Red hair? Chromosomes. Bad eyesight? Chromosomes. A nose like your great-grandpa Stan’s? All thanks to chromosomes.

You get twenty-three ingredients from your dad, and twenty-three from your mom, and together those ingredients make up the Recipe of You. There are two special chromosomes that mean you’re born male or female, and you get one of them from your mom and one from your dad.

Except when you don’t.

The two female chromosome ingredients are XX. That means your mom is XX, and so when you get your twenty-three mom ingredients, you get one of her X’s.

Male ingredients are XY. That means it’s your dad’s ingredients that decide if you’re born male or female. Here’s how it works. Y is the “male” ingredient. Since Dad is XY, if he gives you an X, you get one X from Dad, one X from Mom, and boom bam, XX, you’re a girl. If Dad gives you a Y instead, though, then you’ve got X from Mom, Y from Dad, and XY, happy birthday, it’s a boy.

But it doesn’t always work out like that.

Sometimes you’re just X.

One X and something’s missing.

XO.

Which doesn’t stand for hugs and kisses.

It stands for Missing Ingredient.

It doesn’t bother me too much. In fact, some people who are called boys or girls when they’re born decide that, actually, that word and all its associations doesn’t suit them after all. But I’ve always felt like a girl, all the way from my head to my toes. So I once asked my mom if having only one X meant I might be a boy—and she said it’s not like that, because I don’t have the Y “male” ingredient, and plus I don’t feel like a boy at all. That means I’m just as much a girl as she is.

Here are some more facts: 1 in every 2,500 girls is born XO.

This is called Turner syndrome.

Sometimes when a baby is getting made and the mom’s body senses that it’s missing an ingredient, her body stops making the baby and it dies before it’s even born. That’s called a miscarriage. A miscarriage happens to 99 percent of babies who are being made with a missing ingredient.

That means I’m really lucky.

Dad says it means I’m meant to be here.

There are lots of other ways things can go wonky with the ingredients. For example, it’s also possible to get an extra chromosome. An extra number- twenty-one ingredient from Mom or Dad. That is called Down syndrome.

Maybe it’s miraculous anybody is born “healthy” at all. A miraculous protostar bursting out of a perfect mix of dust and heat.

Turner syndrome fiddles with my body in some ways. Like my ballooned-up heart.

Like my neck that’s a bit thick on the sides.

Like my ribs that are round like a barrel.

Like my low-set ears that don’t hear exactly perfectly.

Like how I have to give myself shots every day.

But that’s just physical stuff. My brain is still intact.