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I have identical twin boys. Both wear glasses for severe farsightedness, one has been found to have astigmatism. What are the chances that his brother will also have one? I understand from your information that their genetics are the same but would this be included?

-A curious adult from California

March 06, 2008

What a fascinating topic! In fact, it is so fascinating that scientists are still hard at work on this one.

What they've been doing is studying twins like yours. They have found that if an identical twin has astigmatism, then the other twin has a 60% chance of having it as well*.

This is definitely a higher risk compared to an unrelated person. But it also tells us that the other twin won't have astigmatism for sure. Let's step back for a moment and try to figure out what's going on.

You have probably heard that your DNA carries the instructions for making you who you are. It sounds like you've also heard that identical twins carry the same set of DNA -- they are nearly identical genetically. This is why identical twins share a lot of similar traits like hair color or blood type.

But then what about the differences? We all know that identical twins aren't really identical in every way. Maybe one likes soccer while the other likes basketball. One might have longer hair than the other. These traits have little to do with DNA but a lot to do with who they're friends with or how long it has been since their last haircut. With traits like these, we say that environment played a big role.

This is where things get more complicated. Many traits we can think of don't fit neatly into "genetics" or "environmental." For example, your height is a trait we often think of as genetic. If you have a tall mom and a tall dad, the child will probably be tall. But we also know that the environment plays a big role too. No matter what that child's DNA says, if he doesn't get enough of the right nutrition when he's growing up, he's probably not going to be very tall.

And height isn't the only trait like this. Think about weight and skin tone for example. Both traits are controlled by genes and what a person does. Same thing with astigmatism.

It turns out astigmatism is one of these complicated traits where genetics and the environment both play a role. Scientists have figured this out by looking at twins just like your boys.

Identical twins have the same DNA. So if something were purely genetic, then if one twin had it, the other would too every time. But this isn't the case.

Let's look at astigmatism in more detail to see how genes and/or the environment could be involved. And how someone might inherit an increased risk of getting it.

Astigmatism is a condition where you get blurred vision because of an irregular shaped cornea. The cornea is the clear, front part of our eye. Usually our cornea is spherical, like a volleyball. It is like this so it can properly focus light.

People with astigmatism have oblong corneas, kind of like a football. Because the shape is a little different, light that passes through doesn't focus and you get blurred vision.

So anything that affects the shape of the cornea is going to affect how someone sees. One thing that can affect the cornea's shape might be genetics. Maybe people with astigmatism are born with football-shaped corneas because that's what was written in their DNA. This would be passed down as an astigmatism.

A "rugby-cornea" might be
halfway towards becoming
the astigmatism causing

Things in the environment can affect a cornea's shape too. For example...well, the truth is, we don't know all the environmental causes. It appears that eye injury or eye surgery carries the risk of leading to astigmatism. However, contrary to popular belief, reading in poor lighting or sitting too close to the TV doesn't seem to be a factor as far as we can tell.

So both genes and the environment can affect the shape of the cornea. But genes might also make something environmental more likely to affect the cornea's shape too.

Imagine a gene that makes the cornea shaped sort of like rugby balls. You know, half-way between a football and a volleyball.

Someone with this cornea won't necessarily have astigmatism. But a "rugby-cornea" might be easier to misshape than a spherical one.

This would be one way for identical twins to share astigmatism more often than two unrelated people. They share genes that give them rugby-corneas. But the environment only affects one twin's cornea.

This is only one possible reason why 40% of the time, only one twin in an identical twin pair ends up with astigmatism*. There are undoubtedly other ways for genes to make the environment more likely to give one twin astigmatism compared to the other.

And astigmatism isn't unique this way. Many, many traits behave similarly. As do many diseases.

For example, when cancer runs in a family that just means someone has an increased risk for developing it. Same thing with heart disease, diabetes and most other common, complicated diseases.

In all of these cases, people can inherit genes that make getting the disease more likely. Our genes and our environment work together to make us who we are.

*This is called the "concordance rate."

Yuya Kobayashi

Identical twin studies can
reveal how much of a trait
comes from genes and how
much from the environment.