A 28-year-old woman comes to the physician for genetic counseling prior to conception. For the past year, she has had intermittent episodes of headache, nausea, abdominal pain, and tingling of her fingers. She also complains of dark urine during the episodes. Her mother and maternal uncle have similar symptoms and her father is healthy. Her husband is healthy and there is no history of serious illness in his family. Serum studies show elevated concentrations of porphobilinogen and δ-aminolevulinic acid. What is the probability of this patient having a child with the same disease as her?
Q2
A newlywed couple comes to your office for genetic counseling. Both potential parents are known to be carriers of the same Cystic Fibrosis (CF) mutation. What is the probability that at least one of their next three children will have CF if they are all single births?
Q3
A Caucasian 32-year-old woman has an uncomplicated vaginal delivery, giving birth to male and female fraternal twins at term. At 2 days of life, the twin sister develops abdominal distension without emesis, and the mother states that she has not noticed the passage of stool for this infant. Genetic testing identifies deletion of an amino acid in a membrane channel for the girl. Both parents are healthy. Assuming that twin brother's disease status/symptomatology is unclear, which of the following best approximates the probability that the twin brother is a carrier of the disease allele?
Q4
A 30-year-old man presents to his primary care physician for a routine check-up. During the appointment, he remarks that he has started noticing some thinning and hair loss without other symptoms. The physician reassures him that he is likely experiencing male-pattern baldness and explains that the condition is largely inherited. Specifically he notes that there are multiple genes that are responsible for the condition so it is difficult to predict the timing and development of hair loss. What genetic principle is being illustrated by this scenario?
Q5
A mother brings her 3-year-old daughter to the pediatrician because she is concerned about her development. She states that her daughter seemed to regress in her motor development. Furthermore, she states she has been having brief episodes of uncontrollable shaking, which has been very distressing to the family. During the subsequent work-up, a muscle biopsy is obtained which demonstrates red ragged fibers and a presumptive diagnosis of a genetic disease is made. The mother states that she has another 6-year-old son who does not seem to be affected or have any similar symptoms. What genetic term explains this phenomenon?
Q6
A scientist is trying to determine the proportion of white-eyed fruit flies in the environment. The white-eyed allele was found to be dominant to the red-eyed allele. The frequency of the red-eyed allele is 0.1. What is the proportion of flies who have white-eyes if the population is in Hardy Weinberg Equilibrium?
Q7
A healthy 29-year-old nulligravid woman comes to the physician for genetic counseling prior to conception. Her brother has a disease that has resulted in infertility, a right-sided heart, and frequent sinus and ear infections. No other family members are affected. The intended father has no history of this disease. The population prevalence of this disease is 1 in 40,000. Which of the following best represents the chance that this patient’s offspring will develop her brother's disease?
Q8
A 26-year-old woman presents to a physician for genetic counseling, because she is worried about trying to have a child. Specifically, she had 2 siblings that died young from a lysosomal storage disorder and is afraid that her own children will have the same disorder. Her background is Ashkenazi Jewish, but she says that her husband's background is mixed European heritage. Her physician says that since her partner is not of Jewish background, their chance of having a child with Niemann-Pick disease is dramatically decreased. Which of the following genetic principles best explains why there is an increased prevalence of this disease in some populations?
Q9
An 11-year-old male with light purple eyes presents with gradual loss of bilateral visual acuity. Over the past several years, vision has worsened from 20/20 to 20/100 in both eyes. He also has mild nystagmus when focusing on objects such as when he is trying to do his homework. He is diagnosed with a disease affecting melanin production in the iris. If both of his parents are unaffected, which of the following represents the most likely probabilities that another male or female child from this family would be affected by this disorder?
