You're right - red-green color blindness is a flaw in a gene on the X chromosome. This is why there are so many men with the condition but so few women. A woman has two X chromosomes, and so would have to get two flawed copies of the gene.

Let's hear it for redundancy!

Vision can vary greatly amoung family members because much of vision quality is environmental. Not to say that genetics aren't important. Unfortunately, vision degrades when you abuse it. But happily, contacts aren't that bad. {grin}