Today’s spotlight is not actually a disease, but a disorder most often caused by affected genes on the X chromosome.
A quick flashback to the magic that was 8th-grade science: a human male receives an X chromosome from mom and a Y chromosome from dad. A human female receives an X from mom and an X from dad. Color-vision is an X chromosome-linked trait, meaning the mutation happens on the X chromosome.
If you remember, a dominant, correct gene will always mask a recessive, faulty gene.
It follows from these bits of information that females will be less likely to be color-blind because they receive two copies of the X chromosome–even if one copy is bad, the other is likely to be good, and the good is the trait that will prevail. Males only have one X chromosome, so if the gene(s) for color-vision are present (genotype), there is nothing to stop it from presenting as a trait (phenotype).
Indeed, research indicates that about 5% (and possibly as high as 8%) of males are color-blind to some degree, while only 0.5% of women are color-blind. Interestingly, there is an idea that a recessive gene characteristic that exists in 5% or more of a population, it may mean that characteristic have some sort of evolutionary advantage in the long-term. Often, color-blind individuals are able to distinguish camouflage patterns from their backgrounds more easily, which certainly may be an evolutionary advantage!
Wall Street Journal (online) posted an excellent article about color-blindness that I want to refer you to. The article contains great information, interactive graphics that allow you to see what the world looks like with various types of color blindness, a video, and even a comments-section full of interesting stories! To get to that article, click on the following link:
