Jack and Jill's Son Jake Has a Severe Case of Myclonic

Essay

Jack and Jill's son Jake has a severe case of myclonic epilepsy and ragged-red fiber (MERRF) syndrome. His case includes frequent and disabling myclonic seizures (involuntary twitching of the muscles) along with hearing loss, exercise intolerance, and poor night vision. Like most cases of MERRF, his case is associated with a mitochondrial mutation that he inherited from his mother Jill. His mother didn't know that she harbors the MERRF mutation among her mtDNA molecules, but she has experienced occasional mild muscle twitching throughout her life and she does not see very well at night.
a. What is the most likely explanation for the difference in the severity of MERRF between Jake and his mother?
b. Jack and Jill would like to have another child. They want to know the probability that their next child will also suffer from MERRF syndrome. How would you counsel them?