1. A scientist is analyzing a pedigree chart to understand the inheritance pattern of a particular disease. Which of the following features in the pedigree chart strongly suggests mitochondrial inheritance?
A. Affected males transmit the disease to all their offspring
B. The disease appears to skip generations.
C. Affected females transmit the disease to all their offspring
D. The disease is predominantly observed in males
E. The disease is equally prevalent in males and females
The correct answer is C. Affected females transmit the disease to all their offspring.
Explanation:
Mitochondrial Inheritance:
Mitochondrial inheritance refers to the transmission of genetic material exclusively through the mitochondria, which are inherited from the mother. This pattern is unique because only females pass mitochondrial DNA to their offspring, while males do not transmit mitochondrial traits to their children.
Key Feature: Affected females transmit the disease to all their offspring, regardless of the offspring’s sex. This occurs because mitochondria are present in the egg cytoplasm and are inherited maternally.
Why Other Options Are Incorrect:
A. Affected males transmit the disease to all their offspring: This does not happen in mitochondrial inheritance because males do not pass on their mitochondria to their children. If an affected male transmits the disease, it would suggest an autosomal or X-linked dominant inheritance pattern, not mitochondrial.
B. The disease appears to skip generations: Mitochondrial inheritance does not skip generations unless the disease exhibits incomplete penetrance. Skipping generations is more typical of autosomal recessive inheritance, where carriers may not show symptoms.
D. The disease is predominantly observed in males: Mitochondrial diseases affect males and females equally because all offspring of an affected mother inherit the mutated mitochondria. The predominance in males is more characteristic of X-linked recessive inheritance.
E. The disease is equally prevalent in males and females: While mitochondrial diseases do affect both sexes equally, this feature alone does not strongly suggest mitochondrial inheritance. This pattern is also observed in autosomal inheritance.
Key Point to Remember:
The hallmark of mitochondrial inheritance is that affected females transmit the disease to all their offspring, while affected males do not pass it on to their children. This unique maternal inheritance pattern is the most definitive clue.
2. A patient presents with a mitochondrial disease that exhibits a range of symptom severity among affected family members. What is the most likely explanation for this variation in disease presentation?
A. Incomplete penetrance of the disease-causing mutation
B. Variable expressivity of the disease-causing mutation
C. Heteroplasmy in the mitochondrial DNA
D. Presence of multiple disease-causing mutations
E. Environmental factors influencing disease severity
The correct answer is C. Heteroplasmy in the mitochondrial DNA.
Explanation:
Heteroplasmy and Its Role in Mitochondrial Disease: Heteroplasmy refers to the coexistence of both normal (wild-type) and mutated mitochondrial DNA (mtDNA) within the same cell.
• The proportion of mutated mtDNA can vary among different tissues and family members due to random segregation of mitochondria during cell division.
• This variation in the proportion of mutated mtDNA influences the severity of mitochondrial diseases.
A higher proportion of mutated mtDNA leads to more severe symptoms, while a lower proportion results in milder or no symptoms.
Why Other Options Are Incorrect:
A. Incomplete penetrance of the disease-causing mutation: Incomplete penetrance means that some individuals with the mutation may not show any symptoms. While incomplete penetrance can occur, it does not explain the variability in severity among affected individuals.
B. Variable expressivity of the disease-causing mutation: Variable expressivity describes differences in the manifestation of symptoms among individuals with the same mutation. While this term could partially describe the scenario, it is not specific to mitochondrial diseases and does not explain the molecular basis of the variation (i.e., heteroplasmy).
D. Presence of multiple disease-causing mutations: Multiple mutations could lead to complex phenotypes, but this is not the primary explanation for variability within a family for a single mitochondrial disease.
E. Environmental factors influencing disease severity: Environmental factors can modulate symptoms, but they are not the primary cause of variability in mitochondrial diseases, which is predominantly driven by heteroplasmy.
Key Point:
Heteroplasmy is the most likely explanation for the range of symptom severity in mitochondrial diseases, as it accounts for the variability in the proportion of mutated mtDNA among different individuals and tissues.
