Question

What would have happened if RBC has mitochondria?

Answer 1

Most of the proteins of mitochondria functioning are coded by the nuclear DNA, so if a mitochondrion is present in a cell having no nucleus (here, RBC) will lack most of its functional and structural capabilities.

As a result, a reduced mitochondrion might exist which will eventually reduce the oxygen-carrying capacity of the RBC by competing with the cell for oxygen and moreover will take the valuable space in RBC which was otherwise be used for closed packing in hemoglobin to increase oxygen carrying efficiency of RBC.