Are Identical Twins Always the Same Sex and Gender

Are Identical Twins Always the Same Sex and Gender

Are Identical Twins Always the Same Sex and Gender

Are Identical Twins Always the Same Sex and Gender?

Recall it for a moment. Have you ever seen identical twins of different genders – in real life? Or even on Instagram, have you ever seen a picture of identical twins boy and girl

*Crickets*

Are you thinking, “Can identical twins be different genders?” We’ve always heard that identical twins are just that — exactly—same face, same hair, same laugh, and yes, usually the same gender. But let’s dive deeper because science has a few surprising twists up its sleeve.

Why Identical Twins Are Usually the Same Sex

Let’s start with the basics. Identical twins come from one single fertilized egg. It’s called a monozygotic twin pregnancy. That one egg splits into two embryos — two babies. And because they come from the same egg and sperm, they share 100% of their DNA. That means they have the same eye color, blood type, hair texture, and the same biological sex. So if that fertilized egg has XX chromosomes, you’ll get two girls. If it has XY chromosomes, you’ll get two boys. That’s why identical twins are usually the same sex — they came from one set of DNA, and that DNA already decided their gender. 

So, when someone asks, “Can you have twins with different genders?” the answer is yes, but usually not when they’re identical. Different-gender twins are usually fraternal twins, not identical. Fraternal twins come from two different eggs fertilized by two different sperm. They’re more like regular siblings who just happened to share a womb.

 

Why Identical Twins Can Be Different Genders

Okay, now here comes the twist. While identical twins, boy and girl, are super rare, it can happen. Here’s how it can happen:

Let’s say an egg with XY chromosomes (which usually makes a boy) splits to form identical twins. In one of the twins, a random mutation causes the Y chromosome to be lost or changed. That twin might develop female characteristics, even though they started as genetically male. It is known as Turner Syndrome (XO chromosomes), and it can result in one twin being a girl while the other still has typical XY chromosomes and becomes a boy. This phenomenon is called genetic mutation and is extremely rare. 

An embryo splits, like with identical twins, but some cells mix or mutate, causing different expressions of gender. One twin might have a genetic pattern that expresses more female traits, the other more male traits — or one might have mixed characteristics. It is called Chimerism. 

When people see twins with different genders, it’s almost always because they’re fraternal twins. Fraternal twins come from two separate eggs fertilized by two separate sperm. They share about 50% of their DNA, just like any other siblings. So one could be a girl, the other a boy. Totally normal. Totally common. They might look alike, or they might not. They might be born with the same eye color or completely different features. It’s kind of like hitting the sibling jackpot — two babies, one delivery.

So when it comes to the question — “Can identical twins be different genders?” — the real answer is Almost never... but sometimes, yes.

 

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