How Chromosomes Relate to Sex and Gender Identity

Chromosomes provide the initial genetic instructions that typically steer the body toward male or female development, but they are only the starting signal in a long chain of events that includes gene activation, hormone exposure, and brain development. A single gene on the Y chromosome usually triggers male development, and its absence usually triggers female development, yet exceptions to this pattern are well documented. Gender identity, meanwhile, appears to involve its own biology, influenced by prenatal hormones and possibly by genetic variants that act on hormone signaling pathways in the brain.

The Y Chromosome’s Master Switch

Most people carry either two X chromosomes (typically developing as female) or one X and one Y (typically developing as male). But the chromosomes themselves do not build sex organs. What matters most is a single gene on the Y chromosome called SRY. When SRY is active during early embryonic development, it switches on another gene called Sox9, and that triggers a cascade of signals that direct the developing gonads to become testes rather than ovaries.¹ Those testes then produce testosterone, which drives the rest of male physical development. Without SRY, the default pathway leads to ovarian development and female anatomy.

This means the Y chromosome’s role in sex determination is remarkably concentrated in one gene. The rest of the Y chromosome has shed most of its original genes over evolutionary time, a process of gradual decay that researchers have traced through comparisons across primate species and other organisms.² Some researchers once projected the Y might disappear entirely within a few million years, but more recent evidence suggests that strong natural selection and internal repair mechanisms have stabilized it after an initial period of rapid gene loss.³

Why Having Two X Chromosomes Does Not Mean Double the Dose

If females have two copies of the X chromosome and males have only one, you might expect females to produce twice as much of every protein coded by X-linked genes. That would cause serious problems. To prevent it, mammals evolved a mechanism called X-chromosome inactivation: early in embryonic development, one of the two X chromosomes in each female cell is largely silenced.⁴ The result is that both males and females end up with roughly one active X chromosome’s worth of gene output.

In mouse embryos, researchers have tracked this process cell by cell and found that as one X chromosome is progressively shut down in female cells, the remaining active X is upregulated to keep its output balanced with the two-copy autosomes.⁵ Male cells, with only one X, similarly upregulate it to match autosomal levels. The system is elegant, but it is not perfect. Some genes on the “silenced” X chromosome escape inactivation and remain active, which is one reason why having an unusual number of X chromosomes produces real physical effects.

When Chromosome Count Differs From the Usual Pattern

Not everyone has the standard XX or XY. Sex chromosome aneuploidies, meaning having more or fewer sex chromosomes than the typical two, are collectively common. They demonstrate how chromosomes influence the body through gene dosage rather than through a simple binary switch.

Turner syndrome occurs when a person has only one X chromosome (45,X) instead of the usual pair. The hallmark feature is short stature. Women with the full 45,X pattern in every cell are on average about 17 cm shorter than XX women, while women who are mosaic, meaning only some of their cells have lost the second X, are about 4 cm shorter on average, with heights ranging widely.⁶ Many Turner syndrome features trace to the loss of SHOX, a gene on the short arm of the X that normally escapes inactivation, so having one copy instead of two reduces its output.⁷

Klinefelter syndrome (47,XXY) affects males who carry an extra X chromosome. People with 47,XYY, an extra Y chromosome, tend to be taller than average and face increased risks of language difficulties, attention problems, and autism-spectrum traits, with psychiatric vulnerability rising during adolescence.⁸ Women with triple X (47,XXX) often go undiagnosed because the effects can be subtle, though they tend to have diminished ovarian reserve and earlier loss of ovarian function compared to XX women.⁹ Men with 47,XYY, by contrast, typically have normal to large testes and far less reproductive impairment than those with 47,XXY.

When Chromosomes and Anatomy Disagree

Perhaps the clearest demonstration that chromosomes alone do not dictate sex is the existence of people whose chromosomal sex and physical appearance diverge sharply. These conditions, grouped under the umbrella term differences of sex development (DSD), are estimated to affect roughly 37 per 100,000 live births across the combined DSD categories tracked in one Swiss registry.¹⁰

One striking example involves people with a 46,XX karyotype who develop as phenotypically male. This is rare, occurring in roughly 1 in 20,000 male births, and about 90% of the time it happens because the SRY gene has been accidentally transferred from the Y chromosome onto an X chromosome during the father’s sperm production.¹¹ These individuals have no Y chromosome, yet they develop testes and male anatomy because SRY is present and functional on one of their X chromosomes.

