55 Opsin genes and their expression in animal models Testing a fairly random theory: How does one gene express two different opsins in two different cells? Mutually exclusive expression of human red and green visual pigment-reporter transgenes occurs at high frequency in murine photoreceptors. Commentary on the stochastic theory of L and M expression Color Vision, Options & Opsins Making a trichromatic mouse.   How do you know if a mouse can see color? Total color blindness in humans that does not involve the opsins.  How can mutations in two different genes cause the same sort of disease?  Does this happen often?