Q10
A 2-month-old boy is presented to the clinic for a well-child visit by his parents. They are concerned with his weak cry and difficulty with feeding. Birth history reveals that the boy was born at the 37th week of gestation by cesarean section due to poor fetal movement and fetal distress. His Apgar scores were 3 and 5 at 1st and 5th minute respectively and his birth weight was 2.5 kg (6 lb). His vital signs include heart rate 120/min, respiratory rate 40/min, blood pressure 90/50 mm Hg, and temperature 37.0°C (98.6°F). Physical examination reveals a malnourished boy with a small narrow forehead and a small jaw. His mouth is small and he has comparatively small genitals. He has a poor muscle tone. After repeated follow-up, he gains weight rapidly but his height fails to increase. Developmental milestones are delayed at the age of 3 years. Genetic testing reveals Prader-Willi syndrome. Which of the following is the most common mechanism for the development of this patient’s condition?
Population genetics principles US Medical PG Practice Questions and MCQs
Question 1: A 28-year-old woman comes to the physician for genetic counseling prior to conception. For the past year, she has had intermittent episodes of headache, nausea, abdominal pain, and tingling of her fingers. She also complains of dark urine during the episodes. Her mother and maternal uncle have similar symptoms and her father is healthy. Her husband is healthy and there is no history of serious illness in his family. Serum studies show elevated concentrations of porphobilinogen and δ-aminolevulinic acid. What is the probability of this patient having a child with the same disease as her?
A. 25%
B. 67%
C. 50% (Correct Answer)
D. 100%
Explanation: ***50%***
- This patient's symptoms (headache, nausea, abdominal pain, tingling, dark urine) and elevated **porphobilinogen** and **δ-aminolevulinic acid** are highly suggestive of **Acute Intermittent Porphyria** (AIP).
- AIP is an **autosomal dominant** disorder. Therefore, there is a **50% chance** that any child of an affected parent will inherit the disease-causing allele.
- Since her husband is healthy with no family history, he does not carry the mutation, making this a straightforward autosomal dominant inheritance calculation.
*25%*
- This probability would be expected in an **autosomal recessive** inheritance pattern when two carrier parents have a child, which is not the case here.
- It would also be the probability for an X-linked recessive disorder if the mother is a carrier and the father is unaffected, and they are discussing a son's inheritance.
- The clinical presentation and family history (mother and maternal uncle affected, consistent with autosomal dominant pattern) rule out this probability.
*67%*
- This probability is seen in specific genetic scenarios, such as the chance of a phenotypically normal sibling of an individual with an autosomal recessive disease being a carrier.
- It's not a standard probability for direct offspring of an affected individual with an autosomal dominant condition.
- This does not apply to the straightforward inheritance question being asked here.
*100%*
- This probability would occur if the disease were inherited in an **autosomal dominant** manner and the affected parent was **homozygous dominant** for the mutation.
- However, this is extremely rare in AIP, as most affected individuals are **heterozygous**.
- The family history pattern (affected mother with unaffected father having an affected child) is consistent with heterozygosity, not homozygosity.
Question 2: A newlywed couple comes to your office for genetic counseling. Both potential parents are known to be carriers of the same Cystic Fibrosis (CF) mutation. What is the probability that at least one of their next three children will have CF if they are all single births?
A. 37/64 (Correct Answer)
B. 0
C. 1/64
D. 1
E. 27/64
Explanation: ***37/64***
- The probability of a child having CF from two carrier parents is **1/4** (recessive inheritance), and the probability of a child not having CF is **3/4**.
- The probability that *none* of the three children will have CF is (3/4)³ = **27/64**. Therefore, the probability that *at least one* child will have CF is 1 - 27/64 = **37/64**.
*0*
- This option is incorrect because there is a **definite statistical probability** for a child to inherit CF when both parents are carriers.
- CF is an **autosomal recessive disorder**, meaning there is a 25% chance per child, not a 0% chance.
*1/64*
- This represents the probability that ***all three children*** would have CF: (1/4)³ = 1/64.
- This is an **underestimation** of the probability for at least one child to be affected, as the question asks about "at least one" not "all three."
*1*
- This would imply that it's an **absolute certainty** that at least one child will have CF, which is incorrect.
