Biological sex isn’t a single trait. It’s a bundle that includes chromosomes, gonads, hormones, and internal and external anatomy, and these usually point the same way.1National Academies Press. Measuring Sex, Gender Identity, and Sexual Orientation In most people the layers line up as clearly male or clearly female, which is why sex usually seems simple. But each layer is built by the one before it, so sex works more like a chain of steps than a single fact fixed at conception. That chain explains why some people’s traits don’t all match, and why medical bodies now describe sex as multidimensional.2American Society for Reproductive Medicine. Just the Facts: Biological Sex
How Chromosomes Start the Process
For the first weeks of development, every embryo has the same unformed gonads, organs that can still become either testes or ovaries. Most people have either two X chromosomes (XX) or one X and one Y (XY). The X or Y alone doesn’t build anatomy, though. Specific genes on them start a chain of events.
The key gene sits on the Y chromosome and is called SRY. It starts male development.3PubMed. Genetic evidence equating SRY and the testis-determining factor People with XY chromosomes but a broken SRY gene develop as female, which shows the gene is required to make testes.4PubMed. Genetic evidence equating SRY and the testis-determining factor SRY is switched on only briefly; in mice it works for about two days of early development.5PubMed. SRY: A transcriptional activator of mammalian testis determination That short burst turns supporting cells in the gonad into the cells that build testes. Without it, those same cells become ovary cells.6PubMed. SRY: A transcriptional activator of mammalian testis determination
SRY then turns on a second gene, SOX9, which drives the male pathway and shuts down female genes.7PubMed Central. A Boolean network model of human gonadal sex determination Female development is an active program too. Without SRY, a different set of genes, including WNT4 and FOXL2, takes over and blocks SOX9.8PubMed Central. A Boolean network model of human gonadal sex determination The early gonad sits balanced between two opposing signals, and SRY tips the scale.9PLoS Biology. Fgf9 and Wnt4 Act as Antagonistic Signals to Regulate Mammalian Sex Determination That is why “female is the default” is a misleading shortcut: ovaries need their own active genes, not just the absence of a Y.
The XX and XY labels are shorthand for the usual pairs. Other combinations exist, such as a single X or XXY, and the SRY gene can sit on an X or be missing, so the labels don’t guarantee anatomy.
Because SRY must act in a narrow window, a delay or weak activation can leave the gonad only partly pushed toward testes. The result can be gonads with mixed tissue, or a gonad that develops as an ovary despite an XY pair.
How Hormones Build the Rest of the Anatomy
Early embryos have two pairs of internal tubes. One pair can become male internal organs, and the other can become the uterus, fallopian tubes and upper vagina.10PubMed. Embryology and endocrinology of genital development Once testes form, they release anti-Müllerian hormone, which makes the female set of tubes shrink away.11PubMed. Development of the genital ducts and external genitalia in the early human embryo They also release testosterone, which turns the other set into the tubes that carry and store sperm.12PubMed. Development of the genital ducts and external genitalia in the early human embryo If anti-Müllerian hormone is absent, the uterus, fallopian tubes and upper third of the vagina form.13PubMed. Development of the genital ducts and external genitalia in the early human embryo
Timing matters here. The female duct system has to be removed during a limited early period, so the testes must make the hormone at the right time. If it’s missing, or the tissue can’t respond, a person with testes can be born with a uterus and fallopian tubes as well as male ducts.
External genitals need a stronger hormone. In certain tissues the body converts testosterone into a more potent form, DHT. DHT is needed to build male external genitals and the prostate.14PubMed. Role of 5 alpha-reductase in health and disease People with XY chromosomes who can’t make DHT have testes and male internal organs but can be born with female-looking or ambiguous genitals.15PubMed. Role of 5 alpha-reductase in health and disease DHT later drives male-pattern facial and body hair and male-pattern baldness.16PubMed. Role of 5 alpha-reductase in health and disease Inside and outside can differ because internal ducts respond to anti-Müllerian hormone and testosterone, while external genitals respond mainly to DHT.