3. A population is in Hardy-Weinberg equilibrium for a gene with two alleles, A and a. The frequency of the A allele is 0.7. What is the expected frequency of heterozygotes (Aa) in this population?
A. 0.09
B. 0.21
C. 0.42
D. 0.49
E. 0.70
The correct answer is C. 0.42.
Explanation: The Hardy-Weinberg equation (p² + 2pq + q² = 1) can be used to calculate the expected genotype frequencies in a population that meets the assumptions of the principle. In this case, the frequency of the A allele (p) is 0.7, and the frequency of the a allele (q) is 0.3 (since p + q = 1). The expected frequency of heterozygotes (Aa), represented by 2pq, is 2 x 0.7 x 0.3 = 0.42.
Why Other Options Are Incorrect:
A. 0.09: This represents q2 (homozygous recessive frequency), not 2pq.
B. 0.21: This is incorrect and does not align with 2pq for the given p and q.
D. 0.49: This represents p2(homozygous dominant frequency), not 2pq.
E. 0.70: This is the frequency of the dominant allele (p), not the heterozygotes.
4. Which of the following conditions would most likely lead to a deviation from Hardy-Weinberg equilibrium in a population?
A. Non-random mating
B. A large population size
C. The absence of mutations
D. No migration into or out of the population
E. No selection for or against any genotype
The correct answer is A. Non-random mating.
Explanation:
The Hardy-Weinberg equilibrium assumes the following five conditions for a population to remain in genetic equilibrium:
1. Random mating.
2. A large population size.
3. No mutations.
4. No migration (gene flow).
5. No natural selection.
When any of these assumptions is violated, the population may deviate from Hardy-Weinberg equilibrium.
Analysis of Options:
A. Non-random mating: Non-random mating occurs when individuals in a population do not mate randomly (e.g., inbreeding or assortative mating). This can alter genotype frequencies (e.g., increase homozygosity in inbreeding) without affecting allele frequencies, leading to deviations from Hardy-Weinberg equilibrium.
B. A large population size: A large population size supports the Hardy-Weinberg equilibrium because it minimizes the effects of genetic drift. Small populations, not large ones, are more prone to random changes in allele frequencies due to drift.
C. The absence of mutations: The absence of mutations is an assumption of Hardy-Weinberg equilibrium. Mutations, if present, can introduce new alleles, causing deviations. However, the absence of mutations does not lead to deviations.
D. No migration into or out of the population: The lack of migration maintains equilibrium by preventing gene flow that introduces or removes alleles from the population. This condition supports equilibrium.
E. No selection for or against any genotype: The absence of selection is a key assumption of Hardy-Weinberg equilibrium. Selection would alter allele frequencies, but the absence of selection ensures equilibrium.
Conclusion:
Non-random mating (A) most likely leads to a deviation from Hardy-Weinberg equilibrium because it disrupts the expected genotype frequencies by favoring certain matings over others.
5. A genetic study examining a particular trait in a population finds that the observed genotype frequencies differ significantly from the expected frequencies based on the Hardy-Weinberg principle. What can be concluded from this finding?
A. The population is evolving
B. The trait is not determined by a single gene
C. The trait is under strong selective pressure
D. One or more of the assumptions of the Hardy-Weinberg principle are not met
E. The population is in equilibrium for this trait
The correct answer is D. One or more of the assumptions of the Hardy-Weinberg principle are not met. This is the most accurate conclusion. Deviations indicate that one or more conditions of the Hardy-Weinberg equilibrium (e.g., random mating, no selection, large population size) are not being fulfilled.
Explanation:
The Hardy-Weinberg principle provides a framework for predicting genotype frequencies in a population under equilibrium. The principle assumes:
1. Random mating.
2. A large population size.
3. No mutations.
4. No migration (gene flow).
5. No natural selection.
Analysis of Options:
A. The population is evolving: While a deviation from Hardy-Weinberg equilibrium could indicate evolutionary processes (e.g., natural selection, genetic drift), the term “evolving” is too broad. The specific cause of the deviation needs to be identified.
