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A woman gives birth to monozygotic twins. One of the twins is completely healthy, while the other twin is diagnosed with myoclonic epilepsy and red fiber disease (MERRF). The affected twin suffers from uncontrollable jerking, deafness, muscle weakness, heart problems, kidney problems, and progressive dementia.

What is the reason behind the sick twin’s medical condition?


These twins are not genotypically identical; therefore, they are most likely fraternal (dizygotic) and not identical (monozygotic).


The twin’s mother was heteroplasmic, and the twins’ genotype depended on the partitioning of mutant mtDNA after fertilization and the tissue that received the mutation after development.


The twin’s mother was heteroplasmic, and the twins’ genotype was a consequence of a random mutation that occurred immediately after fertilization in one of the developing embryos.


The twins’ genomes are identical if they are monozygotic. Therefore, the onset of this disease has to be environmentally triggered, and it ended up affecting one twin, but not the other.

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