The female pattern doesn’t need ovaries to drive it. Female ducts and external anatomy form without ovarian hormones; they appear when the male signals are absent. Ovaries matter later, producing the estrogen at puberty that builds breasts and begins periods.
Hormones keep shaping the body after birth. In the first months of life, babies have a short hormone surge sometimes called mini-puberty.17PubMed Central. Up-To-Date Review About Minipuberty and Overview on Hypothalamic-Pituitary-Gonadal Axis Activation in Fetal and Neonatal Life At puberty, a brain signal reawakens the hormone system, triggering the rise in sex hormones that builds adult traits.18PubMed. Hypothalamo-Pituitary axis and puberty
Why Biological Sex Is Called Multidimensional
The US National Institutes of Health calls biological sex a multidimensional construct based on anatomy, physiology, genetics and hormones.19American Society for Reproductive Medicine. Just the Facts: Biological Sex The National Academies list the traits as external genitals, secondary sex traits like breasts and beard, gonads, chromosomes and hormones.20National Academies Press. Measuring Sex, Gender Identity, and Sexual Orientation Intersex describes people whose sex traits don’t all correspond to the same sex.21National Academies Press. Measuring Sex, Gender Identity, and Sexual Orientation
Listing each trait separately lets a doctor see which layers agree and which don’t, instead of squeezing everything into one label. For most patients the checklist comes back consistent and nothing more is needed. In unusual cases the same list guides which tests to run and which conditions to consider.
Each familiar label gets something right. Binary language captures that most people fall clearly into two groups, but it has no place for people whose traits point different ways. A single spectrum fixes that by adding in-between points, yet it still implies one slider. Some people are typically female in some traits, typically male in others and in between in still others.22The Conversation. Biological sex is neither binary nor a spectrum – a biologist explains how it’s multidimensional They can’t be placed at one point on a single scale, because each trait varies on its own.23The Conversation. Biological sex is neither binary nor a spectrum – a biologist explains how it’s multidimensional
When the Layers Don’t All Match
Some differences start with the chromosomes. Turner syndrome means having one X and no second sex chromosome, and can bring short stature, heart defects and infertility.24PubMed Central. Integrated functional genomic analyses of Klinefelter and Turner syndromes reveal global network effects of altered X chromosome dosage Klinefelter syndrome means a male with an extra X, most often XXY, and can bring tall stature, small testes and low testosterone.25PubMed Central. Integrated functional genomic analyses of Klinefelter and Turner syndromes reveal global network effects of altered X chromosome dosage
Other differences come from a body that can’t respond to testosterone. In complete androgen insensitivity syndrome, a person has XY chromosomes and testes, but the body’s receptor for testosterone and DHT doesn’t work.26PubMed Central. Molecular pathogenesis, diagnosis, and management challenges in complete androgen insensitivity syndrome They look typically female and develop breasts at puberty, but they don’t menstruate, are infertile and have little body hair.27PubMed Central. Molecular pathogenesis, diagnosis, and management challenges in complete androgen insensitivity syndrome Undescended testes in this condition carry a higher risk of tumors, so ongoing medical care matters.28PubMed Central. Molecular pathogenesis, diagnosis, and management challenges in complete androgen insensitivity syndrome
A third group involves the adrenal glands. Congenital adrenal hyperplasia is an inherited group of conditions in which the adrenal glands can’t make certain hormones properly.29PubMed. Congenital adrenal hyperplasia Hormones build up along other routes, and the most common type comes from a missing enzyme.30PubMed. Congenital adrenal hyperplasia Excess male-type hormones can then shape the genitals of XX babies.
These differences come to light at different ages. Some are noticed at birth, when a newborn’s genitals don’t look clearly typical and doctors begin testing. Others surface at puberty, for instance when expected periods never start. Some are found only in adulthood during infertility testing, and a few turn up incidentally during other medical tests. A karyotype, the chromosome test result from a blood sample, shows the sex chromosomes and often explains the rest. These variations are grouped under the terms intersex or differences of sex development (DSD).
