Colour blindness

Colour blindness is a genetic disease. The related gene is located on the sex chromosome. Therefore, it is a sex-linked genetic disease.

There are several types of colour blindness. The most serious one is the total colour-blindness. In total colour-blindness, the patient cannot distinguish colours. They can distinguish dark and light only. The related gene is located on the Y chromosome. Therefore, only males would have the disease.

Other types of colour-blindness can be the red-green colour-blindness and the yellow-blue colour-blindness. The patients of red-green colourblindness cannot distinguish red from green, while yellow-blue colourblindness cannot distinguish yellow from blue. These genes are located on the X chromosome. So, they all are sex-linked genetic diseases and can be transmitted to the next genertaion.

Within these types of colour-blindnesses, the red-green colourblindness has the greatest disturbance to normal daily life. Drivers cannot have this colour-blindess, because if the driver cannot discriminate red from green, they won't know when to move their cars. If all cars move at the same time, there would be some severe car accidences.

Patients of colour-blindness cannot study science subjects, because students of science subjects are required to use colours to identify experimental results.

Example for the red-green colour-blindness.

Let	X be the X chromosome carrying the normal vision gene;
	X be the X chromosome carrying the colour-blindness gene;
	Y be the Y chromosome. There is no related active gene on it.

Then, the phenotypes would be as follow.:

Genotype
XX	is the female with normal visual sense;
XX	is the female with normal vision, but carrying the colour-blindness gene;
XX	is the female with colour-blindness;
XY	is the male with normal vision;
XY	is the male with colour-blindness.

References：

(1)	(web)	Wikepedia
(2)	(web)	KidsHealth
(3)	(web)	Test for colour blindness
(4)	(web)	EyeMDLink
(5)	(web)	StLukesEye

(03.08.2008)