World Health Organisation:
Clearly, there are not only females who are XX and males who are XY, but rather, there is a range of chromosome complements, hormone balances, and phenotypic variations that determine sex.
www.who.int/genomics/gender/en/index1.html
Scientific American:
Nearly everyone in middle school biology learned that if you’ve got XX chromosomes, you’re a female; if you’ve got XY, you’re a male. This tired simplification is great for teaching the importance of chromosomes but betrays the true nature of biological sex. The popular belief that your sex arises only from your chromosomal makeup is wrong. The truth is, your biological sex isn’t carved in stone, but a living system with the potential for change.
Why? Because biological sex is far more complicated than XX or XY (or XXY, or just X). XX individuals could present with male gonads. XY individuals can have ovaries. How? Through a set of complex genetic signals that, in the course of a human’s development, begins with a small group of cells called the bipotential primordium and a gene called SRY.
A newly fertilized embryo initially develops without any indication of its sex. At around five weeks, a group of cells clump together to form the bipotential primordium. These cells are neither male nor female but have the potential to turn into testes, ovaries or neither. After the primordium forms, SRY—a gene on the Y chromosome discovered in 1990, thanks to the participation of intersex XX males and XY females—might be activated.*
Though it is still not fully understood, we know SRY plays a role in pushing the primordium toward male gonads. But SRY is not a simple on/off switch, it’s a precisely timed start signal, the first chord of the “male gonad” symphony. A group of cells (instrument sections) must all express SRY (notes of the chord), at the right time (conductor?). Without that first chord, the embryo will play a different symphony: female gonads, or something in between.
blogs.scientificamerican.com/voices/stop-using-phony-science-to-justify-transphobia/
Picture is from NHS.... but yeah you guys all know better!!