Med School for Writers: Color blindness

Med School for Writers: Color blindness

Color blindness—it’s all about the genes, baby.

Recently, a writer asked me about color blindness.

Color blindness is usually congenital (actually hereditary) and much more common in boys, since it is usually X linked. Women have two X chromosomes and usually only one is defective (a gene that codes for color vision). We all have two genes for each of thousands of traits, one from mom, one from dad.

If a man has a defective gene on the X chromosome, the trait it codes for will be defective. These are called X linked diseases and there are more than a few. Colorblindness is one such entity (although a relatively benign one and not generally referred to as a disease).

If a woman gets a defective gene from her dad (as she must since it is the only X chromosome he has to pass along), the daughter will be a carrier and will have a 50/50 chance of passing it to her children (the boys have a 50/50 chance of getting the bad X and will be color blind if they do; the girls will have a 50/50 chance of getting the bad X from mom—if they get it, they can pass it on, if they don’t their children are in the clear. Hence, we can assume that all daughters from color blind men are carriers, since they have to have the bad X.

In order for a woman to be color blind, she has to have a bad X from both parents. That does happen, though infrequently.

Although it is possible to be entirely color blind, this is quite rare since there are multiple genes coding for color vision (only some of which are on X). Most color blindness is red/green.

Color blindness is not painful, but there is a certain psychological awareness that can be difficult at times. I am color blind so I know this quite well. It is impossible for me to deal with any sort of color reliably, so I mostly ignore it and used to get laughed at in school (although I did not know I had a color vision issue at the time). I had my car painted a bright orange in high school because I thought it was cool. Trust me when I say nobody else thought it was cool.

The real problem with color blindness is that so much of our world is color coded. Folks with normal color vision rarely realize this; color blind folks deal with it all the time. There are many occupations that exclude color blind folks from employment (the aviation industry is a big one; US Naval line officers are another). Incidentally, color blindness does not disqualify a person from medicine or even surgery, and I believe there are a disproportionate number of physicians who are color blind as a result. They are intelligent and high achieving and, finding they could not become an airline pilot or drive ship, they opted for medicine instead. I know many, many color blind physicians and surgeons.

Is it possible to acquire color blindness? Yes, usually later in life. Brain tumors can cause it, and there is an entire differential diagnosis list for acquired color vision. These tend to be quite serious diseases and may include diseases of the optic nerve itself. The eye is a window on overall health though, so acquired color deficiency (which strikes men and women equally) may indicate a bigger problem and should be investigated thoroughly. Such color deficiency, as it tends to be called, may be reversible. Hereditary color blindness is not.

One other thought here worth mentioning. I said above that we have thousands of traits, all coded by our genes. That is a true but inadequate statement. Actually, every trait we possess (from the size of our feet to the efficiency with which our kidneys make urine, to the color of our skin and the way we part our hair) are in large part determined by those damn genes. This means that, statistically speaking, we all have defective genes within us. Generally these are benign. It’s why you might have freckles and your sister not. Or why my skin is slightly less coffee colored than my brother. These are variables that lead to diversity. Genes actually are not on or off, but on to variable degrees. This is called variable pentrance and means that the same defective gene in one person might have less effect in another. This gets quite complicated, since our genes codes for 20-25,000 proteins, which interact to produce the person we are. 

But if you are unlucky enough to get a defective gene in a vital pathway, all bets are off. Sickle cell anemia, muscular dystrophy, cystic fibrosis, sidis inversus (where in the right/left body design is reversed to varying degrees; not a good thing at all), Huntington’s Chorea, and even Alzheimer’s are just a few of the diseases caused by defective genes. 

Genes also determine susceptibility to other diseases, such alcoholism or the flu. The genes that code your immune system determine if you are more or less susceptible to certain infections. This is why some people never get sick, and others succumb to an otherwise benign illness, such as the flu.

There are other influences coded by genes that are probably less determinative and act in concert with environmental factors. Perhaps sexual orientation is one of these, or certain other complex personality traits, such as addiction potential, or an antisocial personality. Or even happiness.

This, by the way, can be seen in animals. Ever wonder why cows are always docile and hyenas less so? Have you ever heard of a person being mauled by their pet rabbit? Why are some breeds of dogs naturally more aggressive than others?

It’s all about the genes, baby. 

With a little help from mom, dad, and our friends.

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