Some women apparently have an extra cone in their eyes, allowing them to perceive more colors than other humans–endowing them with super-human abilities (sort of). Most humans have three types of cone cells in their eyes, which allow people to see red, green, and blue. Color blindness is caused by a mutation in the genes coding for the cone cells, and those genes are located on the X chromosome (which is why color blindness is more common in men; it’s a recessive trait on the X chromosome, and men only have one X chromosome so if they have the mutation it will be presented as color blindness. Women have two X chromosomes and so they need two copies of the mutation to exhibit the color blindness phenotype). It seems that in families with color blind men, women are likely to have an extra cone. So far only one tetrachromat (a person with four cones instead of three) has passed the super-human vision test, although 25 women with four cone types were tested. I don’t really know what that means, but it seems like there may be some superheroes already living among us–and all of them are women!