B. The trait is not determined by a single gene: The Hardy-Weinberg principle applies to single-gene traits. Deviations from expected frequencies do not necessarily mean the trait involves multiple genes; it means the assumptions are not met.
C. The trait is under strong selective pressure: Selection is one possible reason for the deviation, but there are other factors (e.g., non-random mating, mutation, migration). Without additional data, this conclusion cannot be definitively made.
E. The population is in equilibrium for this trait: This is incorrect, as the observed deviation from expected frequencies directly indicates the population is not in equilibrium.
Conclusion:
Deviations from Hardy-Weinberg equilibrium suggest that one or more assumptions of the principle are being violated. Answer D is the most appropriate conclusion.
6. In a population in Hardy-Weinberg equilibrium, the frequency of an autosomal recessive disorder is 1 in 400 individuals. What is the approximate carrier frequency for this disorder?
A. 1 in 10
B. 1 in 20
C. 1 in 100
D. 1 in 200
E. 1 in 400
The correct answer is A. 1 in 10
Explanation: The frequency of the disorder (q²) is 1/400. Therefore, q (the frequency of the recessive allele) is the square root of 1/400, which is 1/20. Since p + q = 1, the frequency of the dominant allele (p) is 19/20. The carrier frequency (2pq) is 2 x (19/20) x (1/20), which is approximately 1 in 10.
7. How does the calculation of allele and genotype frequencies for an X-linked recessive disorder differ from that of an autosomal recessive disorder under the Hardy-Weinberg principle?
A. Allele frequencies in males are directly observed as phenotype frequencies
B. The Hardy-Weinberg equation does not apply to X-linked disorders
C. Genotype frequencies are the same in males and females
D. The frequency of the disorder is higher in females than in males
E. Carrier frequencies cannot be calculated for X-linked disorders.
The correct answer is A. Allele frequencies in males are directly observed as phenotype frequencies.
Explanation:
The calculation of allele and genotype frequencies for X-linked recessive disorders differs from autosomal disorders because males have only one X chromosome (hemizygous), while females have two. This hemizygosity simplifies the observation of allele frequencies in males. Here’s a breakdown:
Key Differences:
1. Allele Frequencies in Males: For an X-linked recessive disorder, the phenotype frequency in males is equal to the allele frequency (q) because males have only one X chromosome. For example, if q is the frequency of the recessive allele, then the proportion of affected males in the population is q.
2. Allele and Genotype Frequencies in Females: Females, having two X chromosomes, follow the Hardy-Weinberg principle: Homozygous recessive females (q2) are affected, Heterozygous females (2pq) are carriers and Homozygous dominant females (p2) are unaffected.
3. Higher Frequency in Males: Since q2 is typically much smaller than q, the frequency of the disorder is usually higher in males than in females.
o In males, if q=0.1, then 10% of males will express the disorder[ the phenotype frequency in males is equal to the allele frequency (q)]
o In females, q2=0.01, meaning 1% of females will express the disorder[In females, the disorder manifests only in homozygous recessive individuals : Frequency of affected females=q2
Analysis of Options:
B. The Hardy-Weinberg equation does not apply to X-linked disorders: Incorrect. Hardy-Weinberg principles apply, but the calculations must account for the different genetic makeups of males and females.
C. Genotype frequencies are the same in males and females: Incorrect. Males only have one X chromosome, so they do not have genotypes like AA or Aa as females do.
D. The frequency of the disorder is higher in females than in males: Incorrect. X-linked recessive disorders are more common in males because males only need one copy of the recessive allele to express the disorder, while females require two.
E. Carrier frequencies cannot be calculated for X-linked disorders: Incorrect. Carrier frequencies for females (2pq) can be calculated using Hardy-Weinberg principles.
Conclusion:
For X-linked recessive disorders, allele frequencies in males directly correspond to phenotype frequencies, simplifying calculations for males compared to autosomal recessive disorders.
8. A small, isolated island population experiences a devastating volcanic eruption that significantly reduces its size. Which of the following evolutionary processes is most likely to have the greatest impact on the allele frequencies in this population following the eruption?
