How Is Gender Determined in Humans? | Chromosomes & the SRY

The SRY gene on the Y chromosome acts as the master switch that directs an embryo toward male development.

When you hear the question “How is gender determined in humans?” most people picture a simple XX or XY formula. You probably learned in school that two X chromosomes make a girl, and an X plus a Y makes a boy. That tidy picture is mostly correct for the vast majority of people — but the process happening inside the embryo is far more intricate.

This article walks through the biological steps that determine sex, what the SRY gene actually does, and why some people’s sex chromosomes don’t match the typical XX or XY pattern. Along the way you’ll see why “sex” and “gender” are related but distinct ideas, and what differences in sex development (DSD) look like.

How Chromosomes Start the Process

Every human cell (except eggs and sperm) carries 23 pairs of chromosomes. The 23rd pair is the sex chromosomes. In the most common arrangement, females have two X chromosomes and males have one X and one Y. But the story really begins at conception.

The mother always contributes an X chromosome through her egg. The father contributes either an X or a Y through his sperm, meaning his cell is the one that determines whether the embryo will develop as genetically female (XX) or male (XY). This is the first fork in the road.

At very early stages — around the first six weeks — both XX and XY embryos have identical gonadal tissue that could become either ovaries or testes. The difference only emerges once specific genes turn on.

Why the XX vs XY Idea Isn’t the Whole Story

Most people assume that if you have a Y chromosome, you’ll develop as male, and if you don’t, you’ll develop as female. That’s true for the vast majority of individuals, but biological sex determination involves multiple criteria — chromosomal, hormonal, and physical — that can interact in surprising ways.

For example, some individuals with XY chromosomes have complete androgen insensitivity syndrome (CAIS), meaning their cells don’t respond to male hormones. Despite having a Y chromosome and testes, a person with CAIS develops female-typical external anatomy. The opposite also occurs: some XX individuals carry a small piece of the Y chromosome containing the SRY gene and develop as male.

Key parts of the process include:

• Sex chromosomes: The X and Y chromosomes carry vastly different gene loads. The X is large and packed with hundreds of genes; the Y is small with only a few dozen, and just one — SRY — is the critical trigger for testis development.
• Meiosis: During sperm formation, meiosis randomly separates the father’s XY pair, so roughly half his sperm carry X and half carry Y. This is why the odds of conceiving a boy or girl are about equal.
• SRY gene location: The SRY gene sits on the short arm of the Y chromosome. It codes for a transcription factor that physically binds to DNA and bends it, activating the testis-forming pathway.
• Downstream targets: SRY turns on the Sox9 gene in the developing gonad, which then drives Sertoli cell differentiation and testis formation. Without SRY, Sox9 stays off and ovarian development proceeds.
• Default pathway: In the absence of a Y chromosome and the SRY signal, the undifferentiated gonadal tissue follows the default route toward ovaries and female reproductive structures.

So the XX vs XY rule holds for the majority, but biological sex is not quite as binary as that simple pairing suggests.

The SRY Gene: The Master Switch Inside the Y Chromosome

The SRY gene — full name sex-determining region Y — is the genetic linchpin for male development. It is located on the Y chromosome and produces a protein that acts as a transcription factor, meaning it binds to specific regions of DNA to turn other genes on or off.

When SRY protein binds to DNA, it bends the molecule dramatically. That physical change helps recruit other proteins to the area and switches on the testis-determining pathway, particularly the expression of Sox9. As Genome.gov puts it, the Y chromosome carries the SRY gene as the key genetic signal for testis development; without it, the developmental pathway defaults to ovaries.

Once the testes form, they begin producing testosterone and anti-Müllerian hormone (AMH). Testosterone directs the development of male internal and external structures, while AMH causes the female reproductive tract precursor to regress. In female development, the lack of a Y chromosome means no SRY, no testis formation, and no AMH, so the Müllerian ducts develop into the uterus, fallopian tubes, and upper vagina.

