About colorblindness

DEAR DR. GOTT: My best friend is colorblind. Her husband is not. They have three children, two girls and a boy. Her daughters are not colorblind, but her son is. How is this possible? Could her girls have colorblind children even though they themselves are not?

DEAR READER: Before I answer your questions, I will give a little background information on the eyes, color deficiency and total colorblindness.

The eyes contain color-sensing materials known as pigments. There are three types of these pigments: One responds to long-wave colors, another to medium-wave colors and the final to short-wave colors. It simply takes a defect in one of these to cause a problem.

The most common form of colorblindness or deficiency is with red-green colors that cause the sufferer to have trouble distinguishing between red and green. However, this does not mean that the person cannot see these colors. Mild defects can cause an inability to differentiate only certain shades of red and green, while severe forms may result in red and green appearing to be the same color.

Blue-yellow color deficiency is less common, and most people with this type also have trouble with red-green colors.

The most severe and rare type is total colorblindness (achromatopsia), in which the sufferer can see only in shades of gray. This disorder is often accompanied by severe light sensitivity, poor vision, lazy eye and small, jerky eye movements known as nystagmus.

Most cases of color deficiency or colorblindness are due to a genetic anomaly and affect men more frequently than women. Hydroxychloroquine, a medication taken for rheumatoid arthritis, can also cause colorblindness.

Problems with color vision due to a genetic problem are inherited. It is considered an X-linked recessive condition. This means that the abnormality is carried on the X-chromosomes. Because men have only one X chromosome, they need only one of the bad genes for it to be active. Because women have two X chromosomes, they require two.

For a woman to be born colorblind, she would have to have received abnormal X genes from both her mother and father. A man would receive it from only his mother. Therefore, if a woman is colorblind, all male children she has will be as well, and all female children will be carriers, unless the father is also colorblind, in which case any children the couple have would be affected.

Your friend’s daughters are carriers for the anomaly, and each has a 25- percent chance of having a colorblind son, a 25-percent chance of having a carrier daughter and a 50-percent chance of having a son or daughter who is neither affected nor a carrier. Her son, on the other hand, has a 50- percent chance of having a normal son and a 50-percent chance of having a carrier daughter.

The good news is that color deficiency does not usually cause disability, and most sufferers lead relatively normal lives. Achromatopsia is more severe and has several accompanying symptoms that can cause disability.

Anyone who is concerned that they or their children may be suffering from a form of color deficiency or blindness should be seen by an ophthalmologist for testing.