Scanning Electron Micrograph of the Human retina. The back of your eye is packed with rod cells (in grey), which are sensitive to brightness, and cone cells (in color) which are sensitive to color. Each cone cell is specific for either red, green, or blue respectively. Those who are color blind most often cannot tell red from green because the genes for red and green pigment are very close to each other on the X-chromosome. Since males only have one X chromosome males most likely express this.
