The abnormality experienced by Robert is color-blindness, a genetic disorder caused by a disparity in the
reaction to certain colors in one or more of the light-sensitive cells located in the retina of the eye.
These cells, called cones, detect light wavelengths which make it possible for the retina to discern colors
from one another. The abnormality is due to the dysfunction of one or more of the three color vision
pigments (red, green, blue) involved. If one pigment is dysfunctional and the other two are functional,
the condition is called dichromatism. An example of dichromatism is redgreen color blindness
The prevalent form of color blindness is due to a gene problem. Thus, redgreen color blindness is
common in males, but not females. About 7% of males have some degree of color blindness, which is
over eight times more common than in females. The basis for this male prevalence is the tandem
configuration of the genes for the red and green photopigments on the X chromosome. Since males
have only one X chromosome, an X-linked mutation is more likely to affect them than females, who are
more likely to have a good gene on one of their two X chromosomes.