Two fully functional variants of the L opsin gene have been found in
humans, giving rise to two L cone opsin photoreceptor proteins that
absorb light of slightly different wavelengths, their absorption max-
ima separated by only a few nanometers.
Human females have two copies of the X chromosome (XX), one
inherited from their mother and one from their father. Human males,
on the other hand, have only one X chromosome and one Y chromo-
some (XY). This means that some human females will have both of
the L opsin variants (a different version on each X chromosome) and
will thus be tetrachromats, possessing a more nuanced ability to dis-
criminate different colors, especially in the orange-red region of the
color spectrum. Males, having only a single X chromosome, would
have either one version of L opsin or the other, but not both, and so
could not have more than trichromatic color vision.
Other sorts of genetic variability also affect color vision. Certain
changes in the amino acid sequence of an opsin protein will leave
the protein functional but slightly change its light-absorbing prop-
erties, and the ability to discriminate different colors. This condition
is called “color anomalous.” Because the M and L opsin genes are on
the X chromosome, if a woman has an altered opsin gene on one of
her X chromosomes, it is likely that the other X chromosome will
contain the normal variant. Thus, enough of the normal variant will
be expressed to yield normal capacity for color discrimination. For a
woman to have the color-anomalous condition, she would need to in-
herit the variant opsin from both parents, a more unlikely occurrence.
However, for males, with their single X chromosome, color anomalous
vision is more likely. The prevalence of the color-anomalous condition
among males is about 6 percent, while its prevalence among females
is only about 0.4 percent. These are sizable prevalences, especially for