Download RG Colorblindness

Colorlblindness, Trichromatic to Dichromatic.
Good Morning, today you’re all going to learn about Red –Green Color Blindness, a Xlinked
recessive disorder that effects millions of people. Raise your hand if you know twenty guys,,
very good. Chances are out of those twenty males at least one will be red-green color blind. I will
explain the mechanism that allows this condition to remain in our population, what exactly the
condition effects, and other things I learned in my research.
First we’ll go over the mechanism and its prevalence. We all are aware of how an x
linked recessive gene works. This is a gene on the X chromosome, males will be hemizygous for
it and display it if they carry it, versus females who would have to inherit the trait from both their
mother and father. Simply by its mechanism, it implys a higher prevalence in males, and a much
rarer one in females. This is realistically reflected when we look at those who have Red green
color blindness. Today, approximately 8% of all men and less then 1 % of all women display the
phenotype for red green color blindess. In addition, there is only a slightly higher prevalence in
Caucasians(8%) then in other ethnic groups(4/5%). Now than we have a better idea of how this
condition is present through our genome, we’ll get into the condition itself.
Let’s look at the condition as it effects in the eye. Here we see where Incoming light
strikes the retina, the retina is the layer of photosensitive cells, both rod and coned shaped. Rod
cells are used in low light situations in in your perriffereals, while cone cells distinguish color.
There are different types of cone cells that perceive different wavelengths of light. Once we
perceive that color, the signal goes through our optic nerve right to our brain. That being said,
Red green color blindness results in a deficiency in one of the cone receptors on your retina.
Normally your cone receptors perceive either the long, middle, or short wavelengths, which
correspond to red, green and blue lights respectively. However with this condition present the red
or green cone receptor isn’t created. Therefore, red or green light and colors close to those aren’t
perceived. Now that we have a better understanding of the eye and how red green color blindness
effects it, is there a cure and can we tell if we’re a carrier?
Currently there is no medical treatment in practice for red green color blindness, but
there are ways to test for its presence. The most common and practical test is Ishihara color
vision plate test. As you can see most of us can distinguish the number in the middle, whereas
someone with the phenotypic expression couldn’t. This is a result of not being able to distinguish
the color of the dots in the middle from the ones around it. As of now, there aren’t any paternal
or carrier tests availble. The only way to figure out if one is a carrier would be to examine a
pedigree. However, some research in gene therapy shows promise. In one study Dr. Jean
Bennete, who studies molecular genetics of inherited retinal degeneration, examines gene therapy that was
conducted on color blinded monkeys, of which 15 to 36% percent had full vision restored. This
of course hasn’t been applied to humans, but it does shows the condition can be changed in
mammals.
Lastly, will look at what this really means to us. As humans with three cone receptors we
are classified as trichromats. Anyone who has two types of functioning cone receptors is a
dichromat. There are many various animals that are dichromats, but being red green color blind
puts you into that category as well. The only significant difference is the amounts of colors the
two groups can see. Its estimated that dichromats see somewhere around 100,000 different
possible colors, and trichromats can see well over 1,000,000. Some people postulate that the
possible reason for this trait may have been for survival. That by seeing less colors, it makes it
easier to see something that is camouflaged which could potentially be a beneficial trait in the
right circumstances.
To wrap up, we should all have a better understanding of the x linked receissve disorder
red green colorblindness. We understand how the prevalence reflects its mechanism in more
males displaying the condition. How the cone receptor is inhibited and why that creates the loss
of color in vision. As well as the lack of genetic testing for the condition, only the practical test
to determine the presence of the phenotype. I hope you’ll know a little more next time you meet
someone who is red-green color blind.
The human genome is riddled with thousands of self-inflicted plights and surprises.
However, few are worse and potentially more embarrassing then Red-Green Color Blindness(RG
color blind). Imagine yourself this Halloween, as you reach for one of your favorite candies like
a starburst. Imagine the delicious taste of watermelon about to bless your taste buds and mind
with nostalgia, but suddenly that thought is pilfered from you right as your eyes deceived you
into thinking that a disgusting orange starburst was actually deliciously flavored with
watermelon. RG color blindness is a fairly common condition and presents a serious
embarrassment not only to the lives but to taste buds of people everywhere.
