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.
Person | Condition | Percent Mitochondria with Mutation |
Proband | A and PN | >90% |
Brother | A and PN | >90% |
Brother | Asymptomatic | 17% |
Mother | PN | 86% |
Maternal uncle | PN | 85% |
Maternal cousin | A and PN | 90% |
Maternal cousin | A and PN | 91% |
Maternal grandmother | Asymptomatic | 56% |
(a) Develop a pedigree that summarizes the information presented in the table.
(b) Provide an explanation for the pattern of inheritance of the disease. What term describes this pattern?
(c) How can some individuals in the same family show such variation in symptoms? What term, as related to organelle heredity, describes such variation?
(d) In what way does a condition caused by mtDNA differ in expression and transmission from a mutation that causes albinism?
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