Discover has an interesting synopsis of a recent study that suggests that the distaff relatives of color-blind males (red-green color blindness is an X-linked recessive trait, which means many female relatives would be carriers for that allele) might have an additional type of cone, which allows them to differentiate colors at a heightened level relative to the typical person:
Researchers suspect, though, that some people see even more. Living among us are people with four cones, who might experience a range of colors invisible to the rest. It’s possible these so-called tetrachromats see a hundred million colors, with each familiar hue fracturing into a hundred more subtle shades for which there are no names, no paint swatches. And because perceiving color is a personal experience, they would have no way of knowing they see far beyond what we consider the limits of human vision…
Color blindness ran in families, affecting men but not women. While color-blind men had two normal cones and one mutant cone, De Vries knew that the mothers and daughters of color-blind men had the mutant cone and three normal cones—a total of four separate cones in their eyes. He suspected the extra cone could be why the women perceived color differently—not because they saw less than most people but because they saw more. He speculated that such women might be using the fourth cone to distinguish more colors than a trichromat, but he buried this insight on the last page of the paper. De Vries never wrote about four-coned women again.
This finding might also shed light on the abnormal prevalence of color-blindedness in humans: a mix of unusual inheritance and carrier effects washing out the detrimental effects on males and allowing the allele to rise in frequency in the population.
Frustratingly, Andrew elsewhere today completely fails yet again to understand pleiotropy, getting hoodwinked into a ludicrous adaptionist story explaining balding as signalling mechanism. Fortunately, actual biologists intervened to set him straight.