Sex-linked Genes

Ishahara tests (Activity)



Monochromatic representation of Ishihara test to a color blind individual as it emerges to something visible to a color-sighted individual

The genes encoding photoreceptor proteins for the long wave-length (reds) and middle wave-lengths (greens) reside on the X chromosome at Xq25. Since the Y-chromosome is not homologous, any mutation to either of these genes that render them non-functional results in an inability to perceive either of those colors. Men are more susceptible to the condition of red-green colorblindness since they are hemizygous. This means that there is no corresponding gene that could complement a deficient red or green photoreceptor gene.

Dr. Shinobu Ishihara published his test for color perception in  1917 and this test is widely used to detect deficits in color perception. Below are  examples of Ishahara plates. Record the number that you perceive in each plate and discuss with the rest of the class.

Ishihara Test


  1.  As you go through the plates above, note the number that you see (if any).
  2. The genes for the Red and Green receptors are on the X-chromosome, who are most affected by mutation? Create a Punnet Square to illustrate how this works.
  3. Can women be color-blind for red/green?
  4. Humans have 3 color light receptors and have trichromatic vision. Some women are described as possibly having tetrachromatic vision (seeing 4 colors) and being able to discriminate colors invisible to the rest of us. Describe a mechanism for why this could happen. Why is there a possible gender bias?

Sequence links