About 300 million people worldwide experience color vision deficiency. This is what the world looks like through their eyes.
The Ishihara test, created in 1917 by Dr. Shinobu Ishihara, uses plates of colored dots to reveal numbers visible only to those with normal color vision. People with red/green deficiency see either a different number or nothing at all.
The plate on the left contains the number 74. If you see 21 instead, you may have a red/green deficiency. If you can't see any number, you may have total red/green color blindness.
Color vision genes sit on the X chromosome. Males have only one X (XY), so a single defective gene causes color blindness. Females have two X chromosomes (XX), so a working copy on either X compensates. A woman must inherit the gene from both parents to be affected.