Gabriele Jordan, colour vision researcher, in her laboratory. Dr Jordan has been investigating four- colour vision or tetrachromacy since 1993. This is a rare genetic mutation (on the X chromosome) that would form an extra type of cone cell on the retina. There are normally only three types of cone cell, and this is called trichromacy. Only female tetrachromats are possible. Women with 4 different types of cone cell do exist, but it is uncertain whether the brain can process the extra signal and whether this would lead to a perception of a wider range of colours. Photographed at the University of Newcastle-upon-Tyne, England, in 2004.