Such conditions have been recognized for thousands of years.31PubMed. DSD/intersex: historical context and current perspectives But in the 1960s and 70s, doctors often kept the diagnosis secret and performed early surgery to make genitals look typical.32PubMed. DSD/intersex: historical context and current perspectives Many people later faced stigma and quality-of-life problems, and approaches have since changed to avoid this.33PubMed. DSD/intersex: historical context and current perspectives
Is Biological Sex Fixed for Life
The outward body is mostly set, but underneath, sex is actively maintained. Anatomy laid down before birth, such as the internal organs and genitals, doesn’t rearrange itself in adult life. Beneath that stable structure, gonad cells keep running gene programs for as long as they live.
Testes and ovaries keep their identity through ongoing gene activity, not a one-time decision.34PubMed Central. Sexual cell fate reprogramming in the ovary by DMRT1 In adult mice, switching off one gene, DMRT1, made testis cells turn into ovary-type cells that produce estrogen.35PubMed Central. DMRT1 prevents female reprogramming in the postnatal mammalian testis Switching DMRT1 on in adult mouse ovaries turned ovary cells into testis-like cells.36PubMed Central. Sex determination and maintenance: the role of DMRT1 and FOXL2 These are lab experiments in mice, not something that happens to people in daily life, and they say little about whole bodies.
Women’s two X chromosomes aren’t fully balanced either. In each cell one X is mostly switched off, so both sexes get a similar dose of X genes.37PubMed Central. Genes that escape from X inactivation But about 15% of X genes escape this switch-off in women and stay active on both copies.38PubMed Central. Genes that escape from X inactivation Those genes are more active in women and may explain some sex differences in health and disease.39PubMed Central. X Inactivation and Escape: Epigenetic and Structural Features
The Y chromosome can change too. As men age, some of their blood cells can lose it.40PubMed Central. Losing maleness: Somatic Y chromosome loss at every stage of a man’s life This loss has been linked to higher risk of Alzheimer’s, cancer and earlier death.41PubMed Central. Losing maleness: Somatic Y chromosome loss at every stage of a man’s life
How Other Animals Decide Sex
Mammals, birds, snakes and most lizards and amphibians use genes or chromosomes to set sex.42PubMed. Environmental versus genetic sex determination: a possible factor in dinosaur extinction? Birds use a reversed system in which females, not males, carry two different sex chromosomes, called Z and W.43Wikipedia. ZW sex-determination system Males carry two matching Z chromosomes, so it is the mother’s egg that determines the chick’s sex. This is a separate system that evolved independently, not a mirror image of the XY pair. The same ZW system appears in butterflies, moths and Komodo dragons.44Wikipedia. ZW sex-determination system The SRY gene exists only in mammals, but the competing male and female signals may be shared across backboned animals.45PLoS Biology. Fgf9 and Wnt4 Act as Antagonistic Signals to Regulate Mammalian Sex Determination
Other species let temperature decide. All crocodilians studied, many turtles and some lizards let nest temperature set hatchlings’ sex.46PubMed. Environmental versus genetic sex determination: a possible factor in dinosaur extinction? In these species, the temperature during a middle part of incubation determines which gonad forms, and eggs in the same clutch can become different sexes depending on how warm their spot in the nest is. The embryo’s chromosomes may play little or no role, yet the same gonad types still form. A lasting temperature change could skew the ratio of males to females.47PubMed. Environmental versus genetic sex determination: a possible factor in dinosaur extinction?
The mix of cues can shift. In one fish, populations at different latitudes adjust how strongly temperature versus genes decides sex.48PubMed. Adaptive variation in environmental and genetic sex determination in a fish Over evolution, switches from temperature control to genetic control happen far more often than the reverse.49PubMed Central. Transitions in sex determination and sex chromosomes across vertebrate species