[Bennett 2009]Today, approximately 8% of all men, and less than 1% of women display
rg color blindness. [USDC 2012]This adds up to a high estimate of approximately 650 million
people in the world who present the disorder. RG color blindness is inherited through an X
Linked Recessive gene. Since males have only one X chromosome, it greatly increases the
probability that they will end up inheriting this trait. For a female to have the trait, her father
would have to be RG color blind, and her mom would have to be a carrier or RG color blind
herself. Thus making the disorder itself hard to come by, especially for females.
[Bennett 2008]As humans we have receptors in our retina that allow us to perceive light
and colors, the relevant receptors are cone photoreceptors that perceive different wave lengths.
As humans we are trichromats, meaning that we have three different kinds of cone receptors that
allow us to view red, green, and blue colors. [Chia et al 2008]The disorder effects genes on the X
chromosome that directly affect either the green or red receptors, either inhibiting them or
dulling their effect. The result is that many see certain colors as other ones instead, such as in the
example given in the introduction.
[Wong 2011]The common and popular test to distinguish if one has RG colorblindness is
referred to as the Ishihara color-vision plate test. The test in cooperates a series of different sized
dots all in close proximity together. Most of the dots are shades of green, but some of the dots
will form a number or letter in a shade of orange. Someone with normal vision can distinguish
the orange figure in the circle, but someone with RG color blindness won’t see the difference
between the dots. [Chia et al 2008] To increase the validity of the test, many are used in a
booklet. These booklets come with control groups, some with and without the contrasting color
to insure that one actually couldn’t see the additional color. There are also other more expensive
and invasive methods of determining to what extent one has different color deficiencies as well.
There is no specific test for prenatal or for carrier of the RG color blindness gene
unfortunately. The easiest way to determine if one is a carry or not is too look at one’s family
tree, possibly if create a pedigree for it. Chances are if no one in your family has it then you’re in
the clear. Some gene therapy has had marginal success in potentially effecting RG color
blindness. [Bennet 2009] In one study gene therapy was applied to primates to have their eyes
produce the third cone photoreceptor to perceive another wavelength and see the full color
spectrum. The procedure was only moderately successful with less than half of the specimens
successfully gaining full vision of the color spectrum. However, it does create potentiality for a
similar procedure in humans.
In conclusion, RG color blindness is an X linked recessive disorder that reduces the
amount of colors one can effectively see. The disorder effects small portion of the population and
no particular ethnic group. The disorder itself is a defect of a cone photoreceptor in the retina that
would allow a normal color spectrum to be perceived, but is instead missing. It however has no
prenatal or carrier test to identify the risk of inheriting it.
Reference List
Chia, A., Gazzard, G., Tong, L., Zhang, X., Sim, E.-L., Fong, A. and Saw, S. M. (2008), Red-
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green colour blindness in Singaporean children. Clinical & Experimental Ophthalmology,
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36: 464–467. doi: 10.1111/j.1442-9071.2008.01799.x. http://onlinelibrary.wiley.com/doi
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/10.1111/j.1442-9071.2008.01799.x/full (Peer Reviewed)
Bennett, J. (2009). Gene Therapy for Color Blindness. The New England Journal of Medicine,
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2009; 361:2483-2484. DOI: 10.1056/NEJMcibr0908643. http://www.nejm.org/doi _
/full/10.1056/NEJMcibr0908643 (Peer Reviewed)
USDC. (2012) World POP Clock Projection. United States Census Bureau. http://www.census .
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gov/population/popclockworld.html
Wong, B. (2011). Points of View:Color Blindness. Nature Methods 8, 441. doi:10.1038/nmeth.
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1618. http://dx.doi.org/10.1038/nmeth.1618 (Peer Reviewed)