Is it possible for a conjoined twin pair to have two different genders?

The short answer is yes!

Conjoined twins are a specific type of identical twin pair. They are formed when a single fertilized egg partially splits apart early in development.

Conjoined twins — and other identical twins — typically have the same DNA. But that doesn’t necessarily mean they’ll have the same gender!

Same DNA = Same gender? Not necessarily!

You maybe remember that most males have XY chromosomes while most females carry XX chromosomes. But what does that mean?

The X and Y are called sex chromosomes. They have a huge effect on a person’s physical development. A person with a Y chromosome will typically develop male anatomy. A person without a Y chromosome will typically develop female anatomy.

But sex and gender are not the same thing. Sex is determined by sexual characteristics such as our reproductive organs. Gender, on the other hand, includes the psychological, behavioral, and social aspects of being a man, woman, or other non-binary identities.

Gender identity is influenced by genetics as well as other factors such as hormones¹. So although identical twins have the same DNA, they can identify as different genders.

There are actually many examples of identical twins who identify as different genders, such as Jonas and Nicole Maines. And there’s even an example of a pair of conjoined twins who have different gender identities.

Lori and George Schappell

Lori and George were born in Pennsylvania in 1961, both assigned female at birth. They are joined by the forehead, which makes them craniopagus twins, an extremely rare condition that occurs roughly 1 in 2.5 million live births².

Ever since their childhood, George felt a deep sense of unease about his gender, a sign of gender dysphoria. “I have known from a very young age that I should have been a boy,” he said during an interview with The Sun³. Later, he came out as a trans man, and his transition made them the world’s first known pair of conjoined twins identifying as different genders.

Despite being conjoined twins and identical in their DNA, Lori and George are known to have very different personalities and lived different lives. Lori is cheerful and warm while George is quiet and independent, according to a New York Times interview⁴.

Currently, George is working as an award-winning country singer performing around the country, while his twin sister Lori acts as his facilitator while working in a laundry. They have appeared in multiple documentaries and talk shows and have even acted in TV series.

Could conjoined twins have different sexes?

Lori and George’s story demonstrates that conjoined twins with identical DNA can have drastically different personalities and identify as different genders. But it’s also theoretically possible for conjoined twins to be assigned different sexes at birth.

We typically think of identical twins as having identical DNA. And for the most part, that’s true. But there are some exceptions to this.

We all started as a single embryo, which divided again and again to form the trillions of cells in us. As our cells divide and copy their DNA, mistakes can happen during the copying process, leading to changes in our DNA.

If a mistake like that happens soon after an embryo splits into identical twins, the identical twins will be not-entirely-identical.

For example, identical twins differ by roughly 5 single-letter differences in their DNA on average⁵. Usually small changes don’t have an effect. But in rare cases, those genetic differences might contribute to some of the differences observed in the twin pair.

But it’s also possible for identical twins to have much larger genetic differences. Sometimes when cells divide, they lose (or gain) entire chromosomes. If a change like this happens after the embryo splits apart into twins, they will be born with different numbers of chromosomes.

While it’s extremely unusual, there have been several dozen examples like this of identical twins who were born different sexes⁶. This usually happens when a male embryo (with XY chromosomes) splits into twins, and then one twin loses the Y chromosome. In those cases, one twin will be born male (XY chromosomes) and the other will be born female (with a single X chromosome).

So far there haven’t been any documented examples of this happening in conjoined twins. Since they’re a type of identical twins, it’s theoretically possible. But it would be extremely rare!