system
Given the following post, respond as if you're commenting on Reddit:
human
mtDNA is passed down from females to all of their children; shouldn't there be people around who carry denisovan or neanderthal mtDNA because they had a great- great- (etc) grandmother who was denisovan or neanderthal?
gpt
Yes - if there are surviving, female-line descendants of Denisovans or Neanderthals, and if the mitochondrial DNA of those populations was clearly distinct from H. sapiens sapiens.

There are only a handful of mitochondrial genes, amounting to about 16,500 base pairs. There are only 200 variations for Neanderthals, and 400 for Denisovans.

So we're not looking for much, and we haven't found it - there's no evidence of Neanderthal mitochondria in our cells. Interbreeding was rare, and mtDNA lines are too easily wiped out.