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Ch. 9 - Extranuclear Inheritance
Klug - Concepts of Genetics 12th Edition
Klug12th EditionConcepts of Genetics ISBN: 9780135564776Not the one you use?Change textbook
All textbooksKlug 12th EditionCh. 9 - Extranuclear InheritanceProblem 19c
Chapter 9, Problem 19c

Researchers examined a family with an interesting distribution of Leigh syndrome symptoms. In this disorder, individuals may show a progressive loss of motor function (ataxia, A) with peripheral neuropathy (PN, meaning impairment of the peripheral nerves). A mitochondrial DNA (mtDNA) mutation that reduces ATPase activity was identified in various tissues of affected individuals. The accompanying table summarizes the presence of symptoms in an extended family.
Table showing family members with Leigh syndrome symptoms and varying mitochondrial DNA mutation levels from 17% to over 90%.
How can some individuals in the same family show such variation in symptoms? What term, as related to organelle heredity, describes such variation?

Verified step by step guidance
1
Understand that the problem involves mitochondrial DNA (mtDNA), which is inherited maternally and can exhibit variability in symptoms among family members due to differences in the proportion of mutated mtDNA.
Recognize that the term describing this variation in symptoms is 'heteroplasmy,' which refers to the coexistence of both normal and mutated mtDNA within the same cell or organism.
Note that the severity of symptoms in mitochondrial disorders like Leigh syndrome is often correlated with the percentage of mutated mtDNA present in an individual's cells. For example, individuals with >90% mutated mtDNA (proband and brother) show severe symptoms, while those with lower percentages (e.g., 17% in the asymptomatic brother) may not exhibit symptoms.
Explain that heteroplasmy arises because mitochondria are distributed randomly during cell division, leading to variable proportions of mutated mtDNA in different tissues and individuals. This random distribution can result in a wide range of symptom severity within the same family.
Conclude that the variation in symptoms among family members is due to heteroplasmy and the threshold effect, where a certain proportion of mutated mtDNA must be exceeded for symptoms to manifest. This explains why some individuals are asymptomatic while others are severely affected.

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Key Concepts

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

Mitochondrial Inheritance

Mitochondrial inheritance refers to the transmission of genetic material found in mitochondria, which is inherited exclusively from the mother. This type of inheritance can lead to variable expression of mitochondrial disorders, as seen in Leigh syndrome, where different individuals may exhibit varying symptoms despite sharing the same mtDNA mutation. The severity of symptoms can depend on the proportion of mutated mitochondria present in different tissues.
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Organelle Inheritance

Heteroplasmy

Heteroplasmy is the presence of a mixture of more than one type of mitochondrial DNA within a cell or individual. In the context of mitochondrial diseases, such as Leigh syndrome, individuals may have varying levels of mutated mtDNA, leading to differences in symptom severity. This variation can explain why some family members are symptomatic while others remain asymptomatic, depending on the percentage of mutated mitochondria in their cells.
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Organelle DNA Characteristics

Penetrance and Expressivity

Penetrance refers to the proportion of individuals with a specific genotype that actually express the associated phenotype, while expressivity describes the degree to which a genotype is expressed in an individual. In the case of Leigh syndrome, the differences in symptoms among family members can be attributed to incomplete penetrance and variable expressivity of the mitochondrial mutation, resulting in a range of clinical manifestations from severe symptoms to asymptomatic individuals.
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Related Practice
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?

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?

Textbook Question

Researchers examined a family with an interesting distribution of Leigh syndrome symptoms. In this disorder, individuals may show a progressive loss of motor function (ataxia, A) with peripheral neuropathy (PN, meaning impairment of the peripheral nerves). A mitochondrial DNA (mtDNA) mutation that reduces ATPase activity was identified in various tissues of affected individuals. The accompanying table summarizes the presence of symptoms in an extended family.

Develop a pedigree that summarizes the information presented in the table.

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Textbook Question

Researchers examined a family with an interesting distribution of Leigh syndrome symptoms. In this disorder, individuals may show a progressive loss of motor function (ataxia, A) with peripheral neuropathy (PN, meaning impairment of the peripheral nerves). A mitochondrial DNA (mtDNA) mutation that reduces ATPase activity was identified in various tissues of affected individuals. The accompanying table summarizes the presence of symptoms in an extended family.

In what way does a condition caused by mtDNA differ in expression and transmission from a mutation that causes albinism?

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Textbook Question

Researchers examined a family with an interesting distribution of Leigh syndrome symptoms. In this disorder, individuals may show a progressive loss of motor function (ataxia, A) with peripheral neuropathy (PN, meaning impairment of the peripheral nerves). A mitochondrial DNA (mtDNA) mutation that reduces ATPase activity was identified in various tissues of affected individuals. The accompanying table summarizes the presence of symptoms in an extended family.

Provide an explanation for the pattern of inheritance of the disease. What term describes this pattern?

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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.

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.