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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 17
Chapter 9, Problem 17

(a) In humans the mitochondrial genome encodes a low number of proteins, rRNAs, and tRNAs but imports approximately 1100 proteins encoded by the nuclear genome. Yet, with such a small proportion from the mitochondrial genome encoding proteins and RNAs, a disproportionately high number of genetic disorders due to mtDNA mutations have been identified [Bigger, B. et al. (1999)]. What inheritance pattern would you expect in a three-generation pedigree in which the grandfather expresses the initial mtDNA defect? What inheritance pattern would you expect in a three-generation pedigree in which the grandmother expresses the initial mtDNA defect?
(b) Considering the description in part (a) above, how would your pedigrees change if you knew that the mutation that caused the mitochondrial defect was recessive and located in the nuclear genome, was successfully transported into mitochondria, and negated a physiologically important mitochondrial function?

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Step 1: Understand mitochondrial inheritance. Mitochondrial DNA (mtDNA) is inherited maternally, meaning it is passed down from the mother to all her offspring. Mutations in mtDNA will only be transmitted to the next generation if the mother carries the mutation. Fathers do not pass on mtDNA to their children.
Step 2: Analyze the inheritance pattern in part (a). If the grandfather expresses the initial mtDNA defect, the mutation will not be passed to his children because mtDNA is not inherited from the father. If the grandmother expresses the initial mtDNA defect, all her children (both sons and daughters) will inherit the mutation, and her daughters will pass it on to their offspring, while her sons will not.
Step 3: Consider the scenario in part (b) where the mutation is recessive and located in the nuclear genome. Unlike mtDNA, nuclear DNA is inherited from both parents. A recessive mutation requires two copies of the defective allele (one from each parent) for the phenotype to be expressed. In this case, the inheritance pattern would follow Mendelian autosomal recessive inheritance.
Step 4: Construct the pedigree for part (b). If the mutation is recessive and located in the nuclear genome, affected individuals would only appear in the pedigree if both parents are carriers of the mutation. The probability of inheritance would depend on the genotypes of the parents. For example, if both parents are heterozygous carriers, there is a 25% chance for each child to inherit the defect.
Step 5: Compare the two scenarios. In part (a), the inheritance pattern is strictly maternal due to mtDNA inheritance, while in part (b), the inheritance pattern is autosomal recessive and depends on the nuclear genome. This distinction highlights the importance of understanding the origin of the mutation (mtDNA vs. nuclear DNA) when analyzing genetic disorders.

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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 exclusively inherited from the mother. This pattern results in all offspring of an affected mother potentially inheriting the mitochondrial traits, while offspring of an affected father do not inherit these traits. Understanding this concept is crucial for analyzing pedigrees involving mitochondrial disorders.
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Organelle Inheritance

Nuclear vs. Mitochondrial DNA

Nuclear DNA is the genetic material located in the cell nucleus, inherited from both parents, while mitochondrial DNA (mtDNA) is found in mitochondria and is maternally inherited. The distinction between these two types of DNA is essential for understanding the inheritance patterns of genetic disorders, especially when considering mutations that may affect mitochondrial function but originate from nuclear genes.
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Organelle DNA Characteristics

Recessive Inheritance

Recessive inheritance occurs when a trait or disorder manifests only when an individual has two copies of a mutated gene, one from each parent. In the context of the question, if a mitochondrial defect is caused by a recessive mutation in the nuclear genome, it would require both parents to carry the mutation for the offspring to express the disorder, altering the expected inheritance patterns in the pedigree.
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Related Practice
Textbook Question

The specification of the anterior–posterior axis in Drosophila embryos is initially controlled by various gene products that are synthesized and stored in the mature egg following oogenesis. Mutations in these genes result in abnormalities of the axis during embryogenesis. These mutations illustrate maternal effect. How do such mutations vary from those produced by organelle heredity? Devise a set of parallel crosses and expected outcomes involving mutant genes that contrast maternal effect and organelle heredity.

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Textbook Question

The maternal-effect mutation bicoid (bcd) is recessive. In the absence of the bicoid protein product, embryogenesis is not completed. Consider a cross between a female heterozygous for the bicoid alleles (bcd⁺/bcd⁻) and a male homozygous for the mutation (bcd⁻/bcd⁻).

Predict the outcome (normal vs. failed embryogenesis) in the F₁ and F₂ generations of the cross described.

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Textbook Question

The maternal-effect mutation bicoid (bcd) is recessive. In the absence of the bicoid protein product, embryogenesis is not completed. Consider a cross between a female heterozygous for the bicoid alleles (bcd⁺/bcd⁻) and a male homozygous for the mutation (bcd⁻/bcd⁻).

How is it possible for a male homozygous for the mutation to exist?

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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

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.

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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.

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

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