Colour Vision Genetics

Approximately 1 in 15 Irish males have a congenital red/green colour vision defect, compared to 1 in 200 females- they are sex linked genetic defects which are carried on the X chromosome. Because males inherit a Y chromosome from their father, if they receive a “defective” X from the mother, they will be colour deficient.

A daughter could inherit a defective X from either parent, but as she has two X chromosomes, and the least defective gene is always used, the other (normal) one would therefore be used and she would be a carrier but would herself be colour normal. This female would have a 50% chance of passing this gene to a child and having a colour defective son, or carrier daughter.

Colour defective males will pass the gene defect to daughters, but their sons cannot inherit an X chromosome from a father. With a colour defective father and carrier mother, a daughter has a 50% chance of being defective, but must be a carrier. Only the daughter with a colour defective mother (with 2 defective Xs) and colour defective father is guaranteed to have a colour vision defect.

Inheritance of Red/Green colour deficiencies

Inheritance of Colour Vision defects

Congenital tritanopia is extremely rare, affecting only 1 in 13000  and it is not sex linked- affecting males and females equally. Because of its rarity, the genetics are far more difficult to determine. Congenital tritanomaly is so rare, that some authorities state it doesn’t happen.

Complete colour blindness- the inability to perceive colour is extremely rare. These people would be termed monochromats as they could match any colour with any other just by adjusting only its brightness. There are two types of monochromats, rod and cone.

Rod monochromats (the only truly colour blind one) have no functioning cones, and only therefore have rod function- this causes a severely poor standard of vision- they have no functioning central vision, which causes nystagmus (wobbling eyes), and extreme light sensitivity.

Cone monochromats vary significantly depending on the type of cones functioning, the incidence is rare- affecting approximately 1 in 100,000 men, 1 in 10 billion women, while for other types the incidence is so rare as to be unknown.

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