Thank you for answering. That explained a lot.I would assume that your boy was the result of an RIR over a sexlink hen. That would give two gold genes and account for the red colouration that sexlink roosters lack. (they're yellow/sort of off-white) and also for the white tail.
So yes, the hen would be a mutt. (EDT: the hen carries gold, the rooster carries gold, all of the chicks are going to be gold. Not sexlinked.)
That's all just a guess, though.