Can identical twins look and act non-identically?

Even with their subtle differences, your daughters are still almost certainly identical twins with identical genetic codes. As you’ve noticed, identical twins can and do show slight differences despite having identical sets of instructions. And this is especially true with identical twin girls. 

Although twins share exactly the same set of DNA, there are many things that can make them just a little different and unique from each other. These things are environmental, as well as genetic. Click here to learn more. 

You hit the nail on the head for one of the big reasons identical twin girls can be different — Lyonization, or as it is also known, X-inactivation.  Twin girls have the exact same genetic code, but can have slight differences because they end up using part of their genetic code differently.

Girls have two X chromosomes while boys have an X and a Y.  What this means is that girls have two copies of each of the 1000 or so genes on the X while boys have just one. This is important because for a lot of genes, you need to have the right number of copies for things to work right.

See, each gene has the instructions for making a protein that does something very specific in the cell. Different numbers of copies can mean different amounts of proteins. And for some proteins, too little or too much protein can cause real problems.

For example, there is a gene called CD40LG located on the X chromosome. The product of the CD40LG gene is the CD40LG protein that sits on the surface of your immune blood cells and tells your body when there is an invader like bacteria or viruses. If you make too much of the CD40LG protein, you can end up with autoimmune disorders like Lupus because your body is constantly sensing that it is under attack.¹ As you can imagine, Lupus is a disease that is more prevalent in women than men.

So, our cells have figured out a way to fix this problem by turning off one of the X chromosomes in women very early on. This is called Lyonization. Thankfully, because of Lyonization, Lupus does not occur in every single woman. It is usually only the case when the second X chromosome is not turned off completely.²

If this is all there was to genes, then this wouldn’t really matter for identical twin girls. One X would pretty much be the same as any other X. But they’re not. 

Girls get one of their X chromosomes from their mom and one from their dad. Each X from mom and dad has pretty much the same set of genes.  But what makes things interesting is that the genes can come in different versions. And a big part of each of our uniqueness comes from our unique combination of different gene versions.

So your twin girls each have two X's that are a bit different. Some of the subtle differences you’re seeing may have come from which X is off in which cells.

Different Xs, Different Twins

Lyonization, or X inactivation, happens when a fetus is made up of only about 70-100 cells. Each of these cells randomly picks an X chromosome to inactivate, by clumping one of the X chromosomes into a tight ball of DNA called the Barr body. All of the genes on that chromosome are essentially shut off. How each cell chooses which X chromosome to shut off is completely random.³

Then, each of the original 70-100 cells goes on to divide over and over becoming vast swathes of cells in the baby.  Now you have some groups of cells that have one X on and the rest that have the other X on.

What this means is that your girls have different sets of cells with different X's off. Which also means they have different cells using slightly different versions of the same genes.

Lyonization doesn’t just happen in humans either. Like us, female cats also have two X chromosomes and they also need to shut off one of the two for the same reason.⁴

You can see the direct effects of Lyonization in a tortoiseshell cat because their X chromosome carries a gene for orange fur.

Very early in development, each of the cats’ cells will pick one X to turn off. Some female cats have a version of this gene that says make orange fur on one X chromosome and a different version on the other X that says to make black fur.⁴

Remember that very early in development, each cell will randomly choose one of the X’s to shut off.  If it happens to be the X that carries the orange fur allele, then that cell will lead to black hair. If it is the OTHER X chromosome that is turned off, then that cell will lead to orange hair.⁴

Each of these early cells will go on to become lots of other cells.  So in this case, big patches of the tortoiseshell cat’s fur will be orange and big patches will be black.  And every female cat, including identical twins, will have a unique pattern because it got a random pattern of cells with different X’s turned off. What results is a tortoiseshell cat with an awesome array of orange and black fur patterns.

The same sort of thing explains why identical twin girls can have a few more differences than identical twin boys.  Now of course they don’t have orange and black hair (although that would be cool).  But they will have some differences because some of the genes will be different between their two X’s.

Lyonization is one of the reasons why identical twin girls are never truly identical! It is a big reason why one of your daughters seems more similar to you and the other to your husband.

Here is a great video about X inactivation.