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MCAT Biology Question — Phenotypes

Myoclonic epilepsy and ragged-red fiber (MERRF) is an extremely rare disorder that affects neuromuscular systems. MERRF results from a mutation in mitochondrial DNA (mtDNA) that impairs protein synthesis, oxygen consumption, and energy production. When an affected male and a normal female reproduce, which of the following best predicts the expected phenotypic ratios of the offspring?


  1. None of the offspring will be affected
  2. All males and no females will be affected
  3. Half of males and half of females will be affected
  4. One-fourth of the offspring will be affected
Click for Explanation

This question is asking the examinee to determine offspring phenotypes for a gene that is located on mitochondrial DNA. Since mtDNA follows maternal inheritance, all the offspring receive their mitochondrial DNA from the mother. In this case, the MERRF-causing mutation is located on the father’s mtDNA. Therefore, the mutations are not passed on to the offspring. Since all of the offspring will receive functioning mtDNA from the mother, none of the offspring will inherit mutated genes causing MERRF. Thus, answer choice A correct.

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