What is the MOST likely explanation for the difference in the severity of MERRF between Frank and his parent?
Some random mutations took place in Frank's mitochondria, which caused MERFF syndrome as his parent does not have full symptoms.
Because mitochondria display heteroplasmic segregation, it is likely that Frank has a higher proportion of mutant mtDNA molecules in his cells compared to his parent.
Because Frank received this mutation from Jon, the severity of the symptoms increases each generation.
Because Frank received this mutation from Maria, the expression pattern of the mutant gene may be different in males than in females.