by Alicia M Prater PhD

Created on: April 14, 2008   Last Updated: March 05, 2011

A person who is color blind is unable to distinguish between particular colors. Color blindness is a sex-linked recessive genetic disorder, so men are more often affected than women and it is present from birth. However, the characteristics of color bindness may also arise when the eye is damaged, particularly the retina, rendering a person unable to dinstinguish between colors later in life.

Types of Color Blindness

Light receptors called rods and cones are present in the back of the eye, the retina. These receptors communicate to the brain via the optic nerve. Rods detect light regardless of wavelength and help the brain determine light intensity as bright or dim. Cones consist of red, blue, or green wavelength detectors and are active in bright light. Each type of cone detects a particular wavelength of light. Color blindness is caused by a person lacking a particular type or types of cones, or if a particular set is non-functioning.

The most common form of color blindness is red-green color blindness, meaning that a person can not distinguish between those two colors. Rather than being "blind", the brain sees both as the same color. Which color depends on which cones are deficient. The disorder is actually categorized as a disability because it can hinder activities, particularly those requiring discernment of color or the reading of signs from a distance. There are many simple tests to determine if a person is color blind that consist of colored dots making a shape among dots of another color - red and green or, in some cases, blue and yellow. Most people do not know they are color blind until an event occurs that points out their inability to distinguish between the colors.

The Genetics of Color Blindness

The genes encoding the cones are on the X chromosome, which is transmitted differently to males and females (i.e. sex-linked). Because males only inherit one X chromosome, they receive the recessive gene from their mothers. Women need to receive two mutated X chromosomes, one from each parent, before they exhibit the disorder. Women who receive only one X with the mutated gene are carriers and do not have the disorder.

Heredity breaks down as follows:

-If a mother is a carrier, her sons have a 50% chance of receiving the X with the mutation and being color blind, but her daughters would not be color blind unless their father was as well. However, the daughters have a 50% chance of being carriers.

-If a mother is color blind then her sons will definitely be color blind and her daughters have a 50% chance of being so as well (Any daughters who are not color blind will be carriers). If their father is also color blind, then all children will be.

-If a father is color blind and the mother is not, then the daughters will be carriers. If the mother is a carrier, then they have a 50% chance of being color-blind, as mentioned above. A father's status does not affect the son because he passes on the Y chromosome, not an X.

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