Chromosomal sex is set at fertilisation, but the body doesn't act on it right away. For roughly the first six to seven weeks of human development, the embryonic gonads are what embryologists call bipotential, or indifferent — built from tissue and duct systems capable of developing into either testes or ovaries, and either set of male or female internal reproductive structures, regardless of chromosomal sex. What happens next depends on a single genetic switch flipping, or not flipping, during a narrow developmental window.
One structure, two possible futures
Every embryo, early on, carries both the Wolffian ducts, which have the potential to develop into male internal structures such as the vas deferens, and the Müllerian ducts, which have the potential to develop into female internal structures such as the fallopian tubes and uterus — present side by side, in every embryo, before either pathway has been chosen. The bipotential gonad itself sits in the same undecided state, containing the tissue types capable of becoming either testis or ovary depending on which signal, if any, arrives.
A single gene switch, then a cascade
The trigger is the SRY gene, carried on the Y chromosome. When SRY is present and activated, it sets off a cascade — activating further genes such as SOX9 — that drives the bipotential gonad to develop as testis. Once testes form, they produce two separate signals: anti-Müllerian hormone, which causes the Müllerian ducts to regress, and testosterone, which supports the Wolffian ducts in developing into male internal structures, with external genitalia later shaped by a related hormone, dihydrotestosterone. Without an SRY signal, the gonad develops along the ovarian pathway instead, the Müllerian ducts persist and develop into female internal structures, and the Wolffian ducts regress.
What we're still unsure about
Textbooks have long described ovarian development as the passive "default" pathway — what simply happens whenever the male-determining signal is absent. More recent molecular research complicates that framing: genes such as WNT4 and RSPO1 have been identified as actively promoting and maintaining ovarian development, rather than the ovary forming merely because nothing told it to become a testis. That means female sexual differentiation appears to be its own actively regulated developmental program, not just an absence of the male one — a correction that's still working its way from research papers into standard teaching. Documented natural variation in this process, historically grouped under the term intersex conditions, continues to show that the underlying biology is more varied at the molecular level than the simplified single-pathway story usually taught.
This sits inside Embryology & Developmental Anatomy, one of seven topics in Anatomy, one of four domains in Medicine, one of seventeen subjects the app can quiz you on.