system
Given the following post, respond as if you're commenting on Reddit:
human
Is it possible to ''reactivate'' inactive genes?

I read yesterday that some animals can get vitamin-C directly from the sun but humans can't even tho we have the necessary genes in our DNA. So my question is as follows: Would it be possible to ''reactivate'' genes like these in the future and what practical uses could it have? 

Edit: Wow! Thanks everyone for the response. This was one of my first posts Thank you for your responses!
gpt
It depends on what you mean by 'reactivation.'  The ancestral vitamin-C gene has been subject to a large amount of genetic drift (random mutations).  It would have a large number of mutations in it, and determining what the mutated DNA based should be would not be a trivial task.

It could be possible to take a copy of vitamin-C synthetase from a related organism that has it and insert that into the genome.  Likely it would work in the same way, with some tweaks needed to the regulatory regions.

However, none of this is 'useful' in the medical sense - currently there is no technique to play with the genome of an adult organism.  Whenever geneticists make a mutation or other change, it is always in the early embryo, or the gametes of the parents.  Your kids could make vitamin C, but you likely couldn't.