Biological sex in humans and nearly all animals is organized around two reproductive roles, male and female, defined by the type of sex cell (gamete) an organism is built to produce. That two-part system is remarkably consistent across the animal kingdom, but it does not mean every individual fits neatly into one category or the other. Chromosomal variations, hormonal differences, and rare developmental conditions create a range of real human bodies that complicate any simple headcount. And the question itself often muddles two separate concepts, sex and gender, that biologists and medical researchers treat as distinct.
Sex and Gender Are Not the Same Thing
In scientific and medical literature, “sex” refers to a person’s biological characteristics, while “gender” refers to socially constructed roles, identities, and norms.1PubMed Central. Differentiating sex and gender in health research to achieve gender equity When people ask “how many biological genders are there,” they usually want to know about biological sex, not the sociological concept of gender. Gender identity, which is a person’s internal sense of being a man, woman, both, or neither, is influenced by biology but is not the same thing as reproductive anatomy or chromosomes. The rest of this article focuses on biological sex: how it is defined, what creates variation, and where the boundaries get blurry.
Why Biologists Say There Are Two Sexes
The foundational definition of biological sex across sexual species comes down to gamete type. Males produce small, mobile gametes (sperm); females produce large, nutrient-rich gametes (eggs). This size difference, called anisogamy, is the criterion biologists use to classify sex in everything from humans to sea urchins to ferns.2PubMed Central. Biological Sex Is Binary and Rooted in Anisogamy There is no third gamete type in any known animal or plant species. You will not find an intermediate sex cell that is halfway between a sperm and an egg. This is why evolutionary biologists describe sex as binary at the level of reproductive strategy: every sexually reproducing species that produces gametes produces exactly two types.3PubMed. Biology should not dispense with sexes
Evidence from green algae suggests that anisogamy evolved from ancestors whose gametes were identical in size, with genes linked to mating-type regions driving the split into two distinct gamete sizes.4PubMed. Evolutionary biology: the origins of two sexes Once that split happened, it stuck. No sexually reproducing animal lineage has ever evolved a stable third gamete type. The two-sex system is, in that narrow sense, universal.
How Sex Gets Determined in Humans
In a typical human embryo, sex development starts with a gene called SRY, located on the Y chromosome. SRY acts as a trigger for male development by switching on a cascade of other genes, most critically Sox9, that steer the developing gonad toward becoming a testis. The testis then produces testosterone, which drives the development of male anatomy.5PubMed. Sry, Sox9 and mammalian sex determination Without SRY, a different set of genetic pathways takes over. Genes including RSPO1, Wnt4, and Foxl2 guide the gonad toward becoming an ovary and promote female anatomical development.6PubMed. Sry and SoxE genes: How they participate in mammalian sex determination and gonadal development?
What makes this interesting is that the male and female pathways actively suppress each other throughout life, not just during embryonic development. If the balance between them is disrupted at any point, the result can be atypical sex development. The system is less like a light switch and more like a tug-of-war that keeps pulling in one direction, and any slackening on one side can shift the outcome.
When Chromosomes Don’t Follow the Usual Pattern
Most humans have either 46,XX or 46,XY chromosomes, but a meaningful number do not. Sex chromosome aneuploidies, where a person has an extra or missing sex chromosome, occur in a significant fraction of the population. In one prenatal study, sex chromosome aneuploidies were identified in about 8% of cases referred for genetic testing, with Turner syndrome (one X, no second sex chromosome) being the most common, followed by Klinefelter syndrome (XXY), 47,XYY, and trisomy X (XXX).7PubMed Central. Prenatal Diagnosis of Sex Chromosome Aneuploidies: A Retrospective Study Using QF-PCR, SNP-Based Chromosomal Microarray Analysis, and NIPT That 8% figure reflects a clinically referred population, not the general birth rate; in unselected newborns, the frequency is much lower but still well above trivial.