The reverse situation also exists. Complete androgen insensitivity syndrome (CAIS) occurs in people with a 46,XY karyotype whose cells cannot respond to testosterone because of mutations in the androgen receptor gene. Despite having a Y chromosome and functioning testes that produce testosterone, these individuals develop female external genitalia and are typically raised as girls.¹² A person with CAIS usually has no uterus, a shortened vagina, and undescended testes, but externally appears female.¹³ The underlying mutations vary widely. In one large Czech family, researchers identified a novel frameshift deletion in the androgen receptor gene that had been passed through multiple generations.¹⁴

Another condition, 5-alpha reductase deficiency, produces a different trajectory. People with a 46,XY karyotype who lack this enzyme are born with ambiguous or female-appearing genitalia because their bodies cannot convert testosterone into its more potent form. At puberty, however, a surge of testosterone can cause significant virilization, and many of these individuals come to identify and live as male.¹⁵ Clinical guidelines recommend deferring sex assignment until puberty when possible, so the individual can participate in the decision.¹⁶

Historically, clinical practice assumed gender identity was malleable and could be shaped by surgery and upbringing alone. That philosophy led to aggressive genital surgery on infants, often without disclosure or follow-up, and frequently resulted in lasting distress.¹⁷ The lessons learned from decades of poor outcomes in DSD care were among the first strong signals that gender identity has a biological basis that cannot simply be overridden by how a child is raised.

How Prenatal Hormones Shape the Brain

If chromosomes set the initial direction and hormones build the body, the brain sits at the intersection of both. The dominant framework for understanding how biology shapes sex-related brain development dates to 1959, when researchers demonstrated that testosterone exposure during a critical window of prenatal development permanently organized the brain’s later sexual behavior and responses. Hormones acting during this early period are called “organizational,” and hormones acting later in life to activate those pre-set patterns are called “activational.”¹⁸

The evidence for this framework is considered solid in non-human mammals and very strong in humans, though studying it directly in people is far more difficult. The preponderance of evidence indicates that the developing fetal nervous system responds to androgens in ways that permanently influence not only sexual behavior but also patterns of cognition, play preferences, and aspects of identity.¹⁹ This does not mean hormones dictate identity in a simple, deterministic way, but it does mean the prenatal hormonal environment matters far more than was appreciated when clinicians were assigning sex to DSD infants based on anatomy alone.

Gender Identity and DSD Conditions

The connection between chromosomes, hormones, and gender identity becomes especially visible in people with DSD. A large European study examined gender dysphoria and gender change across several DSD subgroups, including women with Turner syndrome, men with Klinefelter syndrome, women with XY DSD who had and had not been exposed to androgen effects, and women with congenital adrenal hyperplasia (46,XX individuals exposed to excess androgens before birth).²⁰ The patterns across groups suggested that prenatal androgen exposure influences gender identity independently of chromosomal sex or the sex of rearing, though the relationship is probabilistic rather than absolute. Most people with DSD identify with the gender they were raised as, but the rates of gender change are higher than in the general population, and they vary by condition in ways that track with androgen exposure history.

Searching for Genetic Roots of Gender Identity

For people without any DSD condition, the question of whether gender identity has a genetic component has been explored through twin studies, candidate gene analyses, and more recently through epigenetic research. Family and twin studies consistently point to a heritable component, though no single “gender identity gene” has been found.²¹

The most concrete genetic findings so far involve variants in genes related to sex hormone signaling. A study comparing about 330 transgender women to roughly 280 cisgender male controls found that transgender women were more likely to carry certain versions of genes involved in estrogen and androgen metabolism. Specifically, having a particular genotype in the SULT2A1 gene increased the odds of being transgender by about 1.6 times, and having a short-repeat version of the estrogen receptor alpha gene increased the odds by about 1.65 times.²² A later meta-analysis pooling data from several studies confirmed the overrepresentation of short estrogen receptor alpha alleles in transgender women compared to cisgender men.²³

These effect sizes are modest. They do not come close to predicting who will be transgender and who will not. What they do suggest is that variations in how the body processes sex hormones, particularly estrogen, may subtly shift the prenatal hormonal environment of the developing brain and contribute to the development of gender identity. Researchers in this area consistently note that larger genome-wide studies are needed to move beyond candidate genes and find the broader genetic architecture.

