Single combs are occasionally a byproduct of breeding BLRW. Every once in a while one shows up. I know a good breeder who says 5-10% of their hatchings from BLRW are single comb. But yes, as the person above me mentioned, wyandottes need to have rose combs and technically if it has such a largely different factor such as a single comb it is not technically a wyandotte.
How is it that a single can even pop up....if rose dominates single comb?