You probably recall that back in middle school, you learned about dominant and recessive genes. And you were like "Ohhh, so that's why my hair color is this murky shade of IDEK what!"
Well, a Game of Thrones fan has put that knowledge to good use (by which we mean extremely nerdy use) and made a pedigree chart explaining why Jon Snow's hair is black instead of Targaryen blonde.
And what's more, this handy chart can be used to guess the hair color of Jon and Daenerys' future incest-spawned baby—which, according to #math, has a pretty decent chance of having white hair. See, Reddit agrees:
In case you're confused: Dany will pass on white hair no matter what. So if Jon passes on his white hair gene, the baby will 100 percent have Targaryen locks. If he passes on his black hair gene, the baby will have black hair despite Dany's genes—since black is a dominant hair color.
In conclusion: LOL, Reddit. But also, OMG you're amazing, Reddit.