Color blindness
The "color blind" gene allows non-color blind people to see the difference between red and green. The gene is located on the X chromosome.  This means that a male will be color blind if the single X he inherits from his mother contains the color blind version of the "color blind" gene. A female will inherit color blindness only if she inherits two X chromosomes containing defective (mutant) color gene alleles. In other words, a female needs to inherit "color blind" genes from both parents in order to be color blind.