THE GENETICS OF VISUAL 
PIGMENTS

In the early 1980s Jeremy 
Nathans, while still an M.D.-
Ph.D. student at Stanford,
managed to clone the genes for
the protein portions of human
rhodopsin and all three cone
pigments. He found that all 
four pigments show strong
homologies in their amino acid
sequences: the genes for the 
red and green pigments, which
lie on the X, or sex, 
chromosome, are virtually
identical--the amino acid
sequences of the proteins show
96 percent identity--whereas
the genes that code for the 
blue pigment, on chromosome 
7, and for rhodopsin, on
chromosome 3, show much 
larger differences, from each
other and from the red and 
green genes. Presumably, some
time in the distant past, a
primordial visual pigment gave
rise to rhodopsin, the blue
pigment, and the common
precursor of the red and green
pigments. At a much more 
recent time the X-chromosome
genes for the red and green
pigments arose from this
precursor by a process of
duplication. Possibly this 
occurred after the time of 
separation of the African and 
South American continents, 30 
to 40 million years ago, since 
old world primates all exhibit 
this duplication of cone 
pigment genes on the X-
chromosome, whereas new 
world primates do not.

    Cloning the genes has led to 
a spectacular improvement in 
our understanding of the 
various forms of color 
blindness. It had long been 
known that most forms of color-
vision deficiency are caused by 
the absence or abnormality of 
one or more of the three cone 
pigments. The most frequent 
abnormalities occur in the red 
and green pigments and affect 
about 8 percent of males. 
Because of the wide range of 
these abnormalities the subject 
is complex, but given our 
molecular-level 
understanding, it is fortunately 
no longer bewildering.

    Very rarely, destruction to 
certain cortical areas can cause 
color blindness. Most often this 
occurs as the result of a stroke.