Colorblindness and Color Vision Deficiency

Colorblindness or color vision deficiency alters our ability to perceive certain colors. There are many different types of colorblindness; some are more common in the population than others. It is estimated that 10 – 12 % of the population has some type of color blindness. What is colorblindness and how does colorblindness affect ones daily life? Many people confuse colorblindness with not being able to see any colors at all. However, there are many different types of colorblindness, ranging from seeing in black and white to not seeing one particular hue in the spectrum.

Lesson Objectives:1. Learn the different types of colorblindness/ color vision deficiency and how color blindness affects

vision2. Learn the genetic basis for colorblindness/ color vision deficiency (see Biology Frameworks 3.5)3. Use Punnett square to determine probabilities for a genotypes and phenotypes (see Biology

Frameworks 3.6)

Pre-activity Questions:

1. How important is seeing color to you in your daily life? What things would be difficult to see without color?

2. Are you or do you know someone who is “colorblind”?

3. If so, what are your experiences seeing colors or have you ever asked a colorblind/color vision deficient person what difficulties that they have with colors?(Are there particular colors that you have difficulty with? What do you/they “see” instead of that color?)

4. What do you think are some of the difficulties that people with colorblindness face?

5. List the common types of colorblindness, if any, that you are familiar with? Do you know any famous people who have colorblindness?

Discussion I: A visit to the Optometrist: Testing for colorblindness

The Ishihara test is a common test that optometrists use to see if you are colorblind. It is really simple! All you have to do is read the numbers that are with the circle. If you see a different number in the box or do not see the number at all, then you may have some sort of color vision deficiency. The following tests are for most common form of colorblindness, red - green colorblindness. (If you recognize that you have a difficulty, follow up with an eye exam.)

Activity I: Ishihara test for colorblindness

What are the numbers that you see?

Here are the outcomes for the tests:

Activity II: What is the mode of inheritance for different types of colorblindness?

You decide for a school project that you would like to learn more about how different types of colorblindness is inherited.

First you practice your Punnett square analysis:

If you have a trait that is autosomal dominant, what would be the outcomes?

If you have a trait that is autosomal recessive, what would be the outcomes?

If you have a trait, that is sex-linked dominant, what would be the outcomes?

If you have a trait that is sex-linked recessive, what would be the outcomes?

You decide first to stop random people on the street and test them using the same test above, the Ishihara colorblindness test. You test 100 males and 100 females.

Here are your results:

Ishihara Test Male FemaleNot Colorblind 89 99Colorblind 11 1

What is the percentage of females that are color blind? What is the percentage of males that are colorblind?

Based on your observations, can you predict the mode of inheritance of colorblindness?

You decide that you need more information so you visit a geneticist, who is analyzing the family tree of parents who want to know if their child is at risk for colorblindness.

Pedigree Chart: Here is the family tree of the father. What is the mode of inheritance for colorblindness in the family below?Circles are females, Squares are males.White means that you do not have the trait or carry the trait.Black means that you have the trait.White with a black circle in the middle means that you are a carrier of the trait.

Discussion II: Are there different kinds of colorblindness? Humans are considered trichromats because they perceive colors generated from a three-color system, red, green, and blue light. In our retinas, we have two types of photoreceptors; rods help us see in dim light and cones that are important for color vision. In humans, there are three types of cones, L-cones (long wavelength sensitive), M-cones (medium wavelength sensitive), and S-cones (short-wavelength sensitive). These cones respond maximally to red (~550 nm), green (~500 nm), and blue ~(450 nm) wavelengths of light. (Remember there is a lot of processing beyond the photoreceptors, which include other retinal cells and visual centers in the brain.)

There are many forms of colorblindness.

Monochromacy is the most rare. This is an absence of all rod or all cone photoreceptorsRod or Cone

Dichromacy is if you are missing one type of cone photoreceptor.

Anomalous Trichromacy is when all of the cone photoreceptors are present but one or more respond maximally to a different wavelength.

Resources:

A Site which demonstrates the effects of color vision deficiencies: http://colorlab.wickline.org/colorblind/colorlab/