[adminu]

There actually are more combinations involving three chromosomes. They make binary classification quite... impossible

[CydeWeys]

Or people who have a Y chromosome yet lack the SRY gene, or lack a Y chromosome but have the SRY gene on another chromosome.

It's incredibly complicated and it's frankly amazing that sexual dimorphism and reproduction works even as well as it does.

[marton78]

There's also the possibility of being monosomal: X0