(version 22 August 2005)
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One in 5 million persons has only monochromatic vision seeing only shades of gray.
8% of males and 0.5% of females have dichromatic vision
Contain the pigment Rhodopsin
Rhodopsin bleaches from pink to white when exposed to light
This bleaching converts the light image into shades of gray
Rods responsible for night vision as are very sensitive to low levels of light.
Roughly seven million cones/retina
Typical human eye has three types of cones,
Each type contains a single opsin pigment for either Red, Green, Blue.
Each opsin absorbs light at slightly different wavelengths
Gene for Blue opsin is autosomal
Genes for Red and Green opsins are tightly linked on the X chromosome
Red-Green opsins 96% similar, but both only 43% similar to Blue opsins
Blue and Red/Green opsins diverged about 500 millions years ago
Separate Red and Green opsin genes in human those arose less than 40 million years ago, via gene duplication.
Old-world monkeys are trichromatic but New-world monkeys (South America) are dichromatic (have only blue and one Red/Green opsin). Old-world/new-world monkeys had last common ancestor at approximately 40 MYA.
But most females are trichromatic. Why?
The single Red/Green gene in squirrel monkeys has multiple alleles, some of which are red-sensitive, others which are green-sensitive
Some organisms have opsin genes that can see:
In the far-red (infrared), so can see heat (e.g., rattlesnakes)
In the far-blue (ultraviolet).