[El Gato]

It's called the y chromosome. If she has one in EVERY CELL in her body its a boy. If you have one in NO CELL in your body, you're a girl. Case closed.

What if you also have two X chromosomes to go with that Y? Then what are you? The story is so poorly written we can't tell what the heck the genetic situation is.

[ThreeYearLurker]

Then you are a mutant, but this is not the case here.

Mongoloid he was a mongoloid
Happier than you and me
Mongoloid he was a mongoloid
And it determined what he could see
Mongoloid he was a mongoloid
One chromosome too many
Mongoloid he was a mongoloid
And it determined what he could see
And he wore a hat
And he had a job
And he brought home the bacon
So that no one knew
Mongoloid he was a mongoloid
His friends were unaware
Mongoloid he was a mongoloid
Nobody even cared