- Each child's outcome is independent, and there is always a chance (27/64) that none of the three children will have the disease.
*27/64*
- This calculation represents the probability that **none of the three children will have CF**: (3/4)³ = 27/64.
- This is the **complementary probability** to "at least one child having CF", not the actual answer to the question asked.
Question 3: A Caucasian 32-year-old woman has an uncomplicated vaginal delivery, giving birth to male and female fraternal twins at term. At 2 days of life, the twin sister develops abdominal distension without emesis, and the mother states that she has not noticed the passage of stool for this infant. Genetic testing identifies deletion of an amino acid in a membrane channel for the girl. Both parents are healthy. Assuming that twin brother's disease status/symptomatology is unclear, which of the following best approximates the probability that the twin brother is a carrier of the disease allele?
A. 100%
B. 67% (Correct Answer)
C. 0%
D. 50%
E. 25%
Explanation: ***67%***
- The sister's symptoms of **abdominal distension** without emesis and lack of stool passage, along with genetic testing identifying a **deletion of an amino acid in a membrane channel**, strongly suggest **Cystic Fibrosis (CF)**. CF is an **autosomal recessive disorder**.
- Since the affected twin sister has CF (genotype **aa**), and both parents are healthy, both parents must be **heterozygous carriers (Aa)**. When two carriers (Aa x Aa) have offspring, the probability of any child being a carrier (Aa) is **2/3** among the unaffected offspring. The twin brother is currently unaffected (phenotypically healthy), so the probability of him being a carrier is 2/3, or approximately 67%.
*100%*
- This would only be true if one or both parents were **homozygous affected (aa)**, or if the disease inheritance was **dominant** and the parents were carriers, which is not the case for this autosomal recessive disorder where the parents are healthy carriers and the brother is phenotypically unaffected.
- While both parents *are* carriers, the brother, being unaffected, has a chance of being **homozygous dominant (AA)**, meaning he is not a carrier.
*0%*
- This is incorrect because we know both parents are **obligate carriers** (heterozygous, Aa) for the recessive allele, given their affected child (aa). Therefore, their children have a 75% chance of inheriting at least one disease allele (50% carrier, 25% affected).
- The twin brother being unaffected means he has a 2/3 chance of being a carrier, not 0%.
*50%*
- This probability (1/2) is the chance of a child inheriting a specific allele from one parent, or the chance of being a carrier if one parent is affected and the other is homozygous dominant.
- In an **autosomal recessive** inheritance pattern where both parents are carriers (Aa x Aa) and the offspring is unaffected, the probability of being a carrier is **2/3**, not 1/2.
*25%*
- This is the probability of a child being **homozygous dominant (AA)** from two carrier parents (Aa x Aa), meaning they would neither have the disease nor be carriers.
- It is also the probability of a child being affected (aa) if both parents are carriers. Neither of these scenarios matches the question asking for the probability of the *unaffected* twin brother being a carrier.
Question 4: A 30-year-old man presents to his primary care physician for a routine check-up. During the appointment, he remarks that he has started noticing some thinning and hair loss without other symptoms. The physician reassures him that he is likely experiencing male-pattern baldness and explains that the condition is largely inherited. Specifically he notes that there are multiple genes that are responsible for the condition so it is difficult to predict the timing and development of hair loss. What genetic principle is being illustrated by this scenario?
A. Uniparental disomy
B. Polygenic inheritance (Correct Answer)
C. Pleiotropy
D. Anticipation
E. Heteroplasmy
Explanation: ***Polygenic inheritance***
- This scenario describes male-pattern baldness as being influenced by **multiple genes**, which is the definition of polygenic inheritance.
- The difficulty in predicting the timing and development of hair loss further supports polygenic inheritance, as the combined effect of several genes and environmental factors can lead to a **continuous variation** in phenotypic expression.
*Uniparental disomy*
- This refers to the inheritance of **two copies of a chromosome** or part of a chromosome from **one parent** and no copies from the other parent.
