Tetrachromacy is an enhanced type of colour vision that may allow the individual to see colours that others cannot.

Normal colour vision depends on three types of specialised cells in the eye called cones. These cones are often referred to as blue, green and red cones depending on the particular wavelengths of light that trigger them into action. In comparison, tetrachromats are endowed with a fourth type of cone, which is most sensitive in the yellow-green region of the visible spectrum.

Where does the fourth cone type come from?

The red and green cones are coded by genes lying on the X chromosome. These genes are very similar and are prone to errors during early development of a foetus. 6% of men carry an anomalous gene which produces a different red or a different green cone. We call them anomalous trichromats, because their colour vision is slightly different from normal. 2% of men have either no red or no green cones, because the genetic error was more severe. We call these men dichromats as their colour vision relies on only two types of cone. The blue cones are coded by a gene on chromosome 7 and are rarely subject to genetic errors. A woman, who has two X chromosomes, can carry the normal red and green genes on one of her X chromosomes and an anomalous gene on her other. This genetic pattern enables her to express four types of cone.

How many women might have four types of cone?

12% of women carry such an altered gene, but we do not know how many of these women can use their additional cone type to make colour discriminations that are unachievable for the rest of us.

How do we know whether a woman is a carrier?

The best way to tell whether a woman carries the gene is if she has a son (or father) with a slight colour anomaly (i.e. very mild colour blindness). We can predict with some confidence that the milder the son’s (or father’s) colour anomaly, the better chance there is for the mother (or daughter) to see the world in different colours.

If you think that you may be a candidate for tetrachromacy, click HERE to find out how to get involved.