We think of identical twins as having identical DNA. But a pair of twins born in England shows that's not exactly true.

The two girls started out like all identical twins, developing from the same fertilized egg and the exact same set of DNA. But when they were born last year they didn't look like identical twins. One girl has peach skin and blue eyes and the other has brown sugar skin and brown eyes.

At first blush it might seem impossible they could look so different. After all, while huge differences can and do happen with fraternal twins, identical twins are usually exactly that -- identical in skin, hair and eye color.

But, actually, twins who start out with identical DNA always have slightly different DNA by the time they're born. And they also each use their DNA a bit differently too.

Even though this is true for all identical twins, we can’t usually tell because the changes happen in parts of the DNA that don’t affect how identical twins look. Which isn’t surprising. DNA is large and very little of it has to do with, for example, skin and eye color.

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By chance, the DNA differences for these girls happened to be in the smallish part of their genome that deals with looks. These two girls let us see what goes on in every identical twin. And in every one of us.

DNA Differences

Identical twins do indeed start with identical DNA—they are the result of the same sperm from dad and the same egg from mom.

Even though identical twins come from the same fertilized egg, in the end each twin has slightly different DNA. (Zappys Technology Solutions)

The original fertilized egg divides one or more times before the resulting clump of cells splits into two. Each clump of cells goes on to become one of the identical twins.

In this process of becoming a brand new baby with trillions of cells, the cells in each clump divide over and over again. DNA differences or mutations can happen any time a cell divides.

This is because a cell needs to copy its DNA before it can divide. And while the cellular machinery is astonishingly good at copying DNA, it isn’t perfect. Every now and then it makes a mistake.

All of the new cells that come from the one with the mistake will have that same mistake. One consequence is that if it happens early, the baby will have more cells carrying that mistake.

Think about it like one of those medieval monks patiently copying manuscripts in a monastery somewhere in medieval Europe. Imagine he makes a mistake during copying and the original manuscript is destroyed. Now every new manuscript contains his mistake.

For the English twins, one may have developed a mutation in a gene that affects skin and eye color. Most likely it would be in the lighter child as it is easier to break something than to fix it. And often traits like blue eyes are the result of a gene not working quite right.

But this isn’t the only way this could have happened. Another possibility has to do with how cells read their DNA.

Using DNA Differently

Imagine Louis C.K., Meryl Streep and Kevin Hart are all going up for the same role. Even though they all have the exact same lines to read, odds are there will be real differences in how they say them.

Just like different actors will read the same script differently, so too will different people's cells read the same genes differently. (Wikimedia Commons)

This is sort of what happens in different people’s cells. One person's cells will read a gene one way and another person’s cells will read the exact same gene a different way. If that gene controls skin and/or eye color, then it will affect a person’s skin or eye color.

Here's how it might work: Near the genes there little chemical markers that can, for example, tell the cell how often to read a gene.

In the case of the twins, it could be that a set of these “epigenetic” marks is telling the cells of one twin to read her skin and eye color genes just a little bit. That means she would make less pigment and so have fair skin and blue eyes.

Or her sister might have marks on her DNA telling her cells to read her skin and eye color genes much more often. This would explain her brown eyes and cafe au lait skin.

What is fascinating about these epigenetic marks is that they're reversible. This means that, for example, the twin with blue eyes and fair skin may eventually end up with her sister’s eye and skin color. Which wouldn’t be surprising as it isn’t uncommon for babies to be born with lighter skin and eyes that darken over time.

If the girls' differences are the result of a mutation in the DNA itself, they'll probably go through life as different-looking identical twins. But if the reason is in how their cells read their DNA, these girls might someday look more identical than they do now.

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Why Identical Twins Don't Always Look the Same