Okay, as I rambled about earlier. Wyatt’s DNA (and blood) has traces of demonic DNA within it. @melliwell helped me out with this a lot. But basically, because of the chocolates piper ate before she and Leo conceived Wyatt, he was essentially created from the source’s dark magic and blood. So now, a tiny amount of demonic DNA is apart of Wyatt’s and always will be.
It’s not something that controls him, or say - influences him like being half demon would. But because of it, he does have an affinity towards demons (as we’ve seen in the show) and unwillingly feels almost at ease or welcomed around them. That is probably the only side effect, along with perhaps an even better high resistance than most witches and his inability to get sick or die from ‘natural causes’ like starvation, thirst, disease etc (but a lot of that has to do with his power too). Other than that, it is not something that has ‘corrupted him’ nor will it. It just is. It makes him something other than human and, potentially, witch (but not demon, just like a mutated witch or smth, but not something as similar to part demon or fully demon blooded).
However, as it is directly the sources’ DNA. The only problem it poses is the fact that Wyatt is connected to the source and can be lightly controlled by him. It wouldn’t be something as strong as a total puppeteer action, but more things that influence the subconscious state of mind or reflexes and certain aspects of mobility that we don’t think about when we’re doing it. But one thing we do know in charmed is that those with a direct blood link or magical link to the source cannot kill him or they risk becoming him. Luckily enough, the source was vanquished but when Wyatt brings him back from the dead in Desperate Housewitches i firmly believe that the blood connection between them is what allowed that to happen rather than wyatts actual power (though that certainly helped) because it seems weird to me that Wyatt is able to bring demons back from the dead but not people. But, if he has that connection to not only a demon, but the source of all evil. It makes sense that he can bring back demons who have been vanquished into total nothingness.
No one knows the truth though, and no one ever will (except maybe the triad themselves as they seemed to know a hell of a lot about Wyatt that no one knew). But if anyone were to do tests, they’d find that Wyatts third helix is different to that of other witches and witch-whitelighters, they just wouldn’t know what it was unless they were able to compare it to demons.
Long story short, Wyatt’s DNA/blood has small traces of demonic DNA in them and he’s not human (even though witches weren’t really to begin with).
The source can fuck him up if he ever came back (but! wherever it is that he is he still may have the power to reach Wyatt through nightmares bc of that stupid bond) but it’s not like Wyatt is going to turn into a demon or be corrupted by it.
the demonic dna is almost like a dead vaccine in a way. It isn’t an excuse for what he becomes in the UF and it isn’t the reason for it. He did that and chose that all on his own.
His blood would taste slightly off to any vampire or being that happened to drink it but its not so noticeable that they can recognise it as demonic. that and they just assume the different taste is just simply because of who he is, the twice blessed child.
This ‘demonic dna’ is also NOT why he’s prone to evil or turning evil. As mentioned earlier its just a thing about him. It may add to his ‘darkness’ that he has in him, but that is a matter of who wyatt is, his soul, and the fact that the world wants him to be evil so it can fill the vacuum left by what the source’s child would have posed against the charmed ones. (his destiny may, itself, have been the cause or why he ended up with demonic dna but it never did much to him the long run).
I think if that disease from hulkus pocus ever returned and if wyatt was infected, I think he’d be immune to it seeing as the disease is a result from demon and human blood (i think its been a while since ive seen the ep) mixing while his blood is already like that and had time to properly combine without turning into total chaos. The same would probably go for the croatoan virus in spn - Wyatt would be immune (but that’s just for fun lol it has no bearing here). That would also be another ‘side effect’ of his demonic dna.