X vision. When color blindness occurs one or more

X linked
color blindness (also referred to as color vision deficiency) is a condition
that affects and individual’s perception of color. According to Colour Blind
Awareness approximately 1 in 12 males and 1 in 200 females are affected by
color blindness Red-Green being the most common. A less common and more severe form of color vision
deficiency called blue cone monochromacy causes very poor visual acuity and
severely reduced color vision.  In the eye, there are 3 distinct
kinds of color receptors that are sensitive to different wavelengths of light. The
eyes take in light into all 3 rods to produce normal color.  Mutations in the genes OPN1LW, OPN1MW, and
OPN1SW cause forms of color vision deficiency. The OPN1LW, OPN1MW, and OPN1SW
genes provide instructions for making the three opsin pigment proteins in
cones. These proteins that are produced play a key role in colored vision. When
color blindness occurs one or more of the cones is not functioning. For example,
the disorder tritanomaly (blue- yellow color deficiency), which is rarer,
causes problems distinguishing between shades of blue and green because the S
cone or blue cone is missing.

Color
blindness is passed from mother to son on the 23rd chromosome, which is known
as the sex chromosome because it also determines sex.  Males are more likely to be color blind that
females as for the genes linked with color blindness are at Xq28 on the X
chromosome. Females have two X chromosomes whereas Males have an X and Y
chromosome.  For a male, the mutation
only must be found on his X chromosome whereas for a female to be color blind
the mutation must be present on both X chromosomes.  In addition, this means that a male cannot
pass on the color blindness gene to a son.  Genes on the X chromosome can be recessive or
dominant. Their expression in females and males is not the same. Tritanomaly is
inherited as an autosomal dominant defect, with incomplete penetrance.
Red/Green color blindness is an autosomal dominant treat.

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At the DNA
level, differences in amino acids involved in tuning the spectra of the red and
green cone pigments account for most of the variation. One source of variation
is Ser180Ala polymorphism that accounts for two different red pigments and that
plays a significant role in variation in normal color vision as well as determining
the severity of color blindness. This polymorphism comes from gene conversion
by the green-pigment gene. Another common source of variation is the existence
of several types of red/green pigment with different properties. The red and
green-pigment genes are arranged in a head-to-tail tandem array on the
X-chromosome with one red-pigment gene followed by one or more green-pigment
genes. The high homology between these genes results in recombination’s between
the genes and can lead to irregular pigments. The rearrangements promote
duplications of the red and green genes so that most people have extra pigment
genes. Such events constitute the most common cause of red-green color vision
defects. Only the first two pigment genes of the red/green are expressed in the
retina and therefore contribute to the color vision phenotype. The severity of
red-green color vision defects is inversely proportional to the difference
between the wavelengths of maximal absorption of the photopigments encoded by
the first two genes of the array. Women who are heterozygous for red and green
pigment genes that encode three spectrally distinct photopigments have the
potential for enhanced color vision.

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