Almost everyone has heard or used the term “color-blind,” but what causes color-blindness and what does a color-blind person actually see?

First let’s update our terminology: professionals prefer the term “color deficient” because virtually all “color-blind” people see at least some colors. Second, although there are acquired color vision deficiencies (generally due to diseases such as glaucoma and diabetes), our subject today is hereditary color vision deficiencies. Here are a few facts:

What goes wrong when a person doesn’t see colors properly? A genetic mistake in the chemical structure of either the red or the green receptors prevents the retina from sensing red and green as different colors. In mild to moderate deficiencies (called “anomalies”), red and green are still perceived as being slightly different from one another. People with this condition are referred to as trichromats, because the blue, green and red receptors still function independently.

In more severe deficiencies, red and green colors appear as one and the same. In these severe cases, only 2 receptors are functioning: the blue receptors and the combined red/green receptors. These people are termed dichromats.

Here is how colors are perceived by normals, anomalous trichromats, and dichromats:

Normal trichromat with normal, vibrant color vision
Anomalous trichromat (mild to moderate deficiency) with subdued red and green colors
Dichromat (severe deficiency) with no red or green colors

Look at how differently we see colors as shown by a color vision test known as a D-15 where people are instructed to place the colored caps in order by color:

Color normal people do this
Color deficient people do this

So how do we get color deficiencies? Blame Mom! Mothers carry the defective color gene on their x-chromosome. When they contribute the defective x-chromosome to their sons, the sons have defective color vision (sorry!). When the defective x-chromosome is passed to daughters, the daughters then become carriers. And so it continues into future generations.

In Part II, we will discuss the recent breakthrough in treatment of color vision.