First, the boy had ophathalmoplegia (eye muscle problem) and hypotonia (muscle weakness), indicating that he had muscle related problems. Second, all the problems appeared to come from his mother’s side and the symptoms from different individuals of the family appeared to be very heterogeneous and diverse. All these strongly suggest heteroplasmy, a mitochondrial genetic disease, which is known to show varying degrees of expressivity owing to heterogeneity in genetic mutations in mitochondria. All the mitochondria of one person are inherited from mother’s side (from fertilized eggs) and are different from nuclear chromosome inheritance mechanism. Heteroplasmy is caused by the fact that in each human cells, there are several hundreds mitochondria. In different mitochondria, they may contain different genetic mutations. During genetic transmission, different mitochondria may end up in different eggs / fertilized eggs, leading to mitochondrial genetic disease in the offspring with varying degrees of expressivity owing to heteroplasmy

How to exclude other possibilities: In the narrative, there are genetic diseases two generations in a row in the family. Thus, it is not recessive disease. It came from his mother’s side but affected both sexes. It does not look like a penetrance issue, Additional information - MERRF syndrome (or myoclonic epilepsy with ragged red fibers) is a mitochondrial disease MERRF syndrome affects different parts of the body, particularly the muscles and nervous system. The signs and symptoms of this disorder appear at an early age, generally childhood or adolescence. The primary features displayed on a person with MERRF include myoclonus, seizures, cerebellar ataxia, myopathy, and ragged red fibers (RRF) on muscle biopsy, leading to the disease's name. Secondary features include dementia, optic atrophy, bilateral deafness, peripheral neuropathy, spasticity, or multiple lipomata. Mitochondrial disorders, including MERRFS, may present at any age