Are these bananas ripe enough to eat? Eight percent of men have absolutely no idea. That's about how many men have red-green color deficiency, a condition that makes it hard for them to distinguish the greenish-yellow of an unripe banana from the brownish-yellow of a ripe one.

Women are far less likely than men to be color-deficient in this way. That's because red-green color deficiency is a recessive trait linked to the X chromosome. With two X chromosomes, a woman has a much better chance of getting at least one good copy of the genes necessary for normal color vision.