If having the correct number of copies of each chromosome is so important, how is it possible that both female (with two copies of the X chromosome per cell) and male (with only one copy of the X chromosome per cell) individuals are phenotypically normal?

Mitochondria is the cell organelle that produced the energy that is needed in the body system in form of atp, that is why it is called the power house of the cell. energy is needed by the living organisms to carry out the activities which sustain life.  if the mitochondria is destroyed atp will not be produced. if the mitochondria is destroyed, energy production will stop and the affected living organism will not have the energy to carry out the necessary life processes; this will lead to the death of the organism.