Do mutant females walk among us?

"The theoretical possibility of this secret sorority -- genetics dictates that tetrachromats would all be female -- has intrigued scientists since it was broached in 1948. Now two scientists, working separately, plan to search systematically for tetrachromats to determine once and for all whether they exist and whether they see more colors than the rest of us do. The scientists are building on a raft of recent findings about the biology of color vision."

I am not sure I would consider this a mutation any more than redheads are mutant varities of their more drag counterparts. Just because a gene is not often expressed does not mean it is not part of the genome.