How does maternal versus paternal inheritance influence the presentation of mitochondrial disease?

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Mitochondrial DNA is inherited exclusively from the mother, so an affected mother can pass the mutation to all of her children, but the clinical outcome varies because cells can carry different proportions of mutant mitochondrial DNA (heteroplasmy). The amount of mutant mtDNA in a given tissue determines whether that tissue reaches a threshold to show dysfunction; tissues with high energy demands, like brain and muscle, are often most affected, leading to a wide range of presentations from severe to barely recognizable symptoms. Because the heteroplasmic load can differ among tissues and among offspring, siblings can have very different severities even with the same maternal mutation. Paternal transmission is rare and not the rule, and autosomal inheritance patterns don’t apply to the mitochondrial genome.