- It is typically associated with specific genetic disorders like Prader-Willi or Angelman syndromes and does not explain the general inheritance pattern of male-pattern baldness.
*Pleiotropy*
- **Pleiotropy** occurs when a **single gene** affects **multiple seemingly unrelated phenotypic traits**.
- This principle is incorrect because the scenario explicitly states that male-pattern baldness is influenced by "multiple genes," not a single gene affecting multiple traits.
*Anticipation*
- **Anticipation** is a phenomenon where the symptoms of a genetic disorder become more severe or appear earlier with each successive generation.
- This phenomenon is observed in conditions like Huntington's disease or myotonic dystrophy and is not the genetic principle described for male-pattern baldness.
*Heteroplasmy*
- **Heteroplasmy** refers to the presence of **more than one type of mitochondrial DNA** (mtDNA) within a single cell or individual.
- This principle is exclusive to mitochondrial inheritance and is not relevant to the inheritance pattern of male-pattern baldness, which is generally considered to be affected by nuclear genes.
Question 5: A mother brings her 3-year-old daughter to the pediatrician because she is concerned about her development. She states that her daughter seemed to regress in her motor development. Furthermore, she states she has been having brief episodes of uncontrollable shaking, which has been very distressing to the family. During the subsequent work-up, a muscle biopsy is obtained which demonstrates red ragged fibers and a presumptive diagnosis of a genetic disease is made. The mother states that she has another 6-year-old son who does not seem to be affected or have any similar symptoms. What genetic term explains this phenomenon?
A. Allelic heterogeneity
B. Heteroplasmy (Correct Answer)
C. Locus heterogeneity
D. Phenotypic heterogeneity
E. Genetic heterogeneity
Explanation: ***Heteroplasmy***
- **Heteroplasmy** refers to the presence of more than one type of mitochondrial DNA (mtDNA) within the same cell or individual. The varying proportions of mutated mtDNA can lead to a wide spectrum of disease severity, explaining why the daughter is severely affected while her brother is unaffected.
- The daughter's symptoms (developmental regression, seizures), combined with the muscle biopsy showing **red ragged fibers**, are characteristic of **mitochondrial disorders**, which are often maternally inherited and can manifest with variable expressivity due to heteroplasmy.
*Allelic heterogeneity*
- **Allelic heterogeneity** occurs when different mutations at the *same gene locus* can cause the same disease.
- This term does not explain the differential severity or presence of disease in siblings with a maternally inherited mitochondrial disorder, as it focuses on different mutations within a single gene rather than varying proportions of mutated mitochondria.
*Locus heterogeneity*
- **Locus heterogeneity** describes situations where mutations in *different genes* can cause the same disease phenotype.
- This concept is not applicable here because the clinical picture and red ragged fibers point towards a specific mitochondrial disorder, and the siblings would typically be expected to share the same genetic locus underlying their condition if it were present.
*Phenotypic heterogeneity*
- **Phenotypic heterogeneity** refers to different clinical manifestations or phenotypes resulting from mutations in the *same gene*.
- While there is varying severity of the disease (different phenotypes) between the siblings, this term doesn't specifically explain the underlying *genetic mechanism* of varying mitochondrial mutation loads, which is crucial for mitochondrial disorders.
*Genetic heterogeneity*
- **Genetic heterogeneity** is a broad term encompassing both allelic and locus heterogeneity, meaning that a single phenotype can be caused by mutations at different loci (locus heterogeneity) or by different mutations within the same gene (allelic heterogeneity).
- While mitochondrial disorders can exhibit genetic heterogeneity, this general term doesn't specifically address the mechanism of variable penetrance and expressivity seen in mitochondrial inheritance due to the varying proportions of mutated mtDNA within cells, which is uniquely explained by heteroplasmy.
Question 6: A scientist is trying to determine the proportion of white-eyed fruit flies in the environment. The white-eyed allele was found to be dominant to the red-eyed allele. The frequency of the red-eyed allele is 0.1. What is the proportion of flies who have white-eyes if the population is in Hardy Weinberg Equilibrium?
