What would have happened ? If.............

RBCs had mitochondria.

RBC's perform the major function of transport of oxygen to cells. The pigment present in RBC's called "Haemoglobin" binds to oxygen and then it is transported throughout the body with blood.

Now we know that in Mitochondria oxidation of fats and carbohydrates occurs, this oxidation requires oxygen as one of the electron accepting molecule (in the electron transport chain) . Thus if Mitochondria would be present in RBC's, the oxygen carried by them would be used by themselves and other cells of the body will not get oxygen and then will die.

Hence, Mitochondria are absent in RBC's.