Genetic Mode Of Inheritance Of Colour Blindness Biology Essay

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Colour blindness is the inability of a person to differentiate between some colours. Mostly it is genetic in nature, but can also occur due to nerve, eye or brain damage or due to exposure to any chemical. First of all paper on colour blindness were published by john Dalton who himself was colour blind. That's why this disease is also known as daltonism.

The human retina consists of two types of cell which are known as: rod cells and cone cells. Rod cells are rode like in shape and active in low light while shape of cone cells is conical and are active in daylight. There are three kinds of cone cells, which consist of different type of pigments that get activated on absorption of light. These three kinds of cone cells are:

Anomaloscope is the instrument through which we can easily observe deuteranomaly and protanomaly. It mixes red and green light in variable proportions, for evaluation of a fixed spectral yellow. If this is done in front of a large population of men as the amount of red is increased from a low value, first a small percentage of people will proclaim a match, while most of the addressees see the mixed light as greenish. These are the deuteranomalous observers. Next, as more red is added the majority will say that a match has been achieved. Finally, as yet more red is added, the remaining, protanomalous, observers will declare a match at a point where everyone else is seeing the mixed light as definitely reddish.

Protanomaly: in this case person is having a mutated form of long wavelength pigment, whose peak sensitivity is at shorter wavelength than in normal retina, protanomalous individuals are less sensitive to red light than normal. They are less able to differentiate between colours and they do not see mixed lights as having same colours as normal observer. They also suffer from a darkening of the red end of the spectrum. This causes reds to reduce in intensity to the point where they can be mistaken for black. . Both protanomaly and deuteranomaly are carried on the X chromosome.

DEUTRANOMALY: in this person is having mutated form of medium wavelength pigment. The medium wavelength pigment is shifted toward the red end of spectrum resulting in decrease in sensitivity to green area of spectrum. This is most common form of colour blindness. The deuteranomalous person is considered "green weak". For example, in the evening, dark green cars appear to be black to Deuteranomalous people. deuteranomates are poor at discriminating small differences in the red, orange, yellow, green region of the spectrum.

TRITANOMALY: The persons suffering from this are having a mutated form of short wavelength pigment. The short wavelength pigment is shifted toward green area of spectrum. This is rarest form of anomalous trichromacy colour blindness. This mutation for colour blindness is carried on chromosome 7. Therefore it is equally prevalent in both male and female populations.

OCCURANCE OF COLOUR BLINDNESS

Colour blindness affects a significant number of populations, although exact proportion varies among groups. In Australia for example it occurs in about 8 % males and 0.4% females. In United States about 7% male's population is affected whereas only 0.4% female population is affected. It has been found that more than 95 percent of all variations in human colour vision involve the red and green receptors in male eyes. It is very rare for males or females to be "blind" to the blue end of the spectrum.

Prevalence of color blindness

Men

Women

Total

Overall

-

-

-

Overall (United States)

-

-

-

Red-green (Overall)

7 to 10%

-

-

Red-green (Caucasians)

8%

-

-

Red-green (Asians)

5%

-

-

Red-green (Africans)

4%

-

-

Monochromacy

-

-

-

Rod monochromacy (dysfunctional, abnormally shaped or no cones)

0.00001%

0.00001%

-

Dichromacy

2.4%

0.03%

1.30%

Protanopia (red deficient: L-cone absent)

1% to 1.3%

0.02%

-

Deuteranopia (green deficient: M-cone absent)

1% to 1.2%

0.01%

-

Tritanopia (blue deficient: S-cone absent)

0.001%

0.03%

-

Anomalous Trichromacy

6.3%

0.37%

-

Protanomaly (red deficient: L-cone defect)

1.3%

0.02%

-

Deuteranomaly (green deficient: M-cone defect)

5.0%

0.35%

-

Tritanomaly (blue deficient: S-cone defect)

0.01%

0.01%

-

DIAGNOSIS

Normally colour blindness is diagnosed through Ishihara colour test, it consists of a series of pictures of coloured spots, and the test is most often used to diagnose red-green colour deficiency. A figure is embedded in the picture as a number of spots in a slightly different colour, and can be seen with normal colour vision, but not with particular colour defect. The full set of tests has a variety of figure colour combinations, and enable diagnosis of which particular visual defect is present. The anomaloscope is also used in diagnosing anomalous trichromacy.

Because the Ishihara colour test consist of only numerals, it may not be useful in diagnosing young children, who have not yet learned to use numerals. In the interest of identifying this inconvenience early on in life, alternative colour vision tests were developed using only symbols (square, circle, and car).

http://upload.wikimedia.org/wikipedia/commons/thumb/e/e0/Ishihara_9.png/220px-Ishihara_9.png Example of an Ishihara colour test plate

The numeral "74" should be clearly visible to viewers with normal colour vision. Viewers with dichromacy or anomalous trichromacy may read it as "21", and viewers with achromatopsia may not see numbers.

TREATMENT

There is generally no treatment to heal colour deficiencies. However certain types of coloured filters and contact lenses may help an individual to better distinguish different colours. Optometrists can supply a singular red-tint contact lens to wear on the non-dominant eye. This may enable the wearer to pass some colour blindness tests, but they have little practical use. The effect of wearing such a device is akin to wearing red/blue 3D glasses and can take some time getting used to. Additionally, computer software and cybernetic devices have been developed to assist those with visual colour difficulties such as an eyeborg, a "cybernetic eye" that allows individuals with colour blindness to hear sounds representing colours.

In September 2009, the journal Nature reported that researchers at the University of Washington and University of Florida were able to give trichromatic vision to squirrel monkeys, which normally have only dichromatic vision, using gene therapy

RECENT ACHIEVEMENT RELATED TO COLOUR BLINDNESS

Colour-blindness Cured by Gene Injection in Monkeys

A simple injection of cells has cured monkeys of colour-blindness-giving a green light to future research into improving human vision with gene therapy, a new study says

According to this study monkeys were kept under investigation and it was found that some squirrel monkeys also have a form of colour blindness which is somewhat identical to humans. It was found that eyes of these monkeys lack a pigment gene that make red and green light visible to them.

EXPERIMENT

In the experiment the monkeys were presented to the screen filled with grey dots. Encouraged by a tone, the monkeys were to touch any dots that had changed from gray to another colour. Touching the coloured dots earned a grape juice reward.

Whenever the coloured dots were red or green, though, the colour-blind monkeys acted frustrated-sometimes even shaking the display, Neitz said.

After the initial round of touch screen tests, monkeys were injected a specially engineered virus behind the retinas of two of the colour-blind monkeys.

The virus contained genes for red pigment in cone cells-cells in the eye that responds to light and colour. The virus inserted the red-pigment genes into some of the monkey's green-sensitive cone cells, causing those cells to become red sensitive.

RESULT

Within about 20 weeks, the two monkeys were able to point out red and green, according to the study, to be published tomorrow in the journal Nature.

To previously colour-blind monkeys, the change confers profound abilities, noted visual neurobiologist and colour-vision expert Bevil Conway-for example, the ability to find fruit amid green leaves.

REFRENCES:

Colour-blindness Cured by Gene Injection in Monkeys By: Christine Dell'Amore

National Geographic News, September 16, 2009

http://kidshealth.org/kid/talk/qa/color_blind.html

Colour blindness-A rural prevalence survey by Maya Natu,Laxmi Smruti, 773/1-A, Erandawana, Near Kamla Nehru Park, Pune-41 004, India

www.colblindor.com

www.colorcube.com

principle of genetics by E.J.Gardner, D.Peter Snustad

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