At six weeks of pregnancy, an embryo the size of a lentil already carries the full answer to the question of sex in every cell. But under the microscope, boys and girls look identical. More than that — they look like girls. This is not an accident and not a mistake of nature. It is one of the most elegant solutions in the history of evolution.
Sex is determined at the moment of fertilisation — most people know this. The egg always carries an X chromosome; the sperm carries either an X or a Y. X meets Y: an XY embryo. X meets X: an XX embryo. It would seem that everything is decided.
But the molecular “verdict” delivered at conception is not carried out immediately. Until the sixth week, the genes of the Y chromosome are silent. An XY embryo and an XX embryo develop in complete parallel: the same structures, the same proportions, the same morphology. And those structures, seen without any prior knowledge, look more female than male.
Three key primordia that by week 12 will become fundamentally different organs:
This is precisely why sex cannot be reliably determined by ultrasound before 12 weeks: until that point, differentiation of the external organs is simply not complete.
On days 42 to 49 (weeks 6 to 7), an event occurs in the gonadal cells of the XY embryo that triggers everything else. The gene SRY activates — a tiny region on the Y chromosome, less than one percent of its total length.
SRY encodes a protein that binds to DNA in the cells of the indifferent gonad and switches on the gene SOX9 — located on chromosome 17, shared by both sexes. SOX9 initiates the transformation of the neutral gonads into seminiferous tubules. Leydig cells begin synthesising testosterone. Sertoli cells begin producing anti-Müllerian hormone (AMH).
From there, two hormones take over:
Testosterone acts externally: the genital tubercle elongates into a penis, the labioscrotal folds fuse along the midline to form the scrotum (the seam that can be felt is the trace of this fusion). The urogenital groove closes, forming the urethra.
AMH acts internally: it triggers regression of the Müllerian ducts — the tubular structures that, in the absence of this signal, would become the uterus, fallopian tubes and upper vagina. In a boy, they will be gone.
In girls, things unfold differently — or more precisely, almost nothing happens. The ovaries are silent until weeks 16 to 20. No activating hormonal signals are needed: without testosterone, the genital tubercle remains a clitoris, the folds do not fuse, and the Müllerian ducts continue developing into the uterus and tubes. This is the “default path.”
Evolution did not design two separate blueprints for boys and girls. It designed one — and a switch. No signal means girl. Signal means boy.
The molecular elegance of this system becomes especially clear when something goes wrong. Several clinical scenarios show just how finely calibrated the mechanism is.
A mutation in the SRY gene renders its protein non-functional. The XY embryo receives no signal — and develops along the female path. The result is a person with female genitalia, female appearance and female identity. The only thing absent is ovaries: in their place are “gonadal streaks” of fibrous tissue. The syndrome is typically discovered only in adolescence, when menstruation does not arrive. Karyotype: 46 XY. Frequency: approximately 1 in 80,000.
The reverse: the SRY gene “jumps” onto the X chromosome during sperm formation. The XX embryo receives the signal — and develops along the male path. The result is a person with male genitalia and male appearance. Karyotype: 46 XX. Usually infertile: the genes for spermatogenesis are located in other regions of the Y chromosome that were not carried over. Frequency: approximately 1 in 20,000.
SRY functions normally, testes form, testosterone is produced — but androgen receptors do not work. The testosterone signal goes unheard externally. The genital tubercle does not elongate, the folds do not fuse. The result is a person with an XY karyotype, female external genitalia, and no uterus or ovaries (testes are located in the abdomen or groin). The syndrome is often discovered incidentally during evaluation for inguinal hernia or primary amenorrhoea.
SRY works, testosterone works, external masculinisation proceeds normally. But AMH or its receptors are non-functional. The Müllerian ducts do not regress. The result is a boy with normal male genitalia in whom, during surgery for inguinal hernia, a uterus and fallopian tubes are found.
Each of these syndromes is not an anomaly in the usual sense. It is a demonstration of how many independent steps must fire in sequence to produce a “standard” result.
The body retains the memory of the neutral phase. Several structures that seem self-evident in adults are in fact direct evidence of the common starting point.
Mammary lines — the rudiments of nipples — are laid down at week six, before SRY activates. By the time testosterone begins masculinising the embryo, the nipples are already formed. Remodelling them would be energetically wasteful. Evolution left them in place — a non-functional but completely harmless vestige.
The seam that runs along the midline of the scrotum is the trace of the labioscrotal folds fusing together. In women, the same folds remained separate and became the labia minora. The scrotum and the labia minora are homologous organs, arising from the same embryonic structure.
The nerve endings of the clitoris and the glans penis develop from the same cells — the genital tubercle. This explains the similar density of innervation and corresponding sensitivity of both organs.
From the perspective of evolutionary engineering, the system is elegant for three reasons.
First, economy. Instead of two separate genetic programmes — one blueprint and one switch. This requires less genetic information and fewer steps at each of which an error could arise.
Second, robustness. The default path requires no active signals. If SRY does not fire, development does not stop. The embryo continues along the female path, and for most SRY mutations this path ends in fertility. Male development is a sequence of active steps, any of which can be disrupted. Female development is what happens in the absence of special instructions.
Third, speed. The universal structures are laid down in two weeks. Hormonal transformation takes three. That is fast, even by the standards of embryogenesis.
For comparison: the platypus uses five pairs of sex chromosomes to determine sex. The human managed it with one gene out of 59 million base pairs on the Y chromosome. Elegant — but fragile, as everything built with minimal redundancy tends to be.
The story of sex determination is the story of one gene, two hormones, and three weeks that decide everything. Evolution did not bother inventing two separate blueprints: it took one universal template and added a switch. No signal — girl. Signal — boy. And the fact that this switch sometimes fails explains an entire spectrum of conditions that until recently were considered medical puzzles.
We all begin the same way. Our differences are the result of three weeks of molecular work from a single blueprint.
Module 5 (IVF & Embryo Expertise) explains how preimplantation genetic testing identifies chromosomal abnormalities before embryo transfer. Verified geneticists and reproductive specialists are available in the Partners section.
the gene on the Y chromosome that initiates male embryonic development. Activates at weeks 6–7. Mutations or absence of SRY in an XY karyotype leads to female-type development.
a hormone produced by Sertoli cells in the XY embryo. Causes regression of the Müllerian ducts. In adult women, a marker of ovarian reserve.
organs arising from the same embryonic structures. The clitoris and penis, the labia minora and scrotum, are homologues.
a condition in which an XY embryo develops along the female path due to a non-functional SRY gene. Characterised by female genitalia, absent ovaries and primary amenorrhoea.
embryonic tubular structures that, in the absence of AMH, develop into the uterus, fallopian tubes and upper vagina.