At the most fundamental level, a female is the sex that produces large, energy-rich reproductive cells called eggs. This definition holds across nearly all sexually reproducing species, from humans to ferns to sea urchins. But in humans, “what makes a female” involves layers of biology working together: genetics, hormones, anatomy, and active cellular maintenance that continues throughout life. None of these layers operates as a simple on/off switch.
The Gamete Definition
Biologists define the two sexes by gamete size. Females produce relatively few, large gametes (eggs) that invest heavily in nourishing a potential embryo. Males produce many small gametes (sperm) that are optimized for reaching an egg. This size difference, called anisogamy, is the foundational distinction between male and female across the living world. Everything else we associate with sex, from chromosomes to body shape, evolved in service of these two reproductive strategies.
Large gametes provision the resulting embryo with nutrients and cellular machinery. Small gametes compete for fertilization through sheer numbers. Over evolutionary time, these opposing pressures drove specialization: organisms that tried to make medium-sized gametes were outcompeted by those that committed fully to one strategy or the other.
Genetics: More Than Just Missing a Y
In humans, females typically carry two X chromosomes (46,XX), while males carry one X and one Y (46,XY). The Y chromosome contains a gene called SRY that acts as a master switch for testis formation. It works by binding to DNA and physically bending it out of shape, which changes gene activity and pushes the developing gonad toward becoming a testis. Without SRY, this cascade never fires.
But female development is not simply the absence of male signals. Active “pro-female” genes drive and maintain ovarian identity. A signaling molecule called WNT-4 was the first shown to directly influence the sex-determination cascade. In mouse experiments, deleting WNT-4 from XX embryos caused them to masculinize, developing rudimentary male internal ducts while their female reproductive tract degenerated. WNT-4 works in part by boosting the activity of another gene, DAX1, which directly antagonizes SRY. Both WNT-4 and DAX1 stay active in the developing ovary but shut down in the developing testis.
Another gene, FOXL2, is essential not just for building an ovary but for keeping it an ovary. When researchers deleted FOXL2 from adult mouse ovaries, something remarkable happened: ovarian cells began transforming into testicular cells. Structures resembling seminiferous tubules (the sperm-producing architecture of a testis) started forming in place of follicles. This means female cellular identity requires lifelong genetic maintenance. FOXL2 acts as a continuous suppressor of male programming, even in adulthood.
X-Inactivation
Having two X chromosomes creates a dosage problem: females would otherwise produce twice the X-linked gene products that males do. Early in embryonic development, one X chromosome in each cell is silenced. The choice of which X gets shut down is random in each cell, so most females end up as a mosaic, with some cells using one X and other cells using the other. This ratio can range from a perfect 50:50 split to heavily lopsided patterns where one X dominates. In most women this has no noticeable effect, but highly skewed inactivation can sometimes reveal carrier status for X-linked conditions.
How a Female Body Takes Shape
For the first six weeks of human embryonic development, male and female embryos are anatomically indistinguishable. Both contain a pair of bipotential gonads that can become either ovaries or testes, plus two sets of internal ducts: Wolffian ducts (potential male structures) and Müllerian ducts (potential female structures).
Without signals from SRY and the hormones a testis would produce, the Müllerian ducts survive and develop. They form through three phases: first, precursor cells thicken at the top of the duct region; then these cells fold inward and extend downward; finally, the growing ducts reach and fuse with the urogenital sinus at the base. By around week 10 of gestation, the earliest sign of ovarian differentiation appears as egg precursor cells in the deepest layers of the gonad begin entering the first stage of the cell division that will eventually produce mature eggs.
The Müllerian ducts differentiate into the fallopian tubes, uterus, cervix, and upper vagina. This patterning depends on a series of genes expressed in a head-to-tail sequence along the duct: one gene marks the region destined to become the fallopian tubes, another marks the uterus, another the lower uterus and cervix, and another the cervix and upper vagina. Meanwhile, without testosterone, the Wolffian ducts (which would otherwise form male internal structures) simply wither away.
Hormones and Puberty
Estrogen is the primary hormone driving female puberty, though the ovaries also produce progesterone and small amounts of testosterone. Rising estrogen levels trigger a predictable sequence of changes. Breast development is typically the first visible sign, appearing on average about four months before pubic hair growth begins. About two-thirds of girls follow this “breast-first” pattern.
Even small increases in estrogen have significant effects. Morning estradiol levels in the low early-pubertal range (roughly 4 to 9 pg/mL) are enough to noticeably accelerate growth. By the time estradiol reaches mid-pubertal levels, less than 25% of a girl’s potential pubertal growth spurt remains. Estrogen eventually signals the growth plates in bones to close, which is why females on average stop growing earlier than males.
Beyond puberty, cycling estrogen and progesterone levels regulate the menstrual cycle, prepare the uterine lining for potential pregnancy, and influence bone density, fat distribution, cardiovascular function, and brain chemistry. The characteristic pattern of fat deposition around the hips and breasts, wider pelvic structure, and less pronounced facial hair are all downstream effects of the hormonal environment maintained by functioning ovaries.
When Biology Doesn’t Follow the Typical Path
Sex determination involves many genes, hormones, and developmental steps, and variations can occur at any point. The clearest illustration is Swyer syndrome, in which a person has XY chromosomes but develops female external genitalia, a uterus, and fallopian tubes. Their gonads never fully develop into testes or ovaries, remaining as small, nonfunctional “streak” tissue. This happens because SRY either doesn’t function properly or is absent despite the Y chromosome being present. People with Swyer syndrome are typically raised as girls and may not learn about the condition until puberty fails to start on its own. With hormone therapy, they can develop breasts, menstruate, and in some cases carry a pregnancy using donor eggs.
How common these variations are depends on how you define them. A widely cited estimate of 1.7% includes conditions like Klinefelter syndrome (XXY) and late-onset adrenal hyperplasia, which most clinicians would not classify as intersex because the person’s anatomy is clearly male or female. Using a stricter definition, where chromosomal sex is inconsistent with physical sex or the body is genuinely ambiguous, the prevalence is closer to 0.018%, or roughly 1 in 5,500 births.
These variations reinforce rather than undermine the biological framework. They show that “female” is not determined by any single factor in isolation. Chromosomes set the initial direction, genes actively build and maintain ovarian tissue, hormones shape the body, and anatomy develops through a cascade of precisely timed events. When all of these align, the result is what we recognize as typically female. When one or more steps diverge, the outcome depends on which step was affected and when.