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Ch. 17 - Organelle Inheritance and the Evolution of Organelle Genomes
Sanders - Genetic Analysis: An Integrated Approach 3rd Edition
Sanders3rd EditionGenetic Analysis: An Integrated ApproachISBN: 9780135564172Not the one you use?Change textbook
All textbooksSanders 3rd EditionCh. 17 - Organelle Inheritance and the Evolution of Organelle GenomesProblem 18
Chapter 17, Problem 18

The following pedigree shows a family in which several individuals exhibit symptoms of the mitochondrial disease MERRF. Two siblings (II-2 and II-5) approach you to inquire about whether their children will also be afflicted with MERRF. What do you tell them? 
Pedigree chart showing family members with MERRF symptoms for genetics course.

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1
Analyze the pedigree chart: MERRF (Myoclonic Epilepsy with Ragged Red Fibers) is a mitochondrial disease, which means it is inherited through mitochondrial DNA. Mitochondrial DNA is passed exclusively from mother to offspring, as sperm do not contribute mitochondria during fertilization.
Identify affected individuals: In the pedigree, individuals with filled symbols (black circles or squares) exhibit symptoms of MERRF. Note that all affected individuals are descendants of the female in generation I-1, who is also affected.
Determine inheritance pattern: Since mitochondrial DNA is maternally inherited, only females can pass the disease to their offspring. Males, even if affected, cannot transmit the disease to their children.
Focus on II-2 and II-5: Individual II-2 is female and affected, meaning she has a high likelihood of passing MERRF to all her children, regardless of their sex. Individual II-5 is male and affected, but he cannot pass the disease to his children.
Explain the implications: Inform II-2 that her children are at risk of inheriting MERRF due to maternal mitochondrial inheritance. Inform II-5 that his children will not inherit MERRF, as mitochondrial DNA is not transmitted through males.

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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 traits through mitochondrial DNA, which is inherited exclusively from the mother. This means that only females can pass on mitochondrial diseases to their offspring. In the case of MERRF (Myoclonic Epilepsy with Ragged Red Fibers), if a mother is affected, all of her children may inherit the condition, while the father does not contribute to the mitochondrial DNA.
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Pedigree Analysis

Pedigree analysis is a method used to trace the inheritance patterns of traits or diseases within a family over generations. In a pedigree chart, males are represented by squares and females by circles, with filled shapes indicating affected individuals. By analyzing the pedigree, one can determine the likelihood of offspring inheriting a genetic condition based on the affected individuals in previous generations.
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Risk Assessment for Offspring

Risk assessment for offspring involves evaluating the probability that children will inherit a genetic condition based on parental genotypes. In the case of mitochondrial diseases like MERRF, if the mother is affected, there is a 100% chance that her children will inherit the mitochondrial DNA, and thus the potential for the disease. The father's genetic contribution does not affect the risk of mitochondrial diseases.
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Related Practice
Textbook Question

Consider this human pedigree for a vision defect.

What is the most probable mode of inheritance of the disease? Identify any discrepancies between the pedigree and your proposed mode of transmission, and provide possible explanations for these exceptions. 

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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

What is the most likely mode of inheritance for the trait depicted in the following human pedigree? 

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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

The dodo bird (Raphus cucullatus) lived on the Mauritius Islands until the arrival of European sailors, who quickly hunted the large, placid, flightless bird to extinction. Rapid morphological evolution such as often accompanies island isolation had caused the bird's huge size and obscured its physical resemblance to any near relatives. However, sequencing of mitochondrial DNA from dodo bones reveals that they were pigeons, closely related to the Nicobar pigeon from other islands in the Indian Ocean. Why was mitochondrial DNA suited to the study of this extinct species?

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