Normally, the human retina contains four types of light-sensitive receptors (opsins): three types of cones and one type of rods. Receptors contain proteins-chromoproteins - iodopsin in rods, rhodopsin in cones. The role of the latter in bright light is insignificant, therefore for a person there are three "basic" colors: blue, red, green - all the shades we perceive are formed by their combinations. And what would the world look like if there were not three such colors, but four? (Tetrachromacy is the perception of the visible range of electromagnetic radiation by combinations of four primary colors. The eyes of tetrachromats contain four types of light receptors with different degrees of perception of different subranges of the visible spectrum) The painting "Rainbow Eucalyptus" by the Californian artist Conchetta Antico, who has functional tetrachromacy, makes it possible to appreciate the variety of colors, perceived by people with four-color vision. On the left, for comparison, is a photograph of the landscape shown in the painting.
Many insects, some fish, and most reptiles and birds have four-color vision. The extra pigments allow these animals to see in the ultraviolet range. In humans, tetrachromacy occurs only as a rare genetic abnormality. It does not affect the width of the perceived part of the spectrum, but it significantly increases the sensitivity to shades. However, by the standards of mammals, humans have excellent color vision: many mammals have two-color vision, if not even monochrome. This regression compared to the evolutionary precursors of reptiles was most likely associated with the nocturnal lifestyle of early mammals. In the dark, the effectiveness of color vision drops sharply, and the loss of two types of cones "went unnoticed." As a result, primitive animals retained only two types of receptors - red and ultraviolet. Later, when mammals "came to light" again, some groups were able to restore tricolor vision. For primates, many of whom feed on fruits, this vision is very useful: it allows you to detect brightly colored fruits among green foliage, as well as determine their ripeness. The receptor that perceives the green color arose as a result of a duplication of the “red receptor” gene and subsequent mutation, which shifted its sensitivity to the short-wave region. But the receptor for ultraviolet light for human ancestors has become useless: their lens does not transmit the corresponding wavelengths. But on the basis of this receptor, as a result of a series of mutations, a receptor for blue light arose. Such mutations, which alter the peak of the spectral sensitivity of photoreceptors, can also endow their carriers with four-color vision. However, much more often they make one or another iodopsin non-functional: as a result, dichromacy occurs - color blindness. The genes for "red" and "green" iodopsins are located on the X chromosome, which is present in two copies in the chromosome set of women and only one copy in men. That is why color blindness is predominantly a male ailment: in women, due to the presence of a "reserve" X chromosome, it develops extremely rarely. For the same reason, only women can become tetrachromats: this requires that one of the X chromosomes contains a normal copy of the gene, and the other contains a mutant gene encoding a protein with a shifted photosensitivity peak. Since each of the iodopsins makes it possible to differentiate about a hundred shades, a person with normal vision can potentially distinguish about a million color combinations. The addition of another type of receptor increases this number to one hundred million. Concetta Antico is a carrier of a mutation in the gene of "red" iodopsin, the sensitivity of which has shifted to the short-wave region. Special opportunities are best manifested when distinguishing between reddish-yellowish and violet shades: in the color scheme of her paintings, the emphasis is on these tones.
And here we return to my question: how much will the color perception of my genetically modified people change if I give more than four photoreceptors (5, 6, etc.)?
If the spectrum contains seven primary colors (red, orange, yellow, green, cyan, blue and violet), then if we add each photoreceptor to these colors, we will perceive many more shades?