Timing matters: the SRY gene is expressed for only a narrow window around 6–8 weeks of gestation. If its signal is weak, delayed, or absent, the testes may not develop properly, leading to a range of differences in sex development.

These patterns show that the SRY gene is the trigger, but hormones and receptor function also shape the final anatomy. This is why biological sex is best understood as a cascade of genetic, hormonal, and physical events rather than a single chromosome check.

Biological Sex vs. Gender: Why the Words Matter

A lot of confusion around the question “How is gender determined in humans?” comes from mixing up the terms sex and gender. In biology, sex refers to the chromosomal, gonadal, and anatomical characteristics used to classify individuals as male or female. Gender, on the other hand, is a broader social, psychological, and cultural concept that includes identity, expression, and roles.

Biological sex is typically assigned at birth based on visible genitalia. That assignment matches chromosomal sex for about 99.8% of people, but the remaining 0.2% or so have a difference in sex development (DSD), where chromosomes, hormones, or anatomy don’t line up in the typical way. The NHS lists DSD as a group of rare conditions where a person’s sexual development is atypical.

Gender identity — a person’s internal sense of being male, female, both, or neither — is not determined by chromosomes or genes alone. It is a separate aspect of human development shaped by a mix of biological, psychological, and social factors. This article focuses on the biological mechanisms of sex determination; the science of gender identity involves different lines of research.

When you’re talking about how babies are conceived as male or female, you’re asking about biological sex determination. When you ask about how people identify, you’re asking about gender, which follows a different set of rules.

1. Chromosomal sex: The XX or XY pairing established at fertilization that usually correlates with the type of gonad that develops.
2. Gonadal sex: Whether the embryo develops ovaries or testes, driven by the presence or absence of SRY.
3. Hormonal sex: The type and balance of sex hormones (testosterone, estrogen, AMH) that shape internal and external structures.
4. Anatomical sex: The physical appearance of reproductive organs at birth, which is what doctors use to assign sex on a birth certificate.
5. Gender identity: A person’s deeply held sense of their own gender, which may or may not align with their biological sex assigned at birth.

Each of these layers can sometimes diverge from the others, which is why the simple XX/XY model only covers the most common scenario, not the full spectrum.

What Happens When Sex Determination Doesn’t Follow the Expected Path

Differences in sex development (DSD) occur when the cascade of genetic or hormonal signals deviates from the typical sequence. Some of these are well understood, while others are rarer and less characterized. The SRY gene discovery about 25 years ago helped explain many cases.

According to overview of sex determination, meiosis is the process that determines which sex chromosome the father’s sperm carries. Occasionally, errors in meiosis or in the SRY gene itself can lead to atypical development. For example, an individual with XY chromosomes but a nonfunctional SRY gene may develop as female, while an XX individual with SRY translocated to another chromosome may develop as male.

Other non-SRY genes also play a role. The Sox9 gene, activated downstream of SRY, is required for testis formation — mutations in Sox9 can cause male-to-female sex reversal even with a functioning SRY. Mutations in the androgen receptor gene cause complete androgen insensitivity syndrome (CAIS), where XY individuals develop as female externally despite having testes and SRY.

These conditions are rare, but they illustrate that biological sex is a developmental outcome, not a single yes/no toggle.

The Bottom Line

Biological sex in humans is largely decided by whether the father’s sperm contributes an X or a Y chromosome, but the actual outcome depends on the SRY gene on the Y serving as a master switch for testis development. The process involves a cascade of genetic and hormonal steps that, for the vast majority, produces a male or female body. For a small but significant number of people, differences in these steps lead to variations in sex development that don’t fit the binary box.

If you have questions about your own child’s sexual development or a potential DSD, a pediatric endocrinologist or a genetic counselor can run the appropriate workup — your child’s specific chromosomes, hormone levels, and anatomy will guide the conversation far better than any general rule.