Look at a traffic light. Red on top, green on the bottom. Easy, right? Now imagine both lights look the same muddy yellow. About one in twelve boys sees the world this way, and most of them have no idea until someone it.
Colour vision starts at the back of the eye, in a thin layer called the retina. The retina millions of tiny cells shaped like cones. There are three types, and each type to a different colour of light: red, green, or blue. Your brain reads the from all three types and mixes them into every colour you see.
In colour-blind people, one type of cone does not work . The most form affects the red or the green cones, so red and green look almost the same. The person can still see other colours just fine. Total colour blindness — seeing only black, white, and grey — is extremely .
The passes from parent to child through genes carried on the X chromosome. Boys have only one X, so a single faulty copy is enough. Girls have two, and the healthy copy usually covers for the broken one. That is why colour blindness mainly among males. In a class of forty students, the of at least two colour-blind boys is surprisingly high.
色盲通过 X 染色体上的基因从父母传给孩子。男生只有一条 X，一份坏的就够了。女生有两条，好的那份通常能补上坏的。所以色盲主要在男性中传播。一个四十人的班里，至少出现两个色盲男生的概率高得出乎意料。
In 1917 a Japanese eye doctor named Shinobu Ishihara a simple test. He printed dots of different colours in a circle. People with vision a number hidden in the dots. Colour-blind people see a different number, or nothing at all. The Ishihara test is still used today because it is cheap, fast, and hard to cheat.
Colour blindness cannot be cured, but it rarely causes serious trouble. Most colour-blind people learn to read traffic lights by — red is always on top. Phone and computer screens now offer colour-blind that shift problem colours into safe ones. The is not to "fix" anyone's eyes, but to make sure the information still gets through.
