 Colour blindness
inability to distinguish one or more of the three colours red, green, and blue.
(Ability to see colour exists
in only a few vertebrates, including, among others, man and
the other primates, fish, amphibians,
some reptiles, and some birds; and in bees and butterflies.)
In the retina, the light-sensitive layer of
tissue that lines the back and sides of the eyeball,
there are, in human beings, three types of cones,
the visual cells that function in the
perception of colour. One type absorbs light best in wavelengths of
blue-violet
and another in the wavelengths of green. The third type is most
sensitive to wavelengths of
yellow but is
also sensitive to red.
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Colour-blind
persons may be blind to one, two, or all of the colours red, green, and
blue.
(Blindness to red is called protanopia; to green, deuteranopia; and
to blue, tritanopia.)
Red-blind persons are ordinarily unable to distinguish
between red and green, while blue-
blind persons cannot distinguish between
blue and yellow. Green-blind persons are unable to
see the green part of
the spectrum.
Colour blindness, which affects about 20 times as many males as females,
is a sex-linked
recessive characteristic. A woman must inherit the trait
from both parents to be colour-
blind. A colour-blind man and a woman of
normal colour vision have daughters who have normal
colour vision but are
carriers of the trait—that is, the daughters may have colour-blind
sons and daughters who are carriers. The sons of a colour-blind man and
a woman with normal
vision themselves have normal vision and are unable
to pass the colour-blind trait on to
offspring. The son of a normal man
and a carrier woman may be colour-blind, and the daughter
of such a union
may be a carrier. Thus, colour blindness tends to skip generations. |