Red-green colour blindness: X-linked genes, inheritance and testing

Colour vision deficiency means seeing colours differently from most people and having difficulty telling some apart. The NHS says people can be born with it or it can start at any age, and that it is sometimes caused by eye conditions such as glaucoma, by diabetes, liver disease, multiple sclerosis, an eye or head injury, a stroke or certain medicines. This page is about the common inherited kind.

Colour Blind Awareness corrects a widespread myth. Being red-green colour blind does not mean only confusing red with green. People with it can confuse any colours that have some red or green in them, so blue and purple can be mixed up because the red part of purple cannot be fully seen, and problems can arise across reds, greens, oranges, browns, purples, pinks and greys. The charity also notes that effects can be mild, moderate or severe and that people with severe forms often think theirs is mild.

Children may not complain. The NHS lists signs a parent may notice: using the wrong colours when drawing, difficulty sorting by colour, lack of interest in colouring and smelling food before eating it.

The retina contains cone cells with three kinds of light-sensitive pigment, called opsins, for long, medium and short wavelengths of light. According to MedlinePlus Genetics, a US National Library of Medicine resource used here for gene detail because the UK pages do not name the genes, the opsins for long and medium wavelengths are made from the OPN1LW and OPN1MW genes, and changes involving these genes cause red-green colour vision defects, either by removing one type of cone or by making an abnormal pigment. The blue-sensitive pigment depends on OPN1SW, and changes there cause the much rarer blue-yellow defects.

Colour Blind Awareness puts it in plain terms: the genetic code sequence a person has determines which type and severity of red or green colour blindness they have. Both genes sit on the X chromosome, which is why red-green colour blindness is inherited in a different way from blue-yellow, which Colour Blind Awareness says affects men and women equally because it is carried on a non-sex chromosome.

Males have one X and one Y chromosome, so if the one X carries the colour blindness gene, he is colour blind. Females have two X chromosomes, so a woman with one affected X is a carrier with normal colour vision, and she is only colour blind if both X chromosomes carry it. Colour Blind Awareness explains that this is why the condition is far more common in men.

A carrier mother gives each child one of her X chromosomes. If a son receives the affected one he will be colour blind, and if he receives the other he will not. A colour blind father passes his X chromosome to his daughters only, so he cannot pass the gene to his sons: all of his daughters will be carriers, and a colour blind daughter needs a colour blind father and a mother who is a carrier or colour blind. A colour blind woman passes the affected X to all her sons, who will be colour blind, and to all her daughters, who will be carriers.

The NHS says there are two main tests: the Ishihara test, where you read images made of coloured dots, and colour arrangement tests, where you put coloured objects in order of shade. Colour vision testing is not part of the routine NHS eye test, but you can ask an optician for it. NHS eye tests are free for some people, including children under 18 or under 19 in full-time education. Tests are available online, but the NHS advises getting checked at an optician if there are concerns about vision.

Colour Blind Awareness reports that approximately 40% of colour blind pupils leave school unaware that they are colour blind.

The NHS says people usually adapt well, that it may affect a child's performance at school so the school should be told, and that it may rule out some careers for safety reasons, for example being a pilot or train driver. Colour Blind Awareness has pages on careers, education, food, clothes and sport, and notes that coping strategies mean people can believe they have named a colour correctly when they have not.