Genetics of colour-blindness.

Normal Normal trichromatic colour vision is based on three different kinds of cone cells in the retina, each sensitive to only one of the three primary colours, red, green or blue. Opsins. Each type of cone cell has specific light absorbing proteins called opsins. The genes for red and green opsins are on X chromosome, while the gene for blue opsin is present on autosome 7.

TYPES.

Mutations in opsin genes cause three types of colour-blindness.

Dichromatic.

A dichromatic can perceive two primary colours but is unable to perceive the one whose opsins are missing due to mutation.

 Protanopia is red blindness,

 Deuteranopia is green blindness, while

 Tritanopia is blue blindness.

 Protanomalous and Deuteranomalous.

Some people can detect red and green but with altered perception of the relative shades of these colours. They have abnormal but still partially functional opsins. They are protanomalous and deuteranomalous for red and green weakness respectively.

Monochromatic.

A monochromat can perceive one colour. Monochromacy is true colour-blindness. Blue cone monochromacy It is an X – linked recessive trait in which both red and green cone cells are absent. That is why it is also called red – green colour-blindness. It is a common hereditary disease.

Inheritance.

Like any sex – linked recessive trait, it also zigzags from maternal grandfather through a carrier daughter to a grandson. It never passes direct from father to son. Chances. This type of colour blindness is more common in men than women, because chances for a male to be affected by it are much more than a female.