The genetics behind color blindness: the causes

There are various types of severity of color blindness, and the impairment of color vision is caused by a congenital alteration of the cones, the cellular structures responsible for color perception. In each individual there are 3 different types of cones: 

• long or red ones;
• medium or green ones;
• short or blue ones. 

A genetic alteration of these cones causes an abnormality in vision, which, in contrast, under normal conditions is called trichromatism because it is guaranteed to perceive red, green, and blue.

The most frequent congenital defects of color blindness are: 

• protanopia (insensitivity to red), the most frequent form;
• deuteranopia (insensitivity to green);
• tritanopia (insensitivity to blue), which is much rarer. 

They are all united by the fact that those who are affected experience difficulty in recognizing the shades of these colors. In these cases, in fact, the patient has no discrimination of color shades.

If, on the other hand, there is a total absence of color vision, we speak of bichromatism, a condition in which one of the 3 primary colors is not fully perceived. 

Color blindness is a congenital disorder that is genetically based and, therefore, transmitted by sex chromosomes. Man and woman have 23 pairs of chromosomes: in 22 pairs the chromosomes are the same, while in the 23rd pair (that of sex chromosomes) in woman there are 2 X chromosomes while in man 1 X chromosome and 1 Y chromosome. 

Color blindness is an alteration of the X chromosome: 

• the woman to be colorblind must have 2 X chromosomes, thus a colorblind father and a mother who is either colorblind or a healthy carrier (who has the gene but does not manifest the condition);
• the man to be declared color-blind must have only one X chromosome. 

It goes without saying, therefore, that it is men who are most susceptible to this visual defect with a probability rate of 8% (of these, 5% are unknowingly affected by protanomaly, i.e., mild red color alteration). In contrast, women, despite the fact that they are the ones who pass it on to their children, have a 1 percent risk.

The diagnosis of color blindness is important because, while still maintaining the ability to lead a normal life, some occupations live on color codes: once a person is diagnosed to be color blind (absolute blindness toward a color and not only an abnormality), he or she will not be able to get a job of an electrician, bomb maker, firefighter, fabric sorter, etc. 

The diagnosis is very simple. Ishihara tables are used. These are tables in which a matrix of points of varying size defines a number or symbol (star, circle or triangle): 

• easily recognizable to people with normal color perception;
• less recognizable among those with a chromatic anomaly;
• completely invisible among those with dichromic vision.

At what age is it good to undergo this examination? When the first screening eye examination is done in children with no special disorders, that is, at 3 years of age. 

Then, in cases requiring more in-depth investigation, there is a more complex test, the Farnsworth test, an extremely effective method of assessing a person's aptitude for perceiving and distinguishing colors by matching 100 or 120 tablets (which differ by very small shades of color) according to their gradation.

There is no cure to recover from color blindness. Some recent gene therapies (aided by steady advances in bioengineering) conducted on animals, however, are showing encouraging results. In fact, in certain male dichromatic monkey specimens, they were able through a viral vector to restore normal color sensitivity.