The genes that cause red-green color blindness are inherited on the X chromosome. Red-green color blindness is more frequent in men since it is inherited on the X chromosome. This is due to boys inheriting only one X chromosome from their mother. Men have two copies of the X chromosome, one from each parent. Thus, men have three possible genetic combinations: both parents with color blindness (25% of color-blind people), one parent with color blindness and the other parent without it (50%), or neither parent with color blindness (25%). Women can pass on color-blindness genes without knowing it; this can happen if a woman has a mutation but is not genetically blind herself. Since women only have one X chromosome, they would receive the mutation on that single copy and would be color-blind too. Color blindness is also associated with certain other physical traits and diseases. For example, people who are color blind are more likely to be blue-eyed than others. This is because people who are color blind tend to come from families where the parents are color blind too. If one parent is blue-eyed and the other brown-eyed, then their children will follow suit. Also, people who are color blind are more likely to have dark hair and eyes too.