Colour blindness and gender
An explanation of why colour blindness is more common in males, and its effects on female heterozygotes.
Why is colour blindness less common in women?
Colour blindness occurs in around 8% of the male European population, and around 0.5% of female Europeans. This inequality is due to the association of colour blindness with the X chromosome.
Humans see colour using three different proteins which each react to different wavelengths of light. These proteins are called opsins, and they come in forms which respond to red, green or blue light. Colour blindness is generally caused by a mutation in one or more of the genes encoding these proteins. Depending on the specific mutation, this may produce a non-functional opsin (resulting in dichromacy) or an opsin that does not react to the specific wavelengths that it is supposed to (anomalous trichromacy). The most common form is deuteranomaly, which is caused by a mutation of the green opsin, and affects red-green discrimination.
The genes encoding the red and green opsin are located next to each other on the X chromosome, while the blue opsin gene is found on chromosome 7. This means that females carry two copies of the red and green opsin genes, while males only have one of each. Males with a mutation in one of these genes will exhibit colour blindness to some degree. Females, on the other hand, will only be colour blind if they carry mutations in both copies of one of the genes, which has a much lower chance of occurring.
Interesting effects can occur in females carrying opsin mutations alongside normal versions. Due to X-inactivation, each colour-detecting cone cell in the retina will be utilising only one form of a given opsin gene (X-inactivation is explained here https://www.ukessays.com/ask/what-is-x-inactivation-1026). If the mutations cause anomalous colour responses, rather than inactive products, this can allow affected people to effectively see in 4 or 5 colours, rather than three. Though research on this subject is limited, it is implied that such people can experience and differentiate colours which are unnoticeable to other people, and may be able to distinguish up to 10 billion colours.