Humans can see colour because there are three types of cone, each containing a different form of iodopsin. Each type absorbs light over a range of wavelengths, with its peak at a different point in the spectrum:• blue-sensitive cones: peak about 420 nm
• green-sensitive cones: peak about 530 nm
• red-sensitive cones: peak about 560 nmThe ranges overlap, so light of most wavelengths stimulates more than one type of cone. This is the trichromatic theory of colour vision: the brain works out the colour of light from the relative amounts by which each type of cone is stimulated.For example:• light that stimulates red and green cones about equally is seen as yellow
• light that stimulates all three types equally is seen as white
• light that stimulates mainly blue cones is seen as blueSo just three types of receptor allow us to tell apart a very large number of colours. Colour vision only works in bright light, because cones need a high light intensity.Colour blindness occurs when one type of cone is missing or has faulty iodopsin. The most common form is red-green colour blindness, in which red and green cannot be told apart. The genes for the red and green cone pigments are on the X chromosome, and the faulty alleles are recessive. Males have only one X chromosome, so a single faulty allele causes colour blindness, while females need two. This is why red-green colour blindness is much more common in males (about 8%) than in females (about 0.5%).Many other mammals, such as dogs, have only two types of cone, so they can distinguish fewer colours.
• green-sensitive cones: peak about 530 nm
• red-sensitive cones: peak about 560 nmThe ranges overlap, so light of most wavelengths stimulates more than one type of cone. This is the trichromatic theory of colour vision: the brain works out the colour of light from the relative amounts by which each type of cone is stimulated.For example:• light that stimulates red and green cones about equally is seen as yellow
• light that stimulates all three types equally is seen as white
• light that stimulates mainly blue cones is seen as blueSo just three types of receptor allow us to tell apart a very large number of colours. Colour vision only works in bright light, because cones need a high light intensity.Colour blindness occurs when one type of cone is missing or has faulty iodopsin. The most common form is red-green colour blindness, in which red and green cannot be told apart. The genes for the red and green cone pigments are on the X chromosome, and the faulty alleles are recessive. Males have only one X chromosome, so a single faulty allele causes colour blindness, while females need two. This is why red-green colour blindness is much more common in males (about 8%) than in females (about 0.5%).Many other mammals, such as dogs, have only two types of cone, so they can distinguish fewer colours.