In biology, the sexes are defined by the type of reproductive cell, or gamete, an organism produces. Females produce large gametes (eggs), and males produce small gametes (sperm). This size difference between gametes is called anisogamy, and it is the foundational distinction between male and female across nearly all multicellular life. Species that produce only one size of gamete lack distinct sexes entirely.
How Gametes Define Male and Female
The two-sex system is not unique to humans. It appears independently across plants, animals, and fungi, making it one of the most consistent patterns in biology. The core logic is simple: one sex invests in fewer, smaller reproductive cells (sperm), while the other invests in fewer but larger, energy-rich cells (eggs). Hermaphroditic organisms, like many snails and some fish, can produce both gamete types, either at the same time or at different life stages.
This gamete-based definition is what biologists use because it holds up across species, regardless of how different animals look, behave, or reproduce. A male seahorse still produces sperm even though he carries the young. A female hyena still produces eggs despite having masculinized genitalia. External appearance can vary wildly, but the gamete distinction remains consistent.
How Sex Is Determined in Humans
In humans, sex is set in motion by chromosomes. Most females carry two X chromosomes (46,XX), and most males carry one X and one Y (46,XY). But chromosomes alone don’t build a body. The process depends on a chain of genetic signals, hormones, and tissue responses that unfold over weeks of fetal development.
The key trigger on the Y chromosome is a gene called SRY. Around 41 to 42 days after conception, SRY switches on in XY embryos and directs a set of undifferentiated tissue (the genital ridge, which forms around week five) to develop into testes rather than ovaries. The first visible signs of this divergence appear between weeks six and seven, when testicular structures begin to form. Before that point, the tissue is identical regardless of chromosomes.
Once testes form, they produce hormones that guide further development: deepening the internal reproductive tract toward a male pattern and preventing the formation of a uterus and fallopian tubes. In XX embryos, without SRY activation, the same starter tissue follows an ovarian path instead. By about eight weeks, the gonads are actively producing the hormones that shape the rest of sexual anatomy.
The Role of Hormones
Chromosomes and genes initiate sex determination, but hormones do most of the building. Testosterone and its more potent derivative drive the development of male genitalia, the prostate, and later, puberty changes like facial hair growth, voice deepening, muscle development, and increased height. Estrogen and progesterone drive breast development, the menstrual cycle, and fat distribution patterns typical of female bodies.
These hormones don’t just matter before birth. At puberty, they trigger what are known as secondary sex characteristics: the visible physical traits that distinguish adult male and female bodies. In males, that includes growth of the penis and testes, body and facial hair, a deeper voice, and broader shoulders. In females, it includes breast development, wider hips, and the onset of menstruation.
Critically, the body doesn’t just need to produce the right hormones. It also needs to respond to them. In a condition called androgen insensitivity syndrome, a person has XY chromosomes and testes that produce normal levels of testosterone, but the body’s cells cannot detect or use those hormones. The result is that the person develops external anatomy that looks typically female, despite having a male chromosomal pattern. This illustrates that sex development is a multi-step process where chromosomes, genes, hormones, and cellular receptors all need to align.
Variations in Sex Development
Not everyone fits neatly into the two categories. Variations in sex development (sometimes called differences of sex development, or intersex traits) are a group of conditions where chromosomes, hormones, or reproductive anatomy develop along a less common path. Estimates suggest these variations collectively affect roughly 0.7 to 1.7 percent of people worldwide. Each individual condition is rare, but taken together, they are not uncommon.
Some variations involve chromosomes. In Klinefelter syndrome, a person has 47 chromosomes with an XXY pattern instead of the typical 46. These individuals typically develop as male but often have smaller testes, reduced testosterone production, and taller-than-average stature. Without hormone support, they may experience incomplete puberty, breast tissue growth, lower bone density, and reduced muscle mass. Rarer patterns like 48,XXXY and 49,XXXXY also occur.
Turner syndrome involves a missing or incomplete second sex chromosome (45,X). These individuals develop as female but often face challenges with growth, ovarian function, and certain physical features.
Other variations stem from hormone production or reception. Congenital adrenal hyperplasia can cause an XX individual to produce excess androgens, leading to masculinized genitalia at birth. Androgen insensitivity syndrome, described above, produces the opposite effect in XY individuals. In rare cases called ovotesticular differences of sex development, a person develops both ovarian and testicular tissue.
Clinically, these conditions are grouped by their chromosomal basis: 46,XX variations, 46,XY variations, and sex chromosome variations. The terminology has evolved over time, with “differences in sex development,” “variations in sex characteristics,” and “intersex” all used depending on the medical or personal context.
Sex as a Biological System
Sex is best understood not as a single trait but as a layered system. Chromosomal sex is set at fertilization. Gonadal sex (whether you develop ovaries or testes) is determined in the first weeks of embryonic life. Hormonal sex shapes anatomy both before birth and again at puberty. Phenotypic sex is what the body ultimately looks like.
In most people, all of these layers align in a straightforward way. In some, one or more layers diverge, producing the range of variations described above. The two-sex framework based on gamete production remains the organizing principle in biology, but the developmental pathway from chromosomes to a fully formed body involves enough complexity that outcomes are not always binary.