How are mitochondrial diseases like MERRF inherited?
Mitochondria are primarily thought to be inherited from the mother but there is evidence now that the father can also contribute mitochondria (Schwartz and Vissing, 2002; Luo et al., 2018). 

Even if the mitochondria were purely maternally inherited, a cell (including oocyte) typically contains many mitochondria and not all of the mitochondria may have the (deleterious) mutation. This phenomenon is called heteroplasmy. 

MERRF is associated with a loss of function mutation in mitochondrial tRNAs (See NIH, Genetics Home Reference). The extent to which a cell will be affected would depend on number of mutated mitochondria it contains. Therefore different cells would be affected differently. 

The trait inheritance would therefore not be in the typical Mendelian fashion.