as the child.
The mother is asymptomatic despite carrying the mutation because the UBE3A gene is subject to tissue-specific genomic imprinting. In most of her somatic tissues, which of the following is true?
Explanation: UBE3A shows tissue-specific imprinting. In most somatic tissues, both maternal and paternal alleles are expressed, so the mother's functional paternal allele compensates for her mutated maternal allele. However, in specific neurons of the brain, the paternal allele is silenced (imprinted), and only the maternal allele is expressed. The child inherited the mother's mutated maternal allele, and in his brain neurons, the paternal UBE3A is silenced, leaving no functional UBE3A and causing Angelman syndrome.
A 16-year-old girl is evaluated for recurrent seizures and muscle weakness. She reports frequent, jerky muscle movements, especially in the mornings. Her mother and two older siblings have similar, but less severe, symptoms. A muscle biopsy is performed and microscopic examination reveals abnormally shaped mitochondria accumulating under the sarcolemma of muscle fibers, which stain intensely red with Gomori trichrome stain.
The variable severity of the disease among the affected family members is best explained by which of the following concepts?
Explanation: The patient's symptoms (myoclonus, seizures, myopathy) and the presence of 'ragged-red fibers' on muscle biopsy are characteristic of Myoclonic Epilepsy with Ragged-Red Fibers (MERRF), a mitochondrial disorder. Mitochondrial diseases exhibit variable expressivity due to heteroplasmy. Heteroplasmy is the presence of a mixed population of normal and mutant mitochondrial DNA (mtDNA) within a single cell. The severity of the disease in an individual depends on the proportion of mutant to wild-type mtDNA in different tissues, which can vary significantly among family members due to random segregation of mitochondria during oogenesis and cell division.
A 24-year-old man presents with a 3-month history of progressive, painless, bilateral vision loss. His medical history is unremarkable. Family history reveals that his mother and maternal uncle have similar vision problems that started in their 20s. His father and paternal grandparents are unaffected. Examination shows central scotomas and optic disc pallor. Genetic analysis is pursued to confirm the suspected diagnosis.
The genetic abnormality responsible for this patient's condition is most likely inherited through which of the following mechanisms?
Explanation: This patient's presentation with subacute bilateral optic neuropathy (Leber hereditary optic neuropathy, LHON) and a family history consistent with maternal inheritance (affected mother, maternal uncle, but unaffected father) strongly suggests a mitochondrial disorder. Mitochondrial DNA (mtDNA) is inherited exclusively from the mother, as the ovum contributes the cytoplasm and mitochondria to the zygote. Therefore, affected mothers pass the mutation to all their offspring, while affected fathers do not.
A 6-month-old infant is brought to the pediatrician for poor feeding and failure to thrive. The parents report that he has been a 'floppy' baby since birth. On examination, he has significant central hypotonia and undescended testes. The physician explains that if the infant survives, he is likely to develop an insatiable appetite and obesity in early childhood.
This disorder is most commonly caused by a deletion in the paternally derived chromosome 15. A less common cause, resulting in the same phenotype, is which of the following?
Explanation: The clinical presentation is classic for Prader-Willi syndrome (PWS). PWS is an imprinting disorder caused by the loss of expression of genes that are normally transcribed only from the paternal chromosome 15. The most common cause is a deletion on the paternal chromosome 15. The second most common cause is maternal uniparental disomy (UPD), where the individual inherits two copies of chromosome 15 from the mother and none from the father. Since the relevant genes are imprinted (silenced) on the maternal chromosome, inheriting two maternal copies results in the same lack of gene expression as a paternal deletion.
A geneticist is counseling a couple. The man has Leber hereditary optic neuropathy (LHON), a well-characterized mitochondrial disorder. His partner is unaffected and has no family history of the disease. They are concerned about the risk of passing the condition to their future children.
What is the probability that their son will be affected by this disorder?
Explanation: Mitochondrial disorders are transmitted exclusively through the maternal line. This is because the zygote's mitochondria are derived entirely from the ovum; the sperm's mitochondria are typically destroyed after fertilization. Therefore, an affected male cannot pass a mitochondrial DNA mutation to any of his children, regardless of their sex. The probability of his son or daughter inheriting the disorder from him is 0%.
A 4-year-old girl is diagnosed with Angelman syndrome. Karyotype is normal. Further molecular analysis shows that she has inherited two copies of chromosome 15 from her father and no copy from her mother.
