A More Colourful World

Most humans are trichromats. A trichromat has three color detecting cones in the retina. These cones allow the perception of blue, greenl and red. A trichromat is able to distinguish approximately 100 different shades of those base colours. An individuals brain then combines all of those different colours and gradients to create over a million different hues.
However, recently it has been discovered that some women are tetrachromats instead of trichromats. As a trichromat has three different cones and colours, a tetrachromat has 4 different cones and colours. tetrachromats have an extra cone that is in between the red and green sensors. It is thought to be around an orange colour. This means that tetrachromats are able to perceive a whole other range of colour which means that they are able to distinguish upwards of 100 million colours compared to the million of a regular person.
Researches propose that only women are able to be tetrchromats because the genes coding for pigments of green and red retinal cones lie on the X chromosome. As women have the double XX chromosome, it is only females who have the genetic opportunity to be a tetrachromat. Tetrachromats have one type of red cone activated on one chromosome and the other type activated on the other chromosome. Occasionally some women have a different type of green cone activated on each chromosome.
Despite having two different types of red cones (or green cones) activated, some women are not capable of perceiving additional colours. This is because if the two different cones are too close to the same pigment (colour wavelength) that there is minimal difference.
2-3 % of women are supposed to be tetrachromats that are able to distinguish increased number of hues.
This is an incredible advantage. Imagine the difference that it would make when gathering food. Being able to distinguish and detect more colours would allow faster and more accurate identification. Predators, even camouflaged, would be avoided easier thus increasing the likelyhood of survival. Even in todays world, being able to see more colour could be incredibly beneficial; you note a persons skin looks strange and ensure to keep distance between you, thus avoiding being exposed to the flu or cold virus.

References:
Roth, M. (2006). Some women may see 100 million colors, thanks to their genes.
Pittsburgh Post-Gazette.Retrieved December 14, 2007 from:
http://www.post-gazette.com/pg/06256/721190-114.stm

Wikipedia. (2007). Tetrachromancy. Wikipedia; the free encyclopedia. Retrieved December 14, 2007 from: http://en.wikipedia.org/wiki/Tetrachromacy