In biology, sex is most fundamentally defined by the type of reproductive cell (gamete) an organism produces. Females produce large gametes (eggs), and males produce small gametes (sperm). This two-gamete system, called anisogamy, is the basis for the biological categories of male and female across virtually all sexually reproducing species. But in humans, sex is more than just gametes. It’s a collection of traits, including chromosomes, hormones, reproductive organs, and physical characteristics, that usually align in one of two patterns but sometimes don’t.
The Gamete-Based Definition
The most widely used biological definition of sex centers on gamete size. In any species with sexual reproduction, males are the sex that produces small, mobile gametes (sperm), and females produce large, nutrient-rich gametes (eggs). This distinction exists across animals, plants, and fungi. Some simpler organisms, like certain algae, reproduce sexually but produce same-sized gametes, a state called isogamy. In those species, there are mating types rather than separate sexes.
Humans are anisogamous, meaning we have two distinct gamete types and therefore two sexes defined at the gamete level. This binary framework describes the vast majority of people, but human sex development involves multiple biological layers, and variation exists at each one.
Layers of Biological Sex
The National Institutes of Health defines biological sex as “a multidimensional biological construct based on anatomy, physiology, genetics, and hormones.” These dimensions usually line up neatly, but they’re distinct systems, and understanding them separately helps explain how variation arises.
Chromosomal Sex
Most humans carry either two X chromosomes (46,XX, typically female) or one X and one Y (46,XY, typically male). The Y chromosome carries a gene called SRY, which acts as a master switch for male development. The protein produced by SRY triggers a fetus to develop testes, which then produce hormones that shape the rest of male anatomy. Without a functioning SRY gene, a fetus develops a uterus and fallopian tubes regardless of whether a Y chromosome is present.
This means chromosomal sex and developmental sex don’t always match. A person with XY chromosomes but a nonfunctional SRY gene (a condition called Swyer syndrome) will develop female internal reproductive structures. Conversely, if the SRY gene gets transferred onto an X chromosome during cell division, a person with XX chromosomes can develop male characteristics.
Gonadal Sex
Gonadal sex refers to whether a person has ovaries, testes, or some combination. In typical development, SRY activation leads to testes, and its absence leads to ovaries. But in rare cases, a person can have one ovary and one testis, or gonads containing both ovarian and testicular tissue.
Hormonal Sex
Once gonads develop, they produce sex hormones that drive the rest of sexual differentiation. Testes produce androgens (primarily testosterone), which trigger male genital development in the fetus and later drive puberty changes like facial hair growth, vocal cord lengthening, and sperm production. Ovaries produce estrogens (primarily estradiol), which trigger breast development, wider hips, and egg maturation during puberty. All people produce both androgens and estrogens, just in different ratios.
Hormonal sex can diverge from chromosomal sex. In androgen insensitivity syndrome (AIS), the most common cause of XY sex development variations, a person’s body produces normal levels of testosterone but the cells can’t respond to it. Over 150 different genetic variants can cause AIS, each producing a different degree of insensitivity. Someone with complete AIS typically has XY chromosomes and internal testes but develops female external anatomy and is usually raised as a girl.
Phenotypic Sex
Phenotypic sex is what you can observe: the external genitalia, body shape, and secondary sex characteristics a person develops. Primary sex characteristics (genitalia and reproductive organs) form before birth. Secondary characteristics, like body hair patterns and breast development, emerge at puberty under hormonal influence. Phenotypic sex is what most people think of when they think of sex, but it’s the end result of a chain that starts with chromosomes, runs through genes and hormones, and can vary at any link.
Sex Chromosome Variations
The XX/XY model describes most people, but several other chromosome patterns occur naturally. These are estimated to affect roughly 1 in 1,500 to 1 in 2,000 live births, which translates to somewhere between 200,000 and 330,000 Americans.
Klinefelter syndrome (47,XXY) is among the most common, affecting about 1 in 650 male newborns. People with this pattern typically develop as male but may have reduced fertility, less facial and body hair, and sometimes breast tissue development. Many are never diagnosed because the effects can be subtle.
Turner syndrome (45,X) occurs when a person has only one X chromosome and no second sex chromosome. These individuals typically develop as female but are often shorter than average and don’t go through puberty without hormone therapy. Their ovaries usually don’t function fully.
Other patterns include 47,XYY and 47,XXX. People with XYY are typically taller than average males and usually fertile. People with XXX (triple X syndrome) are typically female, often slightly taller than average, and most have no noticeable symptoms.
Intersex Variations
Intersex is an umbrella term for people whose sex characteristics don’t fit neatly into typical male or female categories. This can involve chromosome patterns, hormone levels, reproductive anatomy, or some combination. Estimates of how many people have intersex traits vary depending on how broadly you define the category. Some researchers place the figure between 1.7% and 4% of the general population when including all measurable variations in sex characteristics, though narrower definitions that focus only on visibly ambiguous genitalia at birth produce much smaller numbers.
One well-known example is 5-alpha-reductase deficiency, a condition where XY individuals can’t convert testosterone into its more potent form. These children are often born with female-appearing genitalia and may be raised as girls, but at puberty, rising testosterone levels trigger masculinization, including muscle growth, voice deepening, and sometimes descent of internal testes. This condition is rare globally but occurs at higher rates in certain populations in the Dominican Republic and Papua New Guinea.
When a difference of sex development is suspected at birth, doctors typically use chromosome analysis (karyotyping), blood tests to measure hormone levels, and imaging studies to examine internal reproductive organs. The goal is to understand the child’s specific biology so that informed decisions can be made about their care.
Sex vs. Gender
Sex and gender overlap in everyday language, but they describe different things. The World Health Organization distinguishes biological sex, which “refers to the different biological and physiological characteristics of females, males and intersex persons, such as chromosomes, hormones and reproductive organs,” from gender, which is a social construct that “varies from society to society and can change over time.”
Gender identity is something different still. It refers to a person’s internal sense of their own gender, which may or may not match their biological sex or the sex they were assigned at birth. A person with typical male biology who identifies as a woman is transgender. A person whose gender identity matches their birth sex is cisgender. These categories describe identity and social experience rather than biology.
Understanding this distinction matters because the answer to “what are the different sexes” depends on which layer you’re asking about. At the gamete level, there are two. At the chromosomal level, there are several naturally occurring patterns. At the level of overall sex characteristics, most people fall clearly into male or female, but a meaningful number of people have bodies that combine traits from both or fully match neither. Biology, in other words, is mostly binary in outcome but not entirely binary in mechanism.