The raged red fibres as they appear under the microscope, courtesy Wikipedia commons
Myoclonic epilepsy with ragged red fibres or MERRF syndrome is a genetic disease which affects the patient's mitochondria. It's name comes from the appearance of damaged mitochondria which align themselves to muscle fibres and appear as red fibres when stained and put under a microscope. The mitochondria lack the ability to create a key protein that is vital to the processing of energy within the cell. It is a rare disease with an incidence of about 1 in 400,000 people. It is due to a defect in the mitochondrial DNA, and as such, an affected mother will pass the defective DNA to all of their children.
However MERRF presents with symptoms varying greatly from patient to patient as the number of mitochondria with the defect in a given egg cell varies greatly from patient to patient. It is perfectly possible for some of the mitochondria in the cell to be perfectly normal while others carry the dysfunctional mutation. As a result, the symptoms generally only progress as a patient ages and more of the defective mitochondria are produced.
MERRF is not dangerous, but it can be dehabilitating and is very difficult to treat. Generally, the symptoms of the disease are treated (such as anti-seizure medication). There are drug therapies, but they are of limited and uncertain effectiveness.