In 1988, neurologists in Australia reported the existence of identical twins (tw
ID: 282628 • Letter: I
Question
In 1988, neurologists in Australia reported the existence of identical twins (two cell masses separated early in the embryonic development to become two individuals) who had developed myoclonic epilepsy in their teens. One twin remained only mildly affected by this condition, but the other twin later developed other symptoms of full-blown myoclonic epilepsy and ragged red fiber disease (MERRF), including deafness, ragged red fibers and ataxia (loss of the ability to control muscles). Explain the phenotypic dissimilarity of these identical twins. (3 points)Explanation / Answer
Myoclonic epilepsy with ragged-red fibers (MERRF) is a disorder that affects many parts of the body, particularly the muscles and nervous system. Affected individuals sometimes have short stature and develop fatty tumors, called lipomas, just under the surface of the skin.
The individual affected with MERRF is characterized by muscle twitches (myoclonus), weakness (myopathy), and progressive stiffness (spasticity). The muscle cells of affected twin usually appear abnormal under microscope. These abnormal muscle cells are called ragged-red fibers. Other features of MERRF include recurrent seizures (epilepsy), difficulty coordinating movements (ataxia), a loss of sensation in the extremities (peripheral neuropathy), and slow deterioration of intellectual function (dementia). The individual with this condition may also develop hearing loss or optic atrophy, which is the degeneration (atrophy) of nerve cells that carry visual information from the eyes to the brain. The individual undergoes progressive neurological deterioration that results in a chronic brain damage syndrome.