FoundationalConceptsNeuroscience

men. Thus, in any large group of individuals, there may be quite a few
men (and a small number of women) who are color anomalous. How-
ever, unless they undergo careful testing of color vision, it is unlikely
they will even know that their color vision is different from the norm.
A more substantial change in color perception occurs if there is
enough of a change in the opsin gene to render the resulting cone
opsin protein nonfunctional. In such cases, one entire type of func-
tional cone receptor cell (S, M, or L) is lost. If this is either an L (red) or
M (green) cone, the resulting condition is called red-green color blind-
ness. It is characterized by a significant loss of ability to distinguish
between various shades of color in the green-yellow-red region of the
color spectrum. Because the L and M cone opsin genes are located on
the X chromosome, there is a difference in prevalence between males
and females: about 2 percent of males and less than 0.1 percent of
females have red-green color blindness. Again, this is a substantial
number of males who have this condition.
There may also be genetic variation in the S cone opsin gene that
renders the S cone opsin protein nonfunctional. This results in a
change in the ability to discriminate between colors in the blue-green-
yellow region of the color spectrum. This condition is called blue-
yellow color blindness. For some reason, the S (blue) opsin gene is
much less likely to be altered to produce a nonfunctional S cone opsin.
Thus, blue-yellow color blindness is relatively rare, occurring in less
than 0.01 percent of the population. Also, because the S opsin gene is
not located on a sex chromosome (it is found on chromosome 7 in hu-
mans), there is no difference in prevalence between women and men.
Cone photoreceptor cells with their cone opsin photoreceptor pro-
teins are sensitive to bright light and have input into the perception
of color. They operate in sunlight or in other sources of sufficiently
bright light. Rod photoreceptor cells with their rhodopsin photore-