Color vision and X-inactivation.
Normal color vision in humans is said to be trichromatic, since there are normally three types of cones in our retinas. The most common types of color deficiencies result from abnormal or missing genes on our X chromosomes. Most of the readers in the Science Group have probably heard that many men, but few women, are "red-green colorblind". This trait is said to be sex-linked and recessive, like baldness and many other abnormalities that affect more men than women.
Roughly speaking, if a man has such a trait and a woman is not a carrier for that trait, their children will not have the trait, but half of their daughters will be carriers. If a woman is a carrier and a man does not have the trait, half of their sons will have the trait, and half of their daughters will be carriers. If a woman is a carrier and a man has the trait, then half of their children of either sex will have the trait, and the daughters who don't have the trait will be carriers. These statements follow from the facts that every female cell has two X chromosomes and every male cell has only one X chromosome.
But the real situation is more complicated. In very precise color-matching experiments, women who are normally considered to be carriers have been shown to have a slight deficit compared to women who are not carriers. In other words, the genes for color deficiencies are not completely recessive. This has to do with the fact that one of the X chromosomes in each cell of a woman's body is inactivated, i.e., most of its genes are not expressed. The reason for X inactivation is to prevent a mismatch between males and females in the amounts of messenger RNA (and hence the amounts of protein) that are produced. In animals (but not plants) if the level of gene expression is off by a factor of two, the results are usually deleterious and often fatal. Even if the level is only off by 50%, as in trisomy 21 (Down Syndrome), there are serious consequences.
A female "carrier" of a sex-linked trait will have clumps of cells in which the X chromosome inherited from her father is inactivated, and other clumps in which the X chromosome inherited from her mother is inactivated. The patches of fur in a calico cat are similarly explained. The retina in each eye will have abnormal patches and normal patches. It's not surprising that such an eye does not perform quite as well as a completely normal eye.
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I am such a carrier. My father was red/green colorblind and my mom had normal vision. So one of my x chromosomes is from each parent, and the one from my dad is abnormal.
I've never had a problem with my red/green vision, thank goodness. Perhaps some sophisticated testing would show otherwise.
Thanx for posting. 
Ah..I see. I especially liked the last part.
The increased sensitivity of PA carriers at lower light levels is consistent with the pooling of signals from the hybrid M' and the M cones and the subsequent stronger inhibition of the rods. The findings suggest that signals from hybrid photopigments may pool
preferentially with the spectrally closest "normal" pigments.
This isn't specifically my field but as an amateur astronomy (and a serious one) This explains a lot. 
...destroy Rhodopsin (almost forgot the chemical). At least that's what the manufacturer says.
"Aww..put 2 more red things on the light and charge them 35 bucks more"
Been thinking of getting this > http://www.daystarfilters.com/Quark.shtml
so a person can see the sun like this > http://www.hydrogenalpha.com/
Our little nocturnal furry mammal ancestors lost the excellent color vision many birds and reptiles still enjoy.
"Trichromacy" in the Superfamily Hominoidea is a wretched evolutionary kludge. It helps me to think of it as "sort-of-good-enough trichromacy."
Further complicating the story, it's recently been discovered that some of the color sorting is done in a structural way, by the shape of optical fibers leading to the light sensitive cells...
Red and green light are funnelled through the (Muller glia) cells, while blue scatters much more
http://www.bbc.com/news/science-environment-31775458
A great deal of color sorting is also done in "software," which means our perception of color isn't at all precise, just a rough guess based on experience and, to put it again in computer terms, some very extreme post-processing which introduces a lot of artifacts into the image we perceive.
The human eye is remarkable in many ways, but it is a terrible camera. What we "see" is a heavily enhanced and "photoshopped" version of the actual reality.
The four pigments in estrildid finches' cones extend the range of colour vision into the ultraviolet
https://en.wikipedia.org/wiki/Tetrachromacy
Notice how neatly spaced the sensitivities are.
Humans:
https://en.wikipedia.org/wiki/Trichromacy
You can see the kludge.
I've never met a parrot who was terribly impressed by television, maybe because to a parrot the colors reproduced make people look like cold corpses living in a world made of mud under a smoggy sky..
One advantage we apes evolved was telling when the fruit was ripe.
Many plants evolved to favor birds over mammals in fruiting because seeds are carried further, and are more likely to pass through birds without being chewed up and digested. One way for plants to favor birds is to use color signals most mammals can't see. Another way is like hot peppers do with Capsaicin. Birds don't mind hot peppers at all, mammals are repelled, well, humans are weird.
Worst pepper burn I ever got was from our parrot who had been happily munching very hot peppers when she flew over, landed on my shoulder, and decided one of my eyelashes was out of place...AAAAAAGGGGGHHHHHH! My eye was on fire!