A. Natural selection
B. Mutation
C. Gene flow
D. Genetic drift
E. Founder effect
The correct answer is D. Genetic drift.
Explanation:
The volcanic eruption significantly reduces the size of the population, creating a situation where random changes in allele frequencies due to genetic drift will have a much greater impact than other evolutionary processes. Here’s why:
Why Genetic Drift is Most Significant:
1. Definition of Genetic Drift: Genetic drift refers to the random changes in allele frequencies in a population, particularly in small populations. When the population size is drastically reduced, random chance plays a much larger role in determining which alleles are passed to the next generation.
2. Bottleneck Effect: The volcanic eruption causes a population bottleneck, where only a small portion of the population survives. The survivors may not have the same allele frequencies as the original population, leading to a significant shift in genetic diversity purely by chance.
3. Post-Eruption Dynamics: In small populations, the effects of genetic drift are magnified, as random events can disproportionately influence which alleles increase or decrease in frequency.
Why Other Options Are Less Likely:
A. Natural selection: Natural selection acts on traits that provide a survival or reproductive advantage. While it may influence allele frequencies, its impact is typically less immediate than genetic drift in a small, isolated population after a bottleneck event.
B. Mutation: Mutation introduces new genetic variation, but its effect on allele frequencies is generally very slow and unlikely to be the primary driver of changes after a sudden population reduction.
C. Gene flow: Gene flow involves the movement of alleles between populations. In an isolated island population, gene flow is minimal or nonexistent, making it irrelevant in this scenario.
E. Founder effect: The founder effect is a specific type of genetic drift that occurs when a small group from a population establishes a new population. While similar to a bottleneck, the founder effect does not apply here, as the population is not forming a new colony but is instead recovering from a severe reduction.
Conclusion:
Following a volcanic eruption that reduces population size, genetic drift—specifically through the bottleneck effect—is the evolutionary process most likely to have the greatest impact on allele frequencies in the population.
9. Which of the following scenarios best illustrates the concept of genetic drift?
A. A population of moths evolves camouflage coloration that matches the bark of trees, providing protection from predators
B. A new allele arises in a population due to a spontaneous mutation, conferring resistance to a deadly disease
C. In a small population of flowers, a rare allele for white petal color increases in frequency over several generations due to random chance.
D. Individuals from a mainland population of birds migrate to a nearby island, introducing new alleles into the island population.
E. A group of individuals from a large population establishes a new colony on a distant island, leading to a different distribution of allele frequencies compared to the original population
The correct answer is C. In a small population of flowers, a rare allele for white petal color increases in frequency over several generations due to random chance. This is the best example of genetic drift, as the increase in frequency of the rare allele occurs by chance rather than through selection or mutation. The small population size amplifies the effects of random fluctuations.
Explanation: Genetic Drift: Genetic drift refers to random changes in allele frequencies in a population, especially in small populations, due to chance events rather than natural selection. These changes are not directed by any adaptive advantage or disadvantage of the alleles.
Analysis of Scenarios:
A. A population of moths evolves camouflage coloration that matches the bark of trees, providing protection from predators: This describes natural selection, not genetic drift, as the change in allele frequency is due to a selective advantage (camouflage provides protection).
B. A new allele arises in a population due to a spontaneous mutation, conferring resistance to a deadly disease: This describes mutation, not genetic drift. While mutations create new alleles, genetic drift refers to the random change in frequencies of existing alleles.
D. Individuals from a mainland population of birds migrate to a nearby island, introducing new alleles into the island population : This describes gene flow, not genetic drift. Gene flow refers to the movement of alleles between populations through migration.
E. A group of individuals from a large population establishes a new colony on a distant island, leading to a different distribution of allele frequencies compared to the original population: This describes the founder effect, a specific type of genetic drift that occurs when a small group finds a new population. While related to genetic drift, it specifically involves the establishment of a new colony and is distinct from random changes within an existing small population.
Conclusion:
The scenario in C best illustrates genetic drift because it involves random changes in allele frequency over time, particularly in a small population, without influence from selection, mutation, or migration.