A. 1%
B. 81%
C. 18%
D. 10%
E. 99% (Correct Answer)
Explanation: ***99%***
- In **Hardy-Weinberg equilibrium**, the frequencies of alleles and genotypes remain constant from generation to generation. The frequency of the dominant allele (W) is represented by 'p', and the frequency of the recessive allele (w) is represented by 'q'.
- Given that the **red-eyed allele is recessive** (w) and has a frequency of **q = 0.1**, then the frequency of the **white-eyed allele (W)**, which is dominant, is **p = 1 - q = 1 - 0.1 = 0.9**.
- The proportion of the population with white eyes includes homozygous dominant individuals (WW) and heterozygous individuals (Ww).
- The genotype frequencies are: WW = p² = (0.9)² = 0.81, and Ww = 2pq = 2(0.9)(0.1) = 0.18.
- Therefore, the proportion of white-eyed flies is **p² + 2pq = 0.81 + 0.18 = 0.99**, or **99%**.
*1%*
- This represents the frequency of the **homozygous recessive genotype (ww)**, which would be (0.1)² = 0.01 or 1%.
- Flies with the **ww genotype** would have **red eyes**, not white eyes.
*81%*
- This represents the frequency of the **homozygous dominant genotype (WW)**, which is p² = (0.9)² = 0.81 or 81%.
- However, white-eyed flies also include **heterozygous individuals (Ww)**, so 81% is an underestimation of the total proportion of white-eyed flies.
*18%*
- This represents the frequency of the **heterozygous genotype (Ww)**, which is 2pq = 2(0.9)(0.1) = 0.18 or 18%.
- This includes only part of the white-eyed population and does not account for the **homozygous dominant (WW) individuals**.
*10%*
- This represents the frequency of the **recessive allele (q)**, which is 0.1 or 10%.
- This is an allele frequency, not a **genotype or phenotype frequency** in the population.
Question 7: A healthy 29-year-old nulligravid woman comes to the physician for genetic counseling prior to conception. Her brother has a disease that has resulted in infertility, a right-sided heart, and frequent sinus and ear infections. No other family members are affected. The intended father has no history of this disease. The population prevalence of this disease is 1 in 40,000. Which of the following best represents the chance that this patient’s offspring will develop her brother's disease?
A. 25%
B. 66%
C. 0.2% (Correct Answer)
D. 0.7%
E. 1%
Explanation: ***0.2%***
- The brother's symptoms (infertility, right-sided heart, frequent infections) are characteristic of **Kartagener syndrome**, a form of **primary ciliary dyskinesia (PCD)**, which has an **autosomal recessive** inheritance pattern.
- Since the patient's parents are obligate heterozygotes (carriers), the patient has a 2/3 chance of being a carrier. Given the population prevalence of 1/40,000 for an autosomal recessive disease, the carrier frequency (2pq) is approximately **2 x sqrt(1/40,000) = 2 x 1/200 = 1/100**. The chance of her child inheriting the disease is (2/3 chance of patient being carrier) x (1/100 chance of partner being carrier) x (1/4 chance of affected offspring) = 2/1200 ≈ **0.00166 or 0.166%**, which is closest to 0.2%.
*25%*
- This would be the risk if both parents were known carriers, and it represents the chance of an affected offspring from two heterozygotes.
- In this scenario, the woman's partner's carrier status is unknown and based on population prevalence, making the overall risk much lower.
*66%*
- This is the probability that the patient (the healthy sister of an affected individual with an autosomal recessive disease) is a **carrier**.
- This value alone does not account for the partner's carrier status or the final Mendelian inheritance probability (1/4) for an affected child.
*0.7%*
- This percentage is too high; it might result from incorrect calculation of the population carrier frequency or misapplication of probabilities.
- The correct carrier frequency for the partner is 1/100, which is significantly lower than what would lead to a 0.7% final risk.
