Tetrachromacy
[8] It has been theorized that females who carry recessive opsin alleles that can cause color vision deficiency (CVD) could possess tetrachromacy.
Female carriers of anomalous trichromacy (mild color blindness) possess heterozygous alleles of the genes that encode the L-opsin or M-opsin.
[10] Another study suggests that as many as 50% of women and 8% of men may have four photopigments and corresponding increased chromatic discrimination, compared to trichromats.
[6][12] Specifically, she has been shown to be a trichromat in the range 546–670 nm where people with normal vision are essentially dichromats due to negligible response of S cones to those wavelengths.
Thus, if S cones of 'cDa29' provide independent color perception dimension as they normally do, that would confirm her being a tetrachromat when the whole spectrum is considered.
The biological basis for this phenomenon is X-inactivation of heterozygotic alleles for retinal pigment genes, which is the same mechanism that gives the majority of female New World monkeys trichromatic vision.
[14][failed verification] Despite being trichromats, humans can experience slight tetrachromacy at low light intensities, using their mesopic vision.
[3] Exceptions include: Some species of birds, such as the zebra finch and the Columbidae, use the ultraviolet wavelength 300–400 nm specific to tetrachromatic color vision as a tool during mate selection and foraging.
Most birds have retinas with four spectral types of cone cell that are believed to mediate tetrachromatic color vision.
The four cone types, and the specialization of pigmented oil droplets, give birds better color vision than that of humans.
[23] Some birds such as corvids, Old and New World flycatchers,[24] as well as most diurnal raptors,[25][26] have little ability to see UV light, with the fourth cone type instead peaking in the violet range.
It is believed that UV vision in raptors is selected against because short-wavelength UVA light contributes highly to chromatic aberration, reducing visual acuity which raptorial birds rely on for hunting.
[citation needed] Some birds (notably pigeons) have five or more kinds of color receptors in their retinae, and are therefore believed to be pentachromats, though psychophysical evidence of functional pentachromacy is lacking.