The effects of these variations are highly variable. A person with Klinefelter syndrome (XXY) is typically raised male, usually develops testes, and may never know about the extra chromosome unless tested. A person with Turner syndrome (45,X) is typically raised female and has ovaries that often regress early. The severity of symptoms depends not just on the chromosomal pattern itself but on broader genetic modifiers and regulatory effects across the genome.8PubMed Central. The multi-omic landscape of sex chromosome abnormalities: current status and future directions Within Turner syndrome specifically, more complex structural abnormalities of the X chromosome tend to produce more severe clinical findings.9PubMed Central. Genotype–Phenotype Correlations in Klinefelter and Turner Syndrome: A Decade of Sex Chromosome Aneuploidy Data From a Single Academic Medical Center
These chromosomal variations do not represent additional sexes in the gamete-based sense. A person with XXY chromosomes still falls on the male side of the gamete divide. But the variations do make the point that chromosomes alone are not a reliable binary sorter of human bodies. The downstream anatomy, hormone levels, and fertility that arise from atypical karyotypes often land somewhere between the textbook descriptions of “male” and “female.”
Differences in Sex Development
Beyond chromosomal variations, there are dozens of conditions where chromosomal, gonadal, or anatomical sex develops atypically. These are collectively called differences (or disorders) of sex development, or DSDs. They range from conditions with very few visible effects to conditions where questions about sex assignment arise at birth.10PubMed Central. Differences/Disorders of Sex Development: Medical Conditions at the Intersection of Sex and Gender
One well-studied example is androgen insensitivity syndrome (AIS). A person with complete AIS has 46,XY chromosomes and internal testes that produce testosterone, but their body’s cells cannot respond to that testosterone because of mutations in the androgen receptor gene. The result is a person who looks female externally, is typically raised as a girl, and may not learn about the condition until puberty when menstruation does not begin.11PubMed Central. A novel androgen receptor gene splice site mutation induces aberrant mRNA splicing and internal in-frame deletion in androgen insensitivity syndrome A study of 30 prepubertal patients with AIS identified 24 different variants in the androgen receptor gene, underscoring how many distinct mutations can produce the same clinical picture.12PubMed Central. Clinical features and genetic analysis of androgen receptor gene variants in 30 prepubertal patients with androgen insensitivity syndrome
Estimates of how common all DSD conditions are collectively vary widely depending on how broadly the category is drawn. One analysis estimated that conditions involving intersex or DSD traits affect roughly 0.7 to 1.7% of people worldwide and found that over 15% of phenotypically detailed genetic disorders in major databases have associations with DSD-related traits.13bioRxiv. Intersex/Differences of Sex Development (I/DSD) Traits: Exploring Their Association with 15% of Phenotypically Detailed Genetic Disorders That 0.7-1.7% figure, if accurate, means DSD conditions are about as common as red hair. They are not exotic anomalies. They are a recurring feature of human biology.
Chimerism and Mixed Gonads
Perhaps the most dramatic illustration of sex’s complexity is chimerism, where a single person carries cells with different chromosomal makeups. In rare cases, two fertilized embryos fuse very early in development, producing one individual with both 46,XX and 46,XY cell lines. The first documented case, reported in 1962, was a child who had one ovotestis (a gonad containing both testicular and ovarian tissue) and one typical ovary. Genetic analysis suggested the child resulted from two separate sperm fertilizing two separate egg cells, which then merged into a single embryo.14OBM Genetics. 46,XX/46,XY Chimerism & Human Sexual Development
People with this type of chimerism are sometimes categorized as having ovotesticular disorder of sexual development. Their gonads can include an ovary on one side and a testis on the other, or mixed ovotestes. Detailed cellular analysis has shown that XX cells can be found in the testicular portions and XY cells in the ovarian portions, meaning the separation between “male tissue” and “female tissue” is not even clean at the cellular level within a single gonad.15PubMed. The Distribution and Cellular Lineages of XX and XY Cells in Gonads Associated with Ovotesticular Disorder of Sexual Development These cases are vanishingly rare, but they demonstrate that a single human body can contain the developmental machinery for both sexes simultaneously.