10. A newborn infant presents with a number of unusual physical features, including a high-pitched, cat-like cry, microcephaly, and congenital heart disease. Karyotype analysis reveals a chromosomal abnormality. Which of the following is the most likely diagnosis?
A. Cri-du-chat syndrome
B. Down syndrome
C. Edward syndrome
D. Klinefelter syndrome
E. Turner syndrome
The correct answer is A. Cri-du-chat syndrome.
Explanation:
The key clinical features described in the question—cat-like cry, microcephaly, and congenital heart disease—are characteristic of Cri-du-chat syndrome, a chromosomal disorder caused by a deletion on the short arm of chromosome 5 (5p deletion).
Analysis of Options:
B. Down syndrome:
Features: i) Hypotonia (low muscle tone), flat facial profile, epicanthal folds, and a single palmar crease.
ii) Congenital heart defects (e.g., atrioventricular septal defects).
iii) Intellectual disability.
Cause: Trisomy 21 (three copies of chromosome 21).
It is incorrect because it lacks the hallmark cat-like cry and microcephaly.
C. Edward syndrome:
Features:
i) Micrognathia (small jaw), clenched fists with overlapping fingers, rocker-bottom feet, and severe intellectual disability.ii) Congenital heart defects are common.
Cause: Trisomy 18 (three copies of chromosome 18).
This is incorrect because cat-like crying is not associated with Edward syndrome.
D. Klinefelter syndrome:
Features:
i) Males with Klinefelter syndrome (47, XXY) often have tall stature, gynecomastia, small testes, and infertility.
ii) Intellectual disability is rare.
Cause: Presence of an extra X chromosome in males.
This is inorrect because it does not present with congenital heart defects, microcephaly, or a cat-like cry.
E. Turner syndrome:
Features:
i) Short stature, webbed neck, lymphedema of the hands and feet, and ovarian dysgenesis.
ii) Congenital heart defects such as coarctation of the aorta may be present.
Cause: Complete or partial loss of one X chromosome (45, X).
This is incorrect because Turner syndrome does not involve microcephaly or a cat-like cry.
Conclusion:
The most likely diagnosis is A. Cri-du-chat syndrome, as it best matches the described symptoms and the chromosomal abnormality.
11. A newborn presents with hypotonia, a single palmar crease, a flat facial profile, and congenital heart defects. Karyotype analysis reveals a chromosomal abnormality. Which of the following is the most likely diagnosis?
A. Down syndrome
B. Turner syndrome
C. Patau syndrome
D. Klinefelter syndrome
E. Cri-du-chat syndrome
Correct Answer: A. Down syndrome
Explanation:
Down syndrome (Trisomy 21) is characterized by hypotonia, single palmar crease, flat facial profile, and heart defects (e.g., atrioventricular septal defect).
Why Others Are Incorrect:
B. Turner syndrome: Occurs in females with features like short stature and webbed neck.
C. Patau syndrome: Features include cleft lip/palate, polydactyly, and severe intellectual disability.
D. Klinefelter syndrome: Affects males and presents with infertility and gynecomastia.
E. Cri-du-chat syndrome: Presents with a cat-like cry and microcephaly.
12. A 16-year-old girl is evaluated for short stature, delayed puberty, and amenorrhea. Physical examination reveals a webbed neck, shield-shaped chest, and lymphedema of the hands and feet. Echocardiography shows coarctation of the aorta. What is the most likely diagnosis?
A. Turner syndrome
B. Klinefelter syndrome
C. Down syndrome
D. Cri-du-chat syndrome
E. Edward syndrome
Correct Answer: A. Turner syndrome
Explanation:
Turner syndrome (45, X) presents with short stature, delayed puberty, webbed neck, lymphedema, and characteristic heart defects like coarctation of the aorta.
Why Others Are Incorrect:
B. Klinefelter syndrome: Seen in males with tall stature, gynecomastia, and small testes.
C. Down syndrome: Features include hypotonia, intellectual disability, and flat facial profile.
D. Cri-du-chat syndrome: Presents with a cat-like cry, microcephaly, and congenital heart defects.
E. Edward syndrome: Associated with clenched fists, micrognathia, and rocker-bottom feet.