by Janet Spencer

It’s estimated that there are about 300 million color blind people worldwide, nearly equal to the population of the U.S. Come along with Tidbits as we look at the facts!

COLOR BLIND FACTS

Color blind people are more accurately called “color deficient.”

• There are many different severities of color vision deficiency, from nearly normal color vision up to complete color blindness.

• It’s estimated that 98% of those with color blindness have red-green color blindness. Of those, 75% have trouble seeing green, while 24% have difficulty seeing red.

• The recessive gene for red-green color blindness is carried on an X chromosome. Women have two X chromosomes, whereas men, with XY chromosomes, have only one. Therefore, women need two of the color blind gene in order to be born color blind. Men, on the other hand, need only one.

• Women are often carriers of the trait without suffering from it themselves. Female carriers pass it down to sons (who will be color blind) and daughters (who will be carriers). A father cannot pass red-green color blindness on to his sons. But any woman who is herself red-green color blind will pass the trait on to her sons.

Red-green color blindness is far more prevalent in men than women, with 1 in 12 men having the condition (8%) while only 1 in 200 women do (0.05%). This means that 95% of color blind people are male.

• Another type of color blindness is blue-yellow. This type is rare. Both men and women carry this genetic anomaly, and they experience it at equal rates.

• Babies are born color blind, with vision improving as they age. Color perception is usually fully developed by the age of 6 months. This is due to the fact that the cones are not yet fully developed at birth, meaning all the colors cannot yet be fully seen.

• It’s possible but rare to be color blind in one eye while having normal vision in the other.

• Specially tinted glasses can help some people distinguish between certain colors, though they will not restore normal color vision, or help pass tests for color blindness. There is no cure for color blindness.

• Color blindness does not improve with age, nor does it get worse.

• Strongly color blind people might only be able to tell about twenty hues apart from each other, while people with normal color vision can see over a hundred different hues and thousands of shades.

• People with red-green color blindness cannot tell when their skin is sunburned or recognize a blush. It’s hard to tell when a banana is ripe, or when meat is thoroughly cooked. They may have difficulty matching socks or recognizing the colors on a stoplight.

• Some countries bar the color blind from certain professions, such as doctors or electricians. In the U.S., pilots cannot be color blind. In some countries, they cannot get a driver’s license. The U.S. military considers some of their jobs off-limits to color blind recruits.

A railroad accident in Sweden in 1875 which killed nine passengers was thought to be caused by a color blind conductor who was unable to read a signal. Afterwards, a method of testing color vision was developed and applied to railroad workers.

WHAT CAUSES IT?

The back of the eye, called the retina, is made up of rods and cones. Rods are found around the outer edges of the eye. They mainly see in black and white, while also serving as motion detectors. Rods work better in the dark than cones do.

• Cones are located in the center part of the retina. They pick up color, and register detail.

• There are three kinds of cones. Each type picks up a different wavelength of light, which registers in the brain as colors. The S-cones register short-length (blue), M-cones register medium-length (yellow-green), and the L-cones register long-length (red).

• The cones are not evenly divided. There are more L-cones collecting red light (65%) than M-cones collecting yellow-green hues (33%). On the far end of the scale are the S-cones, which see blue wavelengths (2%).

• A human with all three of these cones working correctly is said to have trichromatic vision, meaning “three-color” sight. A person with only two of the three types of cones is said to have dichromatic vision.

• A color blind person has cones that are either missing entirely or defective on some level, on a scale from slight to severe. The cones are not uniformly defective from one color blind person to the next, leading to myriad different levels of color blindness. But the more cones that aren’t functioning correctly, the worse the color blindness will be.

COLORBLIND ISLAND”

    Complete color blindness, denoting someone who can see in only shades of black, white, and gray, is rare in normal populations. It’s called achromatopsia, from the Greek “without color.” This is a recessive disorder where both parents need to be a carrier for offspring to inherit it. It’s caused when only rods are present in the eye, without any cones. The disorder affects about one out of every 33,000 people, or 0.003% of the population, in the U.S.

• However, there’s a tiny atoll in the South Pacific called Pingelap Island, where about 10% of the 250 inhabitants have it. This is due to a typhoon that swept the island in 1775, leaving only 20 survivors. One of those survivors was the tribal leader, who carried the gene, though he did not have it himself. As the generations passed, more and more descendants inherited it due to inbreeding. In 1997, a neurologist studied the genetics of Pingelap, and wrote a book called “The Island of the Colorblind.”

TESTING TESTING

One common test consists of a series of circles containing colored dots. The dots are arranged, so a person with normal vision can read the number hidden among the dots, while a color blind person cannot. Called the Ishihara test, it’s named after the Japanese ophthalmologist who designed the test in 1917. Many people with normal color vision can’t pass this test with a perfect score.

• Another test is the anomaloscope, which shows cards of various colors, and a second card with a slider bar allows the user to adjust the color to match the first card.

• There’s also the Farnsworth-Munsell 100-Hue Test. This test consists of 85 pastel-colored chips of similar but slightly different colors, which must be arranged correctly.

• All of these tests are available online.