*1%*
- This value is also too high and likely results from a miscalculation of either the carrier frequency or the overall probability.
- A 1% chance would suggest a much higher population carrier frequency or a different inheritance scenario.
Question 8: A 26-year-old woman presents to a physician for genetic counseling, because she is worried about trying to have a child. Specifically, she had 2 siblings that died young from a lysosomal storage disorder and is afraid that her own children will have the same disorder. Her background is Ashkenazi Jewish, but she says that her husband's background is mixed European heritage. Her physician says that since her partner is not of Jewish background, their chance of having a child with Niemann-Pick disease is dramatically decreased. Which of the following genetic principles best explains why there is an increased prevalence of this disease in some populations?
A. Natural selection
B. Imprinting
C. De novo mutations
D. Gene flow
E. Founder effect (Correct Answer)
Explanation: ***Founder effect***
- The **founder effect** occurs when a new population is established by a small number of individuals, leading to a **reduced genetic diversity** and an increased frequency of certain alleles that were present in the founders. This is particularly relevant in populations like **Ashkenazi Jews**, who descended from a small, isolated group with certain allele frequencies.
- In this scenario, the high prevalence of **Niemann-Pick disease** (and other genetic disorders) in the Ashkenazi Jewish population is due to their historical isolation and intermarriage within a relatively small gene pool, trapping and concentrating certain alleles.
*Natural selection*
- **Natural selection** typically describes the process by which traits that enhance survival and reproduction become more common in a population over time, or deleterious traits become less common.
- While it can influence disease prevalence, it doesn't primarily explain the disproportionately high frequency of rare recessive disorders in specific isolated populations in the manner described.
*Imprinting*
- **Genomic imprinting** refers to the phenomenon where certain genes are expressed in a **parent-of-origin-specific manner**, meaning that only the allele inherited from either the mother or the father is expressed.
- This mechanism explains certain genetic conditions but does not account for the increased prevalence of a recessive disorder due to population history and isolation.
*De novo mutations*
- **De novo mutations** are new genetic alterations that appear for the first time in an individual and are not inherited from either parent.
- While de novo mutations are a source of genetic variation, they do not explain the high prevalence of a specific ancestral allele within an entire population.
*Gene flow*
- **Gene flow** (or migration) is the transfer of genetic material from one population to another, which tends to **decrease genetic differences** between populations and introduce new alleles.
- This principle would suggest a *reduction* in the prevalence of specific rare alleles over time as populations mix, rather than an *increase* in isolated groups.
Question 9: An 11-year-old male with light purple eyes presents with gradual loss of bilateral visual acuity. Over the past several years, vision has worsened from 20/20 to 20/100 in both eyes. He also has mild nystagmus when focusing on objects such as when he is trying to do his homework. He is diagnosed with a disease affecting melanin production in the iris. If both of his parents are unaffected, which of the following represents the most likely probabilities that another male or female child from this family would be affected by this disorder?
A. Male: 25% Female: 25%
B. Same as general population
C. Male: 50% Female: 50%
D. Male: 50% Female: 0% (Correct Answer)
E. Male: 100% Female: 0%
Explanation: ***Male: 50% Female: 0%***
- The symptoms (light purple eyes, gradual vision loss, nystagmus, defective melanin production) are characteristic of **ocular albinism**. This condition is typically **X-linked recessive**.
- If the patient's mother is a **carrier** (XAXa) and the father is unaffected (XAY), there is a **50% chance** that a male child will inherit the affected X chromosome (XaY) and thus be affected, and a **0% chance** for a female child to be affected if the father is unaffected (all female children would either be carriers XAXa or unaffected XAXA).
*Male: 25% Female: 25%*
- This probability pattern would typically suggest an **autosomal recessive** inheritance pattern, where both parents are carriers (Aa x Aa), and there is a 25% chance for any child to be affected regardless of sex.