Which of the following genetic events best describes the cause of this patient's condition?
Explanation: Angelman syndrome results from the lack of expression of the maternal UBE3A gene. In this case, the child has inherited both copies of chromosome 15 from her father and none from her mother. This is known as paternal uniparental disomy (UPD). Because the paternal copy of UBE3A is normally imprinted (silenced) in the brain, the child has no functional UBE3A protein, leading to the Angelman syndrome phenotype. This is the second most common cause of AS after maternal deletion.
A neonate is born with a large tongue (macroglossia), an abdominal wall defect (omphalocele), and is significantly larger than average for gestational age (macrosomia). The pediatrician suspects Beckwith-Wiedemann syndrome, an overgrowth disorder.
This condition is often associated with dysregulation of imprinted genes on chromosome 11. This illustrates that non-Mendelian inheritance can result from which of the following?
Explanation: Beckwith-Wiedemann syndrome is an imprinting disorder, typically involving genes on the short arm of chromosome 11 (11p15), such as IGF2 and CDKN1C. The core pathogenetic mechanism is the dysregulation of parent-of-origin specific gene expression. For example, loss of maternal imprinting (silencing) of the IGF2 gene (a growth promoter) can lead to biallelic expression and overgrowth. This highlights how genomic imprinting, a form of non-Mendelian inheritance, dictates that the effect of a gene depends on whether it was inherited from the mother or the father.
A 30-year-old woman with a known mitochondrial myopathy is concerned about her future children. She experiences exercise intolerance and muscle weakness. Her condition is caused by a point mutation in a mitochondrial tRNA gene. She has a brother who is severely affected and a sister who is nearly asymptomatic.
Which of the following statements most accurately describes the inheritance pattern and risk for her offspring?
Explanation: Mitochondrial DNA is inherited maternally, so an affected mother will pass the mutation to all of her offspring (both sons and daughters). However, the clinical severity of mitochondrial disorders is highly variable due to heteroplasmy—the presence of both mutant and wild-type mtDNA. The proportion of mutant mtDNA inherited by each child is random and unpredictable, leading to a wide range of clinical phenotypes, as seen in her own family. Thus, all her children will inherit the mutation, but their clinical severity cannot be precisely predicted.
A cell biologist notes that disorders arising from mutations in mitochondrial DNA predominantly affect tissues with high metabolic rates, such as the nervous system, heart, and skeletal muscle.
This tissue-specific vulnerability is primarily due to the dependence of these cells on which of the following processes?
Explanation: Mitochondria are the primary sites of oxidative phosphorylation (the electron transport chain and ATP synthase), the most efficient process for generating ATP. Tissues like the brain, heart, and skeletal muscle have extremely high energy (ATP) demands to maintain ion gradients, contract, and perform their functions. Mutations in mitochondrial DNA often impair the components of the electron transport chain, leading to a severe deficit in ATP production. This energy failure disproportionately affects high-demand tissues, explaining the characteristic clinical manifestations of mitochondrial diseases.
Prader-Willi syndrome and Angelman syndrome are distinct clinical disorders that both map to the same chromosomal region, 15q11-q13. The different phenotypes depend on the parental origin of the genetic defect.
Which of the following correctly pairs the syndrome with the parental origin of the defective chromosome?
Explanation: This question tests the core concept of imprinting for these two classic syndromes. Prader-Willi syndrome (PWS) is caused by the loss of gene expression from the paternally inherited chromosome 15. Angelman syndrome (AS) is caused by the loss of gene expression from the maternally inherited chromosome 15. Therefore, the distinct phenotypes arise from the same chromosomal locus but depend entirely on which parent's genetic contribution is missing or silenced.
A genetic phenomenon known as uniparental disomy (UPD) occurs when an individual receives two copies of a chromosome, or part of a chromosome, from one parent and no copies from the other. UPD can lead to disease if the chromosome involved contains imprinted genes.
UPD most commonly arises from which of the following mechanisms?
Explanation: The most common mechanism leading to uniparental disomy is 'trisomic rescue.' This process begins with a nondisjunction event during meiosis, leading to a trisomic zygote (containing three copies of a particular chromosome). In an attempt to correct the aneuploidy, the early embryo may randomly eject one of the three chromosomes. If, by chance, the ejected chromosome is the one from the parent who contributed a single copy, the remaining two chromosomes will both be from the other parent, resulting in UPD.
