According to Dr. Greene, "Colorblindness is almost always a hereditary condition. Red-green colorblindness is a recessive condition passed on the X chromosome. Only one healthy color vision gene is necessary to provide color vision. Since boys have only one X chromosome, it is much easier for them to be colorblind. If their mothers are carriers (having one normal X chromosome and one colorblind X chromosome), the sons have a 50% chance of having the condition. Red-green colorblindness occurs in about 8 per cent of American males. These men cannot pass the condition on to their sons (since they give their sons a Y, not an X, chromosome), but they will pass the gene to their daughters."

The "almost always" is because it is possible to have a genetic mutation that results in colorblindness, though it is rare. In any case it is genetic.

Click here to read Dr. Greene's full article on colorblindness.