Coexpression

A phenomenon where multiple different opsins can be expressed within a single type of cone cell.

For example, an organism may have genes for – and express – two classes of opsin with different peak wavelength, which would suggest they are dichromatic. However, if the opsins are coexpressed in the same cone cells, then the organism is still monochromatic, albeit with a wider visible light band.

Likewise, in some mammals, there could be partial coexpression, where two types of cones express both types of opsins, but do so unevenly, i.e. the SWS1 cone expresses mostly a UVS opsin and a bit of the LWS opsin, and the LWS cone expresses mostly a LWS opsin and a bit of the UVS opsin. In this case, dichromacy is still possible.