A family presents for genetic counseling. A pedigree analysis reveals that an affected mother passes the trait to all of her children, both male and female. However, affected fathers never pass the trait to any of their children. The severity of the condition varies widely among affected individuals in the family.
This pattern of inheritance is most characteristic of a mutation located in which of the following?
Explanation: The described inheritance pattern is classic for a mitochondrial disorder. Key features are: 1) Maternal inheritance: The trait is passed from a mother to ALL of her offspring because mitochondria are inherited via the egg's cytoplasm. 2) Lack of paternal inheritance: An affected father does not pass the trait to his children. 3) Variable expressivity (variable severity): This is due to heteroplasmy, the random segregation of mitochondria during cell division.
A newborn is evaluated for severe intrauterine growth restriction, poor postnatal growth, and a prominent forehead. He has normal intelligence. Genetic analysis reveals hypomethylation at an imprinting control region on chromosome 11, which he inherited from his father.
This condition, characterized by undergrowth, is an example of an imprinting disorder that is functionally opposite to Beckwith-Wiedemann syndrome. What is the most likely diagnosis?
Explanation: Russell-Silver syndrome (RSS) is an imprinting disorder characterized by prenatal and postnatal growth retardation. It is often considered the molecular opposite of the overgrowth disorder Beckwith-Wiedemann syndrome (BWS). One of the primary causes of RSS is hypomethylation of the imprinting control region 1 (ICR1) on the paternal chromosome 11, leading to decreased expression of the growth promoter IGF2. This contrasts with BWS, which can be caused by hypermethylation of the same region, leading to increased IGF2 expression.
A scientist is studying mitochondrial DNA (mtDNA) and comparing it to nuclear DNA (nDNA). She notes several key differences that influence the mutation rate and inheritance patterns of mitochondrial diseases.
Compared to nuclear DNA, mitochondrial DNA has which of the following characteristics?
Explanation: Mitochondrial DNA has a significantly higher mutation rate than nuclear DNA. This is attributed to two main factors: 1) Proximity to the electron transport chain, which produces damaging reactive oxygen species, and 2) A less robust and efficient DNA repair system compared to the nucleus. mtDNA is a circular molecule, lacks introns, is not packaged with histones, and is inherited maternally.
A 2-year-old boy is diagnosed with Prader-Willi syndrome caused by a 4-Mb deletion on his paternally inherited chromosome 15. His parents, who are both healthy, are concerned about the risk of having another affected child.
Assuming this deletion is a de novo event in the father's germline, what is the recurrence risk for this couple's future pregnancies?
Explanation: The majority of deletions causing Prader-Willi syndrome are sporadic, de novo (new) events that occur during gametogenesis in the parent. If the deletion is de novo and the father does not have a chromosomal rearrangement that predisposes to it (which is the most common scenario), the recurrence risk is very low, generally quoted as less than 1%. This small risk accounts for the rare possibility of germline mosaicism in the father.
A child is diagnosed with a genetic disorder caused by inheriting two identical copies of chromosome 7 from his mother, a carrier for cystic fibrosis. The child has cystic fibrosis despite his father having two normal alleles for the CFTR gene.
This scenario, where an autosomal recessive condition is expressed due to inheriting two chromosomes from a single parental source, is an example of which mechanism?
Explanation: This is an example of uniparental disomy (UPD) leading to an autosomal recessive disease. Specifically, it is uniparental isodisomy, where the individual inherits two identical copies of one of the parent's homologous chromosomes (in this case, the mother's chromosome 7 carrying the CFTR mutation). The mother is a carrier, but because the child inherited two copies of her mutant-carrying chromosome and no copy from the father, the child is effectively homozygous for the mutation and expresses the disease.
A patient with a mitochondrial disorder exhibits severe muscle weakness but has relatively normal cognitive function. Her sister, who carries the same mitochondrial DNA mutation, has debilitating seizures and dementia but only mild muscle weakness.
This difference in clinical presentation between the two sisters is best explained by variation in which of the following?
Explanation: The variable expressivity of mitochondrial diseases is largely due to heteroplasmy. During oogenesis and subsequent embryonic development, mitochondria are segregated randomly into daughter cells. This can lead to different proportions of mutant mtDNA in different tissues. The first sister likely has a higher burden of mutant mitochondria in her skeletal muscle, while the second sister has a higher burden in her central nervous system. This variation in tissue distribution of the mutation load accounts for their different clinical phenotypes despite having the same underlying mutation.