- However, ocular albinism most commonly follows an X-linked recessive pattern, and the described clinical features (e.g., light purple eyes due to melanin defect in the iris) are highly suggestive of ocular albinism.
*Same as general population*
- This would only be true if the disorder was not genetic or if the parents' carrier status did not increase the risk for subsequent children.
- Given the heritable nature of albinism and the specific family history (parents unaffected, one affected child), the risk for subsequent children is significantly higher than the general population.
*Male: 50% Female: 50%*
- This pattern would occur in an **autosomal dominant** disorder with 100% penetrance, where one parent is affected (Aa x aa), or in some specific scenarios of X-linked inheritance if the father was affected and the mother was a carrier.
- Ocular albinism is X-linked recessive, not autosomal dominant, and the father is stated to be unaffected.
*Male: 100% Female: 0%*
- This genetic pattern is highly unlikely unless the mother was fully mosaic for the condition or an extremely rare and specialized inheritance pattern was at play.
- In a typical X-linked recessive inheritance with an unaffected father and a carrier mother, there is always a 50% chance for a male child to be unaffected.
Question 10: A 2-month-old boy is presented to the clinic for a well-child visit by his parents. They are concerned with his weak cry and difficulty with feeding. Birth history reveals that the boy was born at the 37th week of gestation by cesarean section due to poor fetal movement and fetal distress. His Apgar scores were 3 and 5 at 1st and 5th minute respectively and his birth weight was 2.5 kg (6 lb). His vital signs include heart rate 120/min, respiratory rate 40/min, blood pressure 90/50 mm Hg, and temperature 37.0°C (98.6°F). Physical examination reveals a malnourished boy with a small narrow forehead and a small jaw. His mouth is small and he has comparatively small genitals. He has a poor muscle tone. After repeated follow-up, he gains weight rapidly but his height fails to increase. Developmental milestones are delayed at the age of 3 years. Genetic testing reveals Prader-Willi syndrome. Which of the following is the most common mechanism for the development of this patient’s condition?
A. Anticipation
B. Heteroplasmy
C. Incomplete penetrance
D. Maternal uniparental disomy
E. Paternal deletion of 15q11-q13 (Correct Answer)
Explanation: ***Paternal deletion of 15q11-q13***
- This is the **most common genetic mechanism** (occurring in about 70-75% of cases) for Prader-Willi syndrome, involving the loss of genetic material from the paternally inherited chromosome 15 in the specified region
- The deletion affects genes that are **normally expressed only from the paternal chromosome** due to genomic imprinting, leading to the characteristic features of hypotonia, feeding difficulties in infancy, subsequent hyperphagia with obesity, hypogonadism, and developmental delays
*Incorrect: Anticipation*
- Anticipation describes a genetic phenomenon where a disorder appears earlier or symptoms become more severe with each successive generation
- This is typically seen in disorders caused by expanding **trinucleotide repeats** (e.g., Huntington's disease, myotonic dystrophy), not applicable to Prader-Willi syndrome
*Incorrect: Heteroplasmy*
- Heteroplasmy refers to the presence of more than one type of mitochondrial DNA within a cell or individual
- This concept is relevant to **mitochondrial genetic disorders** which are maternally inherited, not to Prader-Willi syndrome which is a nuclear chromosomal imprinting disorder
*Incorrect: Incomplete penetrance*
- Incomplete penetrance occurs when individuals carrying a pathogenic mutation do not express the associated clinical phenotype
- Prader-Willi syndrome typically presents with a **consistent set of features** when the genetic defect is present; incomplete penetrance is not the mechanism of disease development
*Incorrect: Maternal uniparental disomy*
- Maternal uniparental disomy (UPD) of chromosome 15 is the **second most common mechanism** for Prader-Willi syndrome (occurring in about 20-25% of cases)
- This involves inheriting **both copies of chromosome 15 from the mother** and none from the father, leading to absence of paternal gene expression in the critical 15q11-q13 region
- While less common than paternal deletion, this is still a significant cause of the syndrome
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