Interestingly it basically is down to a single gene [1] that's called "Testis-determining factor". And while it's usually on the Y-chromosome, there are actuall

Interestingly it basically is down to a single gene [1] that's called "Testis-determining factor". And while it's usually on the Y-chromosome, there are actually a pair of conditions where either that gene ends up on an X-chromosome, making an XX individual outwardly male [2], and another where that gene is missing from a y-chromosome, making an XY individual outwardly female [3]. I didn't mean for this to come across as correcting you, I just thought it was neat, and it seemed worth sharing. [1] https://en.wikipedia.org/wiki/Testis-determining_factor https://en.wikipedia.org/wiki/Testis-determining_factor [2] https://en.wikipedia.org/wiki/XX_male_syndrome https://en.wikipedia.org/wiki/XX_male_syndrome [3] https://en.wikipedia.org/wiki/XY_gonadal_dysgenesis https://en.wikipedia.org/wiki/XY_gonadal_dysgenesis