Table of contents

1. Introduction to Genetics51m
2. Mendel's Laws of Inheritance3h 37m
3. Extensions to Mendelian Inheritance2h 41m
4. Genetic Mapping and Linkage2h 28m
5. Genetics of Bacteria and Viruses1h 21m
6. Chromosomal Variation1h 48m
7. DNA and Chromosome Structure56m
8. DNA Replication1h 10m
9. Mitosis and Meiosis1h 34m
10. Transcription1h 0m
11. Translation58m
12. Gene Regulation in Prokaryotes1h 19m
13. Gene Regulation in Eukaryotes44m
14. Genetic Control of Development44m
15. Genomes and Genomics1h 50m
16. Transposable Elements47m
17. Mutation, Repair, and Recombination1h 6m
18. Molecular Genetic Tools19m
- Genetic Cloning
  9m
- Methods for Analyzing DNA
  9m
19. Cancer Genetics29m
- Overview of Cancer
  21m
- Cancer Mutations
  7m
20. Quantitative Genetics1h 26m
21. Population Genetics50m
- Hardy Weinberg
  19m
- Allelic Frequency Changes
  31m
22. Evolutionary Genetics29m

3. Extensions to Mendelian Inheritance

Organelle DNA

Genetics

3. Extensions to Mendelian Inheritance

Organelle DNA

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2:13 minutes

Problem 18b

Textbook Question

Mutations in mitochondrial DNA appear to be responsible for a number of neurological disorders, including myoclonic epilepsy and ragged-red fiber disease, Leber's hereditary optic neuropathy, and Kearns-Sayre syndrome. In each case, the disease phenotype is expressed when the ratio of mutant to wild-type mitochondria exceeds a threshold peculiar to each disease, but usually in the 60 to 95 percent range.
Compared with the vast number of mitochondria in an embryo, the number of mitochondria in an ovum is relatively small. Might such an ooplasmic mitochondrial bottleneck present an opportunity for therapy or cure? Explain.

Verified step by step guidance

Understand the concept of mitochondrial inheritance: Mitochondria are inherited maternally, meaning they are passed from the mother to the offspring through the cytoplasm of the ovum. Mutations in mitochondrial DNA (mtDNA) can lead to diseases if the proportion of mutant mitochondria exceeds a certain threshold.

Recognize the mitochondrial bottleneck: During oogenesis (egg development), the number of mitochondria in the developing ovum is reduced significantly, creating a bottleneck. This means that only a small subset of the mother's mitochondria are passed on to the egg, which can lead to variability in the proportion of mutant and wild-type mitochondria in the offspring.

Consider the therapeutic potential of the bottleneck: Since the bottleneck reduces the number of mitochondria, it may provide an opportunity to manipulate the mitochondrial population. For example, techniques like mitochondrial replacement therapy (MRT) could be used to replace mutant mitochondria with healthy ones during this stage.

Explain how this could work: In MRT, healthy mitochondria from a donor egg could be introduced into the ovum, replacing the mutant mitochondria. This would reduce the proportion of mutant mitochondria below the disease threshold, potentially preventing the expression of the disease phenotype in the offspring.

Discuss the implications: While the mitochondrial bottleneck presents a potential opportunity for therapy, ethical, technical, and regulatory considerations must be addressed. Additionally, the success of such therapies would depend on precise control over the mitochondrial population and ensuring the safety of the procedure.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Mitochondrial DNA and Mutations

Mitochondrial DNA (mtDNA) is distinct from nuclear DNA and is inherited maternally. Mutations in mtDNA can lead to various diseases, particularly affecting energy-intensive tissues like the nervous system. Understanding how these mutations impact mitochondrial function is crucial for grasping the underlying mechanisms of mitochondrial disorders.

Threshold Effect in Mitochondrial Diseases

The threshold effect refers to the phenomenon where a certain proportion of mutant mitochondria must be present for a disease phenotype to manifest. In mitochondrial disorders, this threshold typically ranges from 60% to 95% mutant mitochondria, meaning that below this level, the wild-type mitochondria can compensate for the dysfunction, preventing disease symptoms.

Ooplasmic Mitochondrial Bottleneck

The ooplasmic mitochondrial bottleneck is a phenomenon during oocyte development where only a limited number of mitochondria are passed from the mother to the offspring. This bottleneck can create an opportunity for therapeutic interventions, as it may allow for the selection of healthier mitochondria, potentially reducing the transmission of mitochondrial diseases to the next generation.

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Textbook Question

What type or types of inheritance are consistent with the following pedigree?

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Textbook Question

What is the endosymbiotic theory, and why is this theory relevant to the study of extranuclear DNA in eukaryotic organelles?

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Textbook Question

A 50-year-old man has been diagnosed with MELAS syndrome. His wife is phenotypically normal, and there is no history of MELAS syndrome in either of their families. The couple is concerned about whether their children will develop the disease. As a genetic counselor, what will you tell them? Would your answer change if it were the mother who exhibited disease symptoms rather than the father?

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Textbook Question

The first person in a family to exhibit Leber hereditary optic neuropathy (LHON) was II-3 in the pedigree shown below, and all of her children also exhibited the disease. Provide two possible explanations as to why II-3's mother (I-1) did not exhibit symptoms of LHON.

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Textbook Question

Mutations in mitochondrial DNA appear to be responsible for a number of neurological disorders, including myoclonic epilepsy and ragged-red fiber disease, Leber's hereditary optic neuropathy, and Kearns-Sayre syndrome. In each case, the disease phenotype is expressed when the ratio of mutant to wild-type mitochondria exceeds a threshold peculiar to each disease, but usually in the 60 to 95 percent range.

Given that these are debilitating conditions, why has no cure been developed? Can you suggest a general approach that might be used to treat, or perhaps even cure, these disorders?

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Textbook Question

In 1918, the Russian tsar Nicholas II was deposed, and he and his family were reportedly executed and buried in a shallow grave. During this chaotic time, rumors abounded that the youngest daughter, Anastasia, had escaped. In 1920, a woman in Germany claimed to be Anastasia. In 1979, remains were recovered for the tsar, his wife (the Tsarina Alexandra), and three of their children, but not Anastasia. How would you evaluate the claim of the woman in Germany?

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Textbook Question

Payne, B. A. et al. (2013) present evidence that a low level of heteroplasmic mtDNA exists in all tested healthy individuals.

What are two likely sources of such heteroplasmy?

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Textbook Question

Payne, B. A. et al. (2013) present evidence that a low level of heteroplasmic mtDNA exists in all tested healthy individuals.

What genetic conditions within a given mitochondrion are likely to contribute to such a variable pool of mitochondria?

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