The incredible discovery of colour blindness by John Dalton in the early 1790’s has lead to many scientific explanations as to the cause of colour blindness. The importance of colour on earth is that it allows human being’s to accomplish tasks in everyday life. Colour vision deficiency affects 5% of males and 0.5% of females (U.S National Library of Medicine 2012). This deficiency is the inability to “see” certain colours, or different perception of colour (State Government of Victoria 2011).
The mechanism of colour vision is based on its absorption of light by visual pigments enclosed within the rod cells and cone cells, which is centralised in the photosensitive structure called Fovia; dense grouping of cone cells, situated in the retina; light sensitive tissue (CinemaSource 2002). Rod cells allow visualisation in low-level lighting, and cone cells are responsible for colour reception (University of Illinois of Chicago n.d.). In the Fovia, there are three types of receptor cells: blue cone (S cones – short wavelength), green cone (M cones – medium wavelength), and red cone (L cones – long wavelength) (Augustine ,GJ, Purves, D, Fitzpatrick, D, et al. 2001). Signals from the Fovia are delivered to the brain allowing visualization in colour through the human eye. The absence of one or more of these receptor cells leads to colour vision deficiency (University of Illinois of Chicago n.d.).
With 23 pairs of chromosomes in the human system, one pair is the sex chromosome where the receptor cells are situated. In a female there are two pairs of X chromosomes, and in a male there is an X chromosome and a Y chromosome (U.S National Library of Medicine 2012). The receptor cells are situated on the X-chromosome. The absence of one or more of the receptor cells is commonly due to a genetic error, known as ‘X-linked recessive genetic disease’, which mainly affects the sex chromosomes X and Y. This disease occurs in the sex chromosome where both copies of a gene must either be mutated or damaged (Health Grades Inc, 2012). Since colour blindness is due to a genetic error, it can be stated that colour blindness is genetically inherited.
For colour vision deficiency to be inherited, both the mother and father have to be carriers of this genetic disease. Inheriting this gene error is usually from mother (carrier) to son, because the mother is the carrier without having any diseases or symptoms of the disease. The likelihood of inheriting vision deficiency is fifty-fifty. However, males are more likely to inherit this trait because generally male cannot have a ‘bad’ X chromosome along with a ‘bad’ gene without getting the disease. Thus, an altered copy of the gene in each cell is enough to cause the disease. Whereas, females typically have a second ‘good’ X chromosome, thus making them the carriers. Therefore, a mutation is required in both copies of X-chromosome to cause the disease. (Health Grades Inc, 2012)
Without colour vision, there are many difficulties in everyday lives that a colour deficient person encounters, such as, traffic lights. This is why the discovery was important in that it has lead to research that lead to the invention of ‘corrective lenses’, which are glasses that are specialised for the specific type of vision deficiency and allows the individual to have normal vision. Therefore, allowing normal functioning in day to day activity.
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Genetics of X-linked Recessive Genetic Diseases, viewed 18th March
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