Do Identical Twins Have the Same DNA?

Identical twin babies!

They’ve got the same tiny fingers, the same tiny toes, and the same tiny button noses, but you might also be wondering (if you’re interested in the science): do identical twins have the same DNA?

Let’s delve into the mysteries of genetics to explain what makes identical twins so hard to tell apart – and how recent research reveals that they may not be quite so similar as we previously thought…

In this article: 📝

Genetics 101: What is DNA?

DNA stands for Deoxyribonucleic acid. 🧬

This is the amazing (yet super tiny!) substance in your body that stores all the crucial information about who you are.

Inside the nucleus (the command center) of each of your cells are carefully organized packages of DNA called chromosomes.

It’s on these chromosomes that you’ll find your genes: pieces of DNA that are responsible for your unique traits.

So, for example, you might have a gene for blue eyes, large feet, or being tall – even qualities like musical ability or athleticism can be affected by your genes.

(Not to say that practice doesn’t still play a big role!)

The entire “recipe” for how to make a human being is written on 23 pairs of chromosomes (that’s up to 25,000 genes in total).

When conception happens, the egg and the sperm provide 23 chromosomes each, with the 23rd pair being the sex chromosomes (X or Y) that determine whether you’ll be hearing “It’s a girl!” or “It’s a boy!” as your baby is born.

Now the egg can only provide the X chromosome, so it’s up to the sperm to decide the sex!

Sperm cells can either have X or Y chromosome.

So the combination of XX is a girl 👧, and XY is a boy 👦.

What percentage of DNA do identical twins share?

Do identical twins have 100% the same DNA?

To answer that question, let’s look at how identical twins come about.

Identical twins (AKA monozygotic twins) both come from one single egg that’s been fertilized by one single sperm.

This is a pretty rare phenomenon: only about 3 in 1,000 births are of identical twins.

When an egg and a sperm cell fuse together, they form what’s called a zygote.

This then divides and multiplies into a bigger clump of cells, beginning its journey to becoming a baby.

Identical twins result when, very early on in this process, the clump of cells splits into two separate individuals.

Because identical twins come from the exact same combination of egg and sperm, they have exactly the same combination of chromosomes.

A DNA test would reveal that they have over 99.9% identical DNA (same as with identical triplets).

But what happened to the 0.1%?, you ask.

How can they be identical twins when their genetic makeup isn’t 100% the same?

Well, more on that later.

But, first, let’s not leave out the non-identical twins.

Do twins have the same DNA when they’re not identical?

Fraternal twins (AKA dizygotic twins) are much more common than identical twins (because they’re hereditary, too).

Out of all the twins born, two-thirds will be fraternal.

Fraternal twins may look and behave very differently to each other (just like siblings), and they may be different sexes (identical twins are always girl-girl or boy-boy).

That’s because they only share 50% of their DNA – the same proportion as any siblings born at separate times.

Why do twins not have the same DNA when they’re fraternal?

Simple: because they come from two different egg-sperm pairings.

Sometimes your ovaries can release more than one egg at a time (particularly if you’re taking fertility medication to stimulate ovulation) and then multiple eggs can be fertilized by multiple sperm.

Each egg-sperm pairing gets a unique combination of chromosomes (23 from mama and 23 from dad), which results in two very different individuals – even if they have shared a uterus!
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Which parent carries the gene for identical twins?

Interestingly, it’s not possible to narrow down a single gene that’s responsible for identical twins.

Scientists don’t actually know what makes that zygote think: Hmmm, being one person is a bit boring – I’ll make two instead.

Although genetics isn’t known to play a part in causing identical twins, fraternal twins do run in families.

If you have a mama or a sister who’s had fraternal twins, you’re twice as likely to have your own double-trouble.

What makes identical twins different?

Identical twins might have near-identical DNA, but that doesn’t mean they’ll look alike in all aspects of their lives.

There are lots of things that can influence the way they can look.

Environmental factors

Even from their time in the uterus, identical twins can be affected in different ways by their environment.

Think of this as the “nurture” part of “nature vs. nurture”.

For example, did you know that identical twins have different fingerprints?

These unique line patterns form when each baby’s tiny, soft fingers brush up against the amniotic sac that’s keeping them safe inside the uterus.

Once they’re in the outside world, identical twins can develop differences depending on a huge range of environmental factors.

Their health outcomes might differ depending on diet, exercise, or time spent in the sun, for instance.

Or they might have unique skills based on their education or how much they were encouraged to pursue an activity.

On the other hand, research carried out by scientist Thomas Bouchard in the 90s found that identical twins often follow very similar paths in life – even if they’ve been raised apart in totally different environments.

Genetic factors

Remember that small difference in identical twins’ DNA?

Well, a recent study in the journal Nature showed that identical twins experience genetic mutations while they’re in the uterus, so their DNA may start off identical but it can change over time.

An emerging scientific field called epigenetics is also helping us to understand more about the way our genes work.

Epigenetic processes are chemical reactions in the body that influence how our genetic code is actually expressed. If we envision our genes as the blueprint for a building, an epigenetic process might cause the building to be constructed slightly differently from the blueprint’s design.

Essentially, scientists think that epigenetic processes can cause our genes to have a stronger or weaker effect on our characteristics, or can even “switch off” a gene entirely.

These processes can be part of normal bodily development, the result of an external factor (such as stress), or sometimes they seem to happen at random.

So, although identical twins have (pretty much) the exact same DNA blueprint, epigenetic factors can come into play and change how that DNA is expressed in each of their bodies.

That could explain, for example, why one twin might develop an inherited illness and another might not – or why one might be a whizz at the cello and the other might go for heavy metal…

If you’re expecting identical twins, you’ve got lots of fun ahead of you!

Inside your uterus, your two babies are already interacting with each other, ready to become the best of friends.

And did you know that many twins make up their own language?

An excellent way to plot mischief without mama knowing about it.

Good luck telling them apart!

And if you’re after some advice from other twin mamas, there’s a whole Community of moms of multiples on Peanut – we know you’ll fit right in.