I'll try to answer the questions regarding color blindness. Hopefully,there is some useful
information embedded in here somewhere:
I suppose it would be best to start with some
background information. As you likely know, the
process of seeing begins when specialized cells in the retina called photoreceptors absorb light energy and change that light energy into neural signals. Also, you probably know that there are two basic types of photoreceptors (rods and cones) and that cones are responsible for color vision.
In order to have normal human color vision a
person must have 3 different cone types each maximally sensitive to light of different
wavelengths (there are other requirements as well...the visual system must be wired in such a way that signals from these different cone types
can be compared).
There is one cone type maximally sensitive to short wavelengths (light you see as blue), one maximally sensitive to medium wavelengths(light you see as green), and
one maximally sensitive to long wavelengths(light you see as red). Because normal humans have 3 different cone types they are referred to as trichromats. The visual system compares signals originating from these different cone types in order to bring about the perception of color.
Within the cones are special
molecules called a photopigments. It is these photopigments within the cones that are the
elements that actually absorb the light. A
component of the photopigment called the opsin is a protein made by the cone photoreceptor. The opsin controls the tuning of the photopigment molecule, that is, it controls the wavelengths to which the photopigment (and therefore, the cone) will be maximally sensitive.
So, there are 3 different cone opsins, each cone expresses just one of the opsin types, and that opsin determines whether the cone will be maximally sensitive to
short, medium or long wavelengths
Production of the opsins is
genetically controlled. That is, there are genes
for each of the 3 cone opsins. The gene for the
short wavelength sensitive cone opsin is autosomal (it is located on chromosome 7). The genes for both the medium wavelength sensitive and long wavelength sensitive cone opsin are on the x chromosome. That means that normally everyone has 2 copies of the short wavelength opsin gene (one paternal copy and one maternal copy). However, because the other 2 opsin genes are on the x chromosome, females have two copies of each (they have 2 x chromosomes...one paternal and one maternal) but males have just a single copy of each gene (they have a single x chromosome from mom).
Most cases of "color blindness" result when a person has only two cone types. For reasons I'll describe below such are person is more accurately described as "color defective." The most common reason a person has only 2 cone types is that there is a defect in one of the cone opsin genes. For example, if a guy has normal short and medium wavelength sensitive opsin genes and a defective long wavelength sensitive opsin gene, they will have just two cone types. Such a person will be red-green color blind and because they have just two cone types are referred to as
dichromats. Obviously, there can be three
different types of dichromats depending on which
gene is defective.
If the short wavelength sensitive opsin is missing, the person is referred to as atritanope, if the medium wavelength opsin
is missing, the person is referred to as a
deuteronope, and if the long wavelength opsin is missing, the person is referred to as a protanope.
Because of the genetics of the opsins (as I
described in the previous paragraph),protanopia
and deuteronopia are far more common than
tritanopia and they are far more common in males than females. This is because if in a male the single copy of either the medium or long
wavelength sensitive opsin is defective, the
person will lack one of the opsin types and so be
color defective (or color blind if you prefer).
If one copy of the gene is defective in a female,
she will have another copy(originating on the
other x chromosome) and so will have normal color
In order for a female to be aprotonope or
deuteronope, she must have 2 defective copies of
either the long wavelength sensitive or medium
wavelength sensitive opsin,respectively.
Likewise, in order for a male or female to be a
tritanope, they must have two defective short
wavelength sensitive opsin genes. That explains why it is rare to find a female who is color blind and equally rare to find someone who is a tritanope.
Deuteronopes and protonopes are both considered red-green color blind because they will confuse these colors. Tritanopes are unable to discriminate blue colors from yellow colors and
so are considered blue-yellow color blind.
However, all dichromats can still discriminate
some colors. They just aren't as good as a normal
trichromatic human. This is why its probably better to refer to dichromats as "colordefective."
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