Reese Fulton Ap Biology X Linked Color Blindness
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 a autosomal dominant treat.
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 most likely 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 has predisposed the locus to relatively common unequal recombination or rearrangement events that give rise to red/green hybrid genes and to deletion of the green-pigment genes. Because of the genes are highly homologous and adjacent to one another, recombination’s between them is common 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 array 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.