Colour Blindness

Red-green colour blindness is the most common abnormal colour vision defect.

By Dr. Y.L.M.

I recently found out that my husband is colour blind. He cannot see the colours on the ringgit bills and frequently mistakes RM50 for RM1. He can drive, but has difficulty distinguishing traffic lights. Someone has to hoot him from behind before he will go. I am afraid for any children we might have. Is colour blindness hereditary?

There are many types of colour blindness and some of it is hereditary.

Abnormal colour vision may be either congenital – which means it has existed prior to or at your birth – or acquired – which means you have acquired it some time during your life after birth.

Red-green colour blindness is inherited through the X-chromosome. Remember, all of us have two sex chromosomes. Men have an XY pair and women have an XX pair.

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The red-green colour blindness gene is X-linked recessive. This means in women, if you carry the red-green colour blindness gene in one of your X chromosomes, you will not manifest the disease.

But in men, since the Y-chromosome is 'empty', if you carry the red-green colour blindness gene in your X-chromosome, then you will manifest the red-green colour blindness.

Females are carriers who pass on the defect to their male children.

Colour blindness is quite a common disorder and not the end of the world! It is estimated that 6% of all white males have red-green colour blindness, and they seem to get along fine. About 0.5% of European females have it.

Red-green colour blindness is the most common abnormal colour vision defect, occurring in 99% of cases. Blue-yellow colour blindness is very rare. Total colour blindness (seeing only shades of grey) is even rarer.

So it’s my husband’s mother’s fault! If we have children, will our male children have colour blindness then?

You can’t really blame your mother-in-law, though it is quite tempting to do so.

The genetics of this is quite simple. Your husband has X (colour blindness) and Y to pass to your children. You have X and X (that is, if you are not a colour blindness gene carrier).

If you have a girl, she will inherit one X chromosome from you and one X (colour blindness) chromosome from your husband. Therefore, your daughter will be a red-green colour blindness carrier.

However if you have a son, he will inherit the Y chromosome from your husband and one X chromosome from you. (He has to inherit the Y-chromosome, otherwise he won’t be a boy!) He will not be colour blind.

Most individuals with X-linked colour blindness have deuteranomaly, which means they are primarily blind to the colour green. A smaller percentage of individuals with X-linked colour blindness have protanomaly, which means they are primarily blind to the colour red.

Now, it is not true that only males will manifest this disease. Female carriers have variable degrees of red-green colour deficiency, but most are only mildly affected.

Are there any other types of colour blindness that are inherited or acquired in other ways?

The hereditary progressive cone (the cells of your eye that receives colour light signals) degenerations are a group of disorders inherited in an autosomal dominant fashion, which means the gene is carried through your 'normal' chromosomes and not your sex chromosomes.

Dominant means you only need one of the genes to manifest the disease, not two. There have also been non-inherited cases of this disease.

Patients usually manifest worsening vision at age 20 and sensitivity to bright light. They may have difficulty adapting from a bright to a dark environment. By the time they are 30 years old, their vision has dropped to 20/200 and colour vision has been significantly affected.

Some acquired colour blindness can be caused by :

* Certain diseases which affect the eye, such as diabetes, macular degeneration, leukaemia, Parkinson’s disease

* Some medications – certain high blood pressure drugs, antibiotics etc

* Old age – you ability to differentiate colours generally gets worse when you get older because your cones start degenerating, like the rest of your vision

* Certain chemicals – fertilisers etc

These diseases/agents affect the cones, which are the cells in your retina that encode for colour. Each cone contains pigments sensitive to either red, light or blue. All other colours are variations of these.

What does it mean to be colour blind? Can I go through life normally?

Yes, you can go through life, but you won’t be able to do certain jobs that require distinguishing of colours.

You will have difficulty reading traffic lights, reading glowing indicators, reading maps, reading money if the bank notes are of different colours, seeing nature around you.

When you are out sunbathing, you can’t even tell if you are getting sunburnt (red!). You might have difficulty telling apart socks.

Jobs that involve colour differentiation, such as a pilot, artist or designer, may not be suitable for you. (Unless you plan to paint or design clothes only in black and white.)

More info on COLOUR BLINDNESS here.

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