Researchers examined a family with an interesting distribution of Leigh syndrome symptoms. In this disorder, individuals may show a progressive loss of motor function (ataxia, A) with peripheral neuropathy (PN, meaning impairment of the peripheral nerves). A mitochondrial DNA (mtDNA) mutation that reduces ATPase activity was identified in various tissues of affected individuals. The accompanying table summarizes the presence of symptoms in an extended family.

Person                 Condition           Percent Mitochondria with
                                                       Mutation
Proband                A and PN            >90%
Brother                  A and PN            >90%
Brother                  Asymptomatic      17%
Mother                   PN                       86%
Maternal uncle       PN                       85%
Maternal cousin     A and PN             90%
Maternal cousin     A and PN             91%
Maternal                Asymptomatic      56%
grandmother


How can some individuals in the same family show such variation in symptoms? What term, as related to organelle heredity, describes such variation?