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

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

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

The most common form of colour-blindness or deficiency is with red-green colours that cause the sufferer to have trouble distinguishing between red and green. However, this does not mean that the person cannot see these colours. 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 colour.

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

The most severe and rare type is total colour-blindness (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 colour 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 colourblindness.

Problems with colour 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 colour-blind, 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 colour-blind, all male children she has will be as well, and all female children will be carriers, unless the father is also colour-blind, 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 colour-blind 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 colour 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 colour deficiency or blindness should be seen by an ophthalmologist for examination and testing.