With apologies to J.R.R. Tolkien …
Achromatopsia is a disorder characterized by a partial or total lack of color vision sometimes occurring with other vision problems such as sensitivity to light (photophobia), involuntary back-and-forth eye movements (nystagmus), and nearsightedness (myopia). Three genes of special interest in achromatopsia are OPN1LW, OPN1MW and OPN1SW where the fifth characters stand for long, medium and short. Most people have one copy of the OPN1LW gene and one or more copies of the OPN1MW gene on each X chromosome. A nearby region of DNA, known as the locus control region (LCR), regulates the activity of these genes. Only the two opsin pigment genes nearest the LCR, generally the OPN1LW gene and the first copy of the OPN1MW gene, are active in the retina and contribute to color vision. Usually, the OPN1LW gene’s location is described as Xq28 [q being the long arm of the X chromosome] at about base pairs 154,144,224 to 154,159,032 while the OPN1MW gene’s location is at base pairs 154,182,596 to 154,196,861. Things never are simple: the OPN1SW gene is located at 7q32.1 base pairs 128,772,489 to 128,775,790. This remoteness is not necessarily a bad thing – as we shall see, translocations of parts of OPN1LW and OPN1MW cause all sorts of trouble.
Opsins are a collection of proteins that are light sensitive – when struck by a photon they can emit an electron so that a useful signal can be processed. The earlier prokaryotic life-forms such as Archaea (the so-called simple single celled organisms) and Bacteria have type I opsins as do some eukaryotic forms like green algae. Type I opsins react to light faster and more reliably than type II opsins like those used by eukaryotes like us.
Most animals with eyes have retinas, which are the light-sensitive tissue at the back of the eye. The retina contains two types of light receptor cells, called rods and cones, that transmit visual signals from the eye to the brain. Rods provide vision in low light. Cones provide vision in bright light, including color vision. Most of the time there are three types of cones, each containing a specific opsin that is most sensitive to particular wavelengths of light. The brain combines input from all three types of cones to produce normal color vision. The opsin made from the OPN1LW gene is more sensitive to light in the yellow/orange part of the visible spectrum (long-wavelength light), and cones with this pigment are called long-wavelength-sensitive or L cones. The opsin made from the OPN1MW gene is more sensitive to light in the middle of the visible spectrum (yellow/green light), and cones with this pigment are called middle-wavelength-sensitive or M cones. The opsin made from the OPN1SW gene is more sensitive to light in the blue/violet part of the visible spectrum (short-wavelength light), and cones with this pigment are called short-wavelength-sensitive or S cones.