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What Are Mirror Image Twins, And How Are They Different From Identical?

by Team Scary Mommy

Updated: March 26, 2021

Originally Published: Dec. 3, 2020

Mirror Image Twins

Paper Boat Creative/Getty Images

You can’t always tell the types of twins based solely on appearance. After all, despite the fact one type is literally called identical, even they can still have physical variations. And, let’s be real anyway, how many of us actually know all of the different types? For example, do you know what mirror image twins are? Yep, that’s a thing — and yet another testament to the mind-blowing magic of human genetics.

Twins are cool for a million reasons, but especially because you get two babies for the price of one pregnancy (it’s a steal!). It’s a lot like ordering food and realizing they gave you a large instead of a small. Granted, learning that you’re having two kiddos at once can throw you for a loop. Just remember, though, that you’ve given them the gift of each other and a special sibling bond only twins can share.

Before we define mirror image twins, it helps to take a look first at the two “main” types of twins.

How are identical and fraternal twins formed?

Fraternal (or dizygotic) twins are the most common type of twins. They develop when two separate sperm fertilize two separate eggs. Then, both of those fertilized eggs implant in the uterus. They share about 50 percent of their DNA, making them effectively the same in a genetic sense as non-twin siblings.

With identical (or monozygotic) twins, a single egg gets fertilized by a single sperm. Then, the egg splits into two and grows into two separate embryos. Identical twins share 100 percent of their DNA, hence them being genetically identical — although environmental factors can cause slight variations in appearance.

So, what is a mirror image twin?

Now that you have a better sense of what makes fraternal and identical twins, let’s explore mirror image twins. Which is to say let’s further explore identical twins, because mirror image twins are identical twins. A little confusing; we get it. Still, stay with us on this. Not all identical twins are mirror image twins, but all mirror image twins are identical twins.

How does this even happen? Well, the process starts the same as with identical twins in that a single egg is fertilized by a single sperm and then splits. It’s believed the key difference is that the fertilized egg splits more than a week after conception in mirror image twins. So, mirror twins aren’t a real category of twins so much as they are an offshoot of identical twins.

What do mirror image twins look like?

As you may have surmised from the name, these twins appear as if they are looking in a mirror when facing one another. In other words, they have reverse asymmetric birthmarks and physical features. For example, one twin may have a dimple on their left cheek while the other has a dimple on their right cheek. One twin may have a birthmark just above their right eye, while the other has the same birthmark just above their left eye. Other potential asymmetric physical characteristics of mirror twins may include moles, freckles, cowlicks, and the shape of eyes, eyebrows, ears, and nostrils.

Do mirror image twins have the same fingerprints?

If your evil (mirror) twin leaves their DNA at the scene of a crime, you might be going to jail. However, if they’re silly enough to leave fingerprints behind, you can’t be framed. Yes, mirror twins share the same genetic makeup, but their fingerprints are not identical. No one’s fingerprints in the world are the same. Mirror image twins would have similar fingerprints, but they’d be the opposite pattern, in the same way twins’ birthmarks or cowlicks may mirror each other.

What do “functional mirroring” and “medical mirroring” mean?

Amazingly, mirror image twins get even more interesting! In addition to having asymmetrical physical features, they might exhibit two other types of mirroring: functional and medical. With functional mirroring, gestures or movements can manifest in opposite ways. So, one twin might lead with their left foot while walking, while the other twin leads with their right. Or when sleeping, one twin might default to their left side while the other defaults to their right.

Medical mirroring, on the other hand, describes a history of opposite medical presentations, i.e. the twins have a history of opposite dental issues or opposite tendencies in blood pressure.

What is situs inversus?

In extremely rare cases, mirror image twins present with a condition called situs inversus. This means that their internal organs — heart, liver, lungs, stomach, etc. — are found on the opposite side of the normal anatomical position. The normal positioning of organs is known as situs solitus.

How common are mirror image twins?

Per Laura Segal, director of the Twin Studies Center at California State University in Fullerton, the mirror image phenomena occurs in approximately 25 percent of all identical twins.

Who are the most famous mirror image twins?

In Hollywood, twins have always had the “it” factor. The mischief, fun, and confusion they cause in their on-screen lives are always hilarious. They have a way of making the rest of us yearn for a twin too. So, without further ado, here’s a list of famous twins and celebs you probably didn’t know were twins. And before you @ us, Lindsay Lohan from the Parent Trap is not on this list because, contrary to popular belief, she is not a twin (just in case you didn’t know).

This article was originally published on Dec. 3, 2020