A mature oocyte (egg cell) has nearly half a million copies. This means that a mutation, if present in only a few copies, might be masked by numerous normal copies. But it can be much more confusing. The same mitochondrial mutation often produces differing outcomes at different doses, or if set against different nuclear genes, or in distinct tissues, or if interacting with other mitochondrial DNA in the same cell, much of which exhibits tortuously complex behaviour, making mitochondrial diseases elusive and esoteric to this day.