How Other Organisms Handle Sex
If you look beyond mammals, the relationship between biology and sex gets even more varied. In many egg-laying reptiles, sex is not written into the chromosomes at all. Instead, the temperature at which an egg incubates determines whether the offspring develops as male or female.16PubMed. Temperature-dependent sex determination in reptiles: proximate mechanisms, ultimate outcomes, and practical applications Some species sit at a midpoint between purely genetic and purely temperature-driven sex determination, with underlying genetic factors that can be overridden by extreme temperatures. In the jacky dragon, for instance, ovotestes appear during development at rates that suggest a system intermediate between chromosomal and temperature-based sex control.17PubMed Central. Ovotestes suggest cryptic genetic influence in a reptile model for temperature-dependent sex determination Researchers have argued that genetic and environmental sex determination in reptiles should be understood as a continuum rather than an either-or system.18PubMed. The ends of a continuum: genetic and temperature-dependent sex determination in reptiles
Many fish species go further by changing sex entirely during their lifetimes. Sequential hermaphrodites start life as one sex and transition to the other, often triggered by social context. Anemonefish, for example, begin as males and can become female. Other species switch from female to male. In the New Zealand spotty wrasse, complete female-to-male transition can occur in about 60 days when social conditions change.19PubMed Central. A new experimental model for the investigation of sequential hermaphroditism In all these cases, the switch replaces the production of one gamete type with the other, so the organism moves from one binary sex category to the other rather than occupying a third.20PubMed Central. Resistance of Adult Kryptolebias marmoratus Hermaphrodites to Irreversible Sex Change by Exogenous Androgens
Then there are species that have dispensed with males entirely. Six species of whiptail lizards in the genus Cnemidophorus reproduce without males at all, consisting entirely or almost entirely of females who produce offspring through parthenogenesis.21PubMed. All-Female Species of the Lizard Genus Cnemidophorus, Teiidae These lizards still technically produce eggs, so they are “female” by the gamete definition, but they have eliminated the male role from their species entirely.
Fungi and the Thousands of Mating Types
If you stretch the question beyond animals and plants, the answer changes dramatically. Fungi do not have male and female sexes at all. Instead, they use mating types, which are determined by genetic differences at specialized chromosomal regions. In many basidiomycete fungi (the group that includes mushrooms), two independent genetic loci combine to produce thousands of possible mating types.22PubMed. Tetrapolar fungal mating types: sexes by the thousands A given individual can mate with any other individual that differs at those loci, which means most encounters between unrelated individuals are compatible. The system maximizes outbreeding without relying on the large-gamete/small-gamete distinction that defines sex in animals.23PubMed Central. Fungal sexual reproduction and mating-type loci
Some people cite fungal mating types as evidence that sex is not binary. That reasoning gets the biology a bit wrong. Mating types are a separate system from sexes. Fungi that have thousands of mating types still do not produce two different gamete sizes; their reproductive cells are typically identical. The correct takeaway is that nature has multiple ways of organizing sexual reproduction, and the male-female system is specific to organisms with anisogamy. It is not the only game in town, but it is the system that applies to all animals, including humans.24PubMed. The Rewiring of Who Mates with Whom: Evolutionary Transitions of Mating Systems, Mating Classes and Their Genetic Architecture
What the Brain Adds to the Picture
Sex differences are not limited to gonads and chromosomes. The human brain develops differently depending on its hormonal environment during pregnancy, with testosterone playing a central organizing role. In a typical male fetus, testosterone steers brain development in one direction; in its absence, the brain develops along a different trajectory.25PubMed. Sexual differentiation of the brain and behavior Steroid hormones including testosterone, estrogen, and dihydrotestosterone shape brain structures and affect everything from neuroendocrine function to behavioral tendencies.26PubMed Central. The effects of prenatal sex steroid hormones on sexual differentiation of the brain
This is where the question of gender identity starts to intersect with biology. One neuroimaging study of transgender women (before hormone therapy) found that their brain anatomy, as measured by a probabilistic classifier, fell between that of cisgender men and cisgender women, though still closer to cisgender men. The differences from both groups were statistically significant.27PubMed Central. Brain Sex in Transgender Women Is Shifted towards Gender Identity Studies of children and adolescents experiencing gender incongruence have found some evidence supporting the idea that their brain development followed a less typical sex-differentiation pattern, though the results are more mixed than early hypotheses predicted. Some findings suggest transgender individuals may present a unique brain phenotype rather than simply being shifted toward the “opposite” sex.28PubMed. Neurobiological characteristics associated with gender identity: Findings from neuroimaging studies in the Amsterdam cohort of children and adolescents experiencing gender incongruence
Research on broader populations has also found that gender identity correlates with measurable brain differences. In people assigned female at birth, a more feminine identity was linked to reduced cortical thickness in several brain regions, while in people assigned male at birth, masculine and feminine identities were associated with distinct patterns of cortical area and thickness.29Brain‐X. Heterogeneity in brain morphology and psychological, cognitive, and contextual factors of gender identity None of this means the brain has a clean “male version” and “female version.” The neuroscience increasingly points toward overlapping distributions with lots of individual variation rather than two discrete categories.
