Genetically altered humans

That's an interesting one, bags, esoecially the question as to whether a third biological parent will arise from this method.

I'm wondering if the sperm bank system couldn't then be used whereby the donor is anonymous?

This isn't like tampering with DNA in the nucleus, where most of our genetic material rests.
Mitrchondrial DNA, unlike the rest of our DNA, does not rest inside the cell nucleus but in a mitochondrial sack in the protoplasm. It is always inherited from your mother. So women pass it in to their sons and daughters, but only daughters pass it on to their children.

If you only have sons therefore your mitochondrial DNA is never passed on to your grandchildren.

It makes sense to replace this if it is faulty and causing birth defects as the 'good' mitochondrial matter will them pass down through the female line.

PS. I don't see why mitochondrial DNA from the father could not be introduced rather than a third person being involved,

Interesting. In theory the replacement of mtDNA seems to be a rational plan, although I'd like to see further development of the ethical issues that would surround such modification.

A tricky balance of medical advancement and ethical issues that push the boundaries is always difficult. It's important and a good thing that one addresses and questions such issues.