Star Wars

Time for a Parasite Eve style event where Sith are shown to have competing mitochondria against the Jedi mitochondria?

You can increase your midi-chlorian count but that doesn't change your Force sensitivity - it's what Moff Gideon's experiments involved.

Ultimately it's kept a bit vague possibly so creators can hand-wave away possible issues that arise if yiu could merely make new Jedi or Sith.

The Technical Commentaries for TPM dig into the implications:

Many observers assume that midi-chlorians are the cause of the Force or else the exclusive means by which sapient beings interact with the Force. This is not necessarily true. They provide one kind of objective test for Force-sensitivity, but correlation does not imply causation. It may be that certain people with strong potential to use the Force tend to attract midi-chlorians, rather than the midi-chlorians being responsible for the talent. (This attraction may be medical and heritable, or a direct manifestation of Force power.) Alternatively, the talent and the midi-chlorian concentration may be symptoms of some other, deeper cause.

...

The mere presence of midi-chlorians cannot be the only condition for Force sensitivity. Otherwise the Jedi or the Sith could cultivate midi-chlorians and then simply infuse them into the bodies of ordinary volunteers to create countless initiates with arbitrarily high potential.

...

The practices and beliefs of the Jedi must be taken into account in any discussion of the midi-chlorians. The STAR WARS civilisation is ancient and technologically static; scientific tools exist, but scientific practice appears to be finished and absent. It appears to be a society that depends on artisan engineers and mystics only. The "midi-chlorians" may in part be invented Jedi jargon. So far as we've seen, they serve only to provide an objective measurement of Anakin's messianic potential. Or at least the Jedi read it objectively according to their superstitions — there's no reason why the midi-chlorian count must be any more "real" than the use of tarot cards.

Jedi from different schools of thought might disagree about the significance or reality of the midi-chlorians. In The Phantom Menace novel, Qui-Gon Jinn admits a bias towards a "living Force" interpretation, at the expense of knowledge of the "unifying Force". Perhaps this means that he is sensitive to biological interpretations of the Force to the point where he regards the midi-chlorians as a cause rather than an effect. Perhaps he drives his analogies too far. This may partly explain the consternation of some of the members of the Jedi Council. Perhaps to them he seems to have eccentric biases or holes in his understanding, making him a frustrating dissident whose practical skills compensate for his weakness in some theoretical areas.