Can a joke turn into real discrimination?
In my personal experience, it has seemed more that jokes are instead a sign of direct or latent discrimination. Your'e unlikely to poke fun of a group unless you've already figured out that group is "not us".
And, for the ginger thing - Morrus mentions a migration of Scots in the 1970s. Not that I'm a scholar of British history, but the Irish are also known for their redheads, and the Irish Independence movement was pretty violent, and ran for decades. Might no the anti-redhead thing be a vestige of that?