[Kat Clements | Contributing Writer]
In the first part of “Seeing in colour” we looked at the myths surrounding tetrachromacy. On the other end of the spectrum, we talk a lot about colour blindness. Any time someone fails to distinguish two colours we see as distinct, we accuse them of being colour blind, in much the same way as people who can’t sing are referred to as tone-deaf. Both appellations are inaccurate; a colour-blind person is just as capable of distinguishing most shades as everyone else, depending on what type of colour blindness they suffer. Tone-deafness is also not at all like being a poor singer, but that’s another question.
Red-Green colour blindness
Red-Green colour blindness is actually a combination of 2 different types of colour blindness. Both of them, as well as the rarer blue-yellow colour blindness, are referred to as “anomalous trichromacy”, as those affected have 3 types of cones, but one or more is faulty. The degree of function can vary – some people will be almost totally unable to perceive shades of the relevant colour, while others will be only a little less sensitive than people with 3 functioning cones. It affects approximately 8% of XY individuals and 0.5% of XX individuals, and is more common in Caucasian populations. Around half of colour blind individuals have a serious form of the disorder, while around half have a milder manifestation.
Deuteranomaly is a reduced sensitivity to green light. It’s the most common type of colour blindness, often just referred to as red-green, and occurs when the cone responsible for detecting light in the green region of the spectrum is faulty.
Protoanomaly is a reduced ability to perceive red light. It often occurs together with deuteranomaly, and thus the two are known colloquially as red-green colour blindness. Like deuteranomaly, it occurs when one cone – in this case, the one responsible for detecting red light – is faulty.
According to the Colour Blind Awareness site: “People with deuteranomaly and protanomaly are collectively known as red-green colour blind and they generally have difficulty distinguishing between reds, greens, browns and oranges. They also commonly confuse different types of blue and purple hues.”
Both deuteranomaly and protoanomaly are genetic, and inherited via the X chromosome. XX individuals are less frequently affected because they have two copies of the X gene, and so the recessive gene for colour blindness will be overruled by the allele for normal vision unless two copies of the faulty allele are inherited. XY individuals have only one X chromosome, so if they inherit the colour-blindness allele they have no “normal” allele to override it. This means that the allele is going to be expressed, so XY individuals are much more likely to express red-green colour blindness than XX people. XX people can, however, be carriers and pass it on to their children.
Dichromacy is where one cone is completely absent. It’s similar to red-green colour-blindness and often the two are confused, because the majority of dichromats are lacking either their red or green cone.
Like Red-Green, there are two types: Deuteranopia and protanopia. Deuteranopes lack their green cones, protanopes their reds and – like tritanomaly, as you will see below – tritanopes lack their blues.
Again according to the Colour Blind Awareness site:
“Protanopes are more likely to confuse:-
Black with many shades of red
Dark brown with dark green, dark orange and dark red
Some blues with some reds, purples and dark pinks
Mid-greens with some oranges
Testing Colour Vision
Deuteranopes are more likely to confuse:-
Mid-reds with mid-greens
Blue-greens with grey and mid-pinks
Bright greens with yellows
Pale pinks with light grey
Mid-reds with mid-brown
Light blues with lilac
Testing Colour Vision
The most common colour confusions for tritanopes are light blues with greys, dark purples with black, mid-greens with blues and oranges with reds.”
Tritanomaly is an extremely rare condition in which people have a reduced sensitivity to blue light. People with tritanomaly are likely to struggle distinguishing blue and yellow colours. This is an inherited condition, but because it has such a low incidence there has not yet been sufficient research to fully establish inheritance patterns, although it does not appear likely that it is sex-linked. (Reference: Schmidt, I. (1970). On congenital tritanomaly. Vision research, 10(8), 717-743.)
Testing Colour Vision
True colour blindness
“True” colour blindness is a condition where a person cannot perceive colour at all. It’s known as monochromacy, or achromatopsia. People who have achromatopsia will see in black and white – or, more accurately, in greyscale. It’s extremely rare, occurring in approximately 1 in 33,000 people.
People who are truly colour blind may also be very photosensitive, meaning that bright lights – or even normal indoor lighting – are very painful, and they may need to wear dark glasses even inside.
In the next part of “Seeing in colour” we’ll be investigating how animals see colour, so check back soon!