The Binary-vs.-Spectrum Debate in Science
There is a genuine scientific disagreement about how to characterize sex. On one side, evolutionary biologists emphasize that biological sex, defined by gamete type, is binary. There are two types of gametes, and the existence of individuals who produce neither (due to infertility or developmental conditions) does not create a third sex any more than a person who is blind creates a third category of vision. This camp argues that chromosomes, hormones, and anatomy are components of a sex-determination system, not definitions of sex itself. Variation in those components is real but does not change the number of sexes.2PubMed Central. Biological Sex Is Binary and Rooted in Anisogamy
On the other side, researchers in medical genetics and anatomy argue that the gamete-only definition is too narrow for clinical purposes. They point out that sex encompasses chromosomes, hormones, secondary sexual characteristics, and anatomy, and that insisting on a strict binary based on gametes obscures the reality of patients whose bodies do not align neatly with either category.30PubMed Central. Is sex still binary? Some have called for anatomy curricula to present sex as a continuum and to formally introduce intersex and DSD as categories rather than treating them as exceptions to a rule.31PubMed. Beyond the Sex Binary: Toward the Inclusive Anatomical Sciences Education
Both positions are defensible, and they are not really contradicting each other as much as talking about different things. The evolutionary biologists are correct that no third gamete exists. The clinical researchers are correct that a binary classification system fails many real patients. Whether you call sex “binary” or “a spectrum” depends entirely on whether you mean the reproductive strategy or the full suite of biological traits that make up a sexed body. This is less a factual dispute than a framing dispute, which is why it generates so much heat and so little resolution.
Why the Distinction Matters for Medicine
The practical reason to care about sex complexity is that it affects health. Women and men differ in immune function, with women being more susceptible to many autoimmune disorders, a difference tied in part to having two X chromosomes and the gene-dosage effects that come with it.32PubMed Central. When the balance is broken: X-linked gene dosage from two X chromosomes and female-biased autoimmunity Sex differences in how the body absorbs, distributes, metabolizes, and excretes drugs are significant enough that researchers have called for sex-specific pharmacological guidelines, though most current prescribing practices remain sex-neutral.33PubMed Central. Biological sex differences in pharmacokinetics and adverse drug reactions Sex-based differences also extend to cardiovascular disease, where the prevalence, symptoms, and outcomes differ between men and women, driven by both hormonal and hormone-independent mechanisms.34PubMed Central. Epigenetic regulation of sex dimorphism in cardiovascular health
For people with DSD conditions or atypical chromosomal patterns, cookie-cutter medical advice based on a binary sex classification can miss the mark. A person with complete androgen insensitivity syndrome has XY chromosomes but may face health risks more typical of female patients in some respects and unique to their condition in others. Understanding the layers of biological sex, rather than treating it as a checkbox, leads to better care for everyone, not just people with rare conditions. The evidence is clear that male and female bodies behave differently at almost every level of physiology, and the people who fall between those categories need medicine that accounts for where they actually are, not where a form says they should be.35PubMed Central. Sex Differences in Pharmacokinetics and Pharmacodynamics