Why is our son colour blind when both of us are not? Isn’t colour blindness hereditary?

I had an interesting case recently where a young boy, Adam, was brought in by his Mum and Dad for an eye exam to see if he was colour blind.

As he was quite young the paediatric colour vision test was used. This test uses easy to recognise shapes such as a circle, square and star, and pictures such as a car, boat, and house. This is a child friendly alternative to the Ishihara Colour Vision Test (the standard colour vision test used in eye exams), which uses number presentations to assess colour vision.

It was found that Adam had a red-green colour deficiency (technical name is Deutan, the most common form of colour vision defects) which means that he mixes up all colours which have some red or green as part of the whole colour.

It was explained that this was a genetic defect that would not improve or worsen. The best management is to minimise the impact the colour deficiency has on everyday life for Adam. Adam’s pre-school teacher should be made aware of the colour deficiency so that she could help him when required. His parents were directed to the very informative website www.colourblindawareness.org for further information.

Adam’s parents wanted to know how he could be colour deficient, when neither of them was colour deficient. It was explained that the red/green colour deficiency was passed from Adam’s mother to Adam on the 23rd chromosome, which is known as the sex chromosome because it also determines sex. Colour deficiency is a recessive, sex-linked trait. Recessive meaning that if you have another healthy chromosome, it won’t show up.

Chromosomes are structures which contain ‘genes’. The colour defective ‘gene’ is carried on one of the X chromosomes. Men have only one X chromosome as they are XY, and women have two X chromosomes and they are XX. If the man’s X chromosome carries the colour defective ‘gene’ he will be colour defective. If the woman’s X chromosome carries the defective ‘gene’ she will be a carrier. She will only have defective colour vision if both her X chromosomes carry the defective ‘gene’. Adam would have received the Y chromosome from his father, and his father can only pass an X chromosome to his daughters. In Adam’s case, his mother gave the X chromosome with the faulty colour defective ‘gene’ to Adam.

Colour defective vision is far more common in men than women because it is a recessive, sex-linked trait on the X chromosome. It affects approximately 1 in 12 men (8%) and 1 in 200 women in the world. Due to the high percentage of men affected, and the impact colour deficiency can have on career choice and career promotion, it makes sense to routinely check for colour deficiency in all young children, and particularly young males. Consequences of inaccurate colour discrimination could include mistakes made in identification of coloured signals in railway, shipping or aviation, which potentially could result in accidents and loss of life. Future career choices would be a consideration, and Adam’s parents were advised to steer Adam away from interests or future career options that would require good colour discrimination, such as civil aviation, train drivers, fire service officers, armed services, and work in paint, textile, photography and fine art.

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