Within the world of science women seem to have two X-chromosomes, while men have one X and one Y-chromosome. Comparing them the Y is missing one of the four legs of the X. Why does it matter one might ask. But this leg can perceive the wave of the red light waves. If mens one X is defect they don’t have a second to come up with it in order to perceive red. Since red and green have the same value of light one cannot compensate by the light and dark contrast in order to distinguish them. So men have sometimes the small problem not to see the difference between the colour of a sweet strawberry and an unripe fruit. Still – they survive.