Genetics

Color Blind



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WHAT IT MEANS TO BE COLOR BLIND

For a small percentage of planet Earth's population the world looks much different than it does for the rest of us. They are the 5% to 8% of men and .05% of women who have Color Vision Deficiency. They are Color Blind.

The human eye is a marvelous instrument. Most of the time we take it for granted. We think in terms of 'what you see is what is there', but that is not always true.

The retina, located in the back of the eye, is composed of rods and cones. The rods do not show us much in the way of color but they allow us to see at night. The cones show us color. Each cone is equipped with light sensitive pigment. This pigment differentiates between colors. Genes carry coded messages to the cones. Color blindness results when the genes deliver the wrong messages.

Although aging and some eye diseases can cause problems with color vision most color blindness is inherited through the X chromosome. You are born with it. A man has one X and one Y chromosome. A woman has two X chromosomes. A man will be color blind if his one X chromosome is color deficient. For a woman to be color blind both X chromosomes must be color deficient.

This does not mean that they see no color at all, that they see everything as black or white or shades of gray. That comes with total color blindness, monochromasy, which is extremely rare. There are many different degrees of color blindness. There are also many different types.

Trichromasy and Dichromasy

Trichromasy is normal, three color vision. Each of the three primary colors, red, green and blue, travels to the retina on a different wavelength of light, stimulating a particular pigment. Normal, healthy cones and light sensitive pigment allow us to see these colors in all their various combinations.

If there is a slight problem with even one pigment it will cause a mild color deficiency. This is anomalous trichromasy which includes protanomaly (red-weakness) and deuteranomaly (green-weakness). If there is a larger problem with even one pigment it will cause a severe color vision deficiency. This is dichromasy, or two color vision, which includes protanopia and deuteranopia.

Protanomaly and Deuteranomaly

It is reported that 99% of those who are deficient in color vision will fall into one of two categories, red-weak or green-weak, sometimes referred to as 'red-green color blind'. Out of every 100 males 1 will be protanomalous, meaning that what the normal eye sees as red, orange or yellow shifts to green for him. Five out of every 100 males will be deuteranomalous, meaning that what the normal eye sees as green, orange or yellow shifts to red for them. Often the shifts are so slight that they never realize they have a color vision deficiency, but, sometimes it can cause problems with traffic lights.

Protanopia and Deuteranopia

Although people with dichromasy fit into the same red and green categories as those with anomalous trichromasy, the color deficiency is much more severe. The 2 out of 100 males with protanopia and the 2 out of 100 males with deuteranopia know they have a color vision problem. They are confused by the names others give to color because to them everything looks pretty much the same. The biggest difference between the protanope and the deuteranope is the degree of brightness in their vision. Things look brighter to the deuteranope.

Testing for color blindness is very simple. Usually Ishihara Plates are used. Each plate is a circle displaying a number composed of colored dots. All the patient has to do is identify the numbers.

A special set of plates has been designed for very young children. Since they might not know their numbers familiar shapes such as stars, circles, squares, and animals are displayed instead. It is important for children to be tested before starting school. Color blindness can create problems for them in the classroom where color plays such an important role in their education.
colorvisiontesting.com/

critiquewall.com/2007/12/10/blindness
www.eyecaresource.com/conditions/color-blindness/

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