Beyond the DNA sequence itself, an epigenome-wide analysis found differences in DNA methylation patterns between transgender and cisgender individuals, concentrated in genes involved in brain development and gene expression regulation.²⁴ Epigenetic modifications do not change the DNA code but alter how actively genes are read, and they can be influenced by the hormonal environment. This finding is preliminary but fits with the broader picture: the path from chromosomes to gender identity runs through hormones and their effects on how genes are expressed in the brain.

What Brain Imaging Shows

Neuroimaging studies have tried to determine whether the brains of transgender individuals more closely resemble those of their identified gender or their sex assigned at birth. The answer, based on the largest study to date, is that the patterns are complicated and do not reduce to a simple “male brain in a female body” narrative. The ENIGMA Transgender Persons Working Group analyzed structural MRI data from over 800 people who had not received hormone therapy, including transgender men, transgender women, cisgender men, and cisgender women. They found that transgender individuals differed from cisgender individuals in brain volumes and surface area, but the specific patterns varied by brain region and by the direction of gender identity.²⁵

A separate study used machine-learning classifiers trained to distinguish male from female brains based on structural MRI. When these classifiers were applied to transgender women who had not yet taken hormones, they misclassified them at a much higher rate than they misclassified cisgender men. The classifier correctly identified about 93% of cisgender men but only about 56% of transgender women as male, suggesting their brain structure was already shifted away from a typical male pattern before any hormonal intervention.²⁶

Functional brain imaging adds another dimension. A study of resting-state brain activity found that transgender women showed distinct patterns of local activity and network connectivity compared to cisgender participants. In most measures, cisgender men fell between transgender women and cisgender women, suggesting that while transgender women’s brain function does differ from cisgender patterns, it aligns more closely with their sex assigned at birth than with their identified gender in these particular metrics.²⁷ The overall picture from neuroimaging is that transgender brains are not simply the “wrong sex” brain in the “wrong” body. They show a unique pattern, with some features shifted toward the identified gender, some toward the sex assigned at birth, and some that are distinct from either cisgender group.

How Hormone Therapy Changes Brain Structure

Once transgender individuals begin gender-affirming hormone therapy, their brains change measurably. A study tracking both transgender men (receiving testosterone) and transgender women (receiving estrogen and anti-androgens) found significant structural shifts after a median of about four and a half months. Transgender men showed increases in gray matter density in several brain regions, including the fusiform gyrus, cerebellum, and lingual gyrus. Transgender women showed decreases in gray matter density in overlapping but distinct regions, including the insula, cerebellum, and posterior cingulum.²⁸ These changes moved brain structure in the direction of the identified gender, which is consistent with the organizational-activational framework: hormones continue to shape brain tissue even in adulthood.

Mosaic Loss of the Y Chromosome in Aging

An unexpected development in sex chromosome research involves what happens to the Y chromosome as men age. Cells throughout the body can gradually lose their Y chromosome, a phenomenon called mosaic loss of Y (mLOY). Until recently this was considered a harmless byproduct of aging. It is now linked to an increased risk of cardiovascular disease, neurodegenerative conditions, and several types of cancer.²⁹ The Y chromosome clearly does more than just trigger male development in the embryo. Its ongoing presence in cells throughout life appears to matter for immune function and disease surveillance in ways researchers are still working to understand. This finding complicates the older view that the Y chromosome is a genetic backwater with little relevance beyond sex determination.

Sex Determination Across the Animal Kingdom

The XY system is not the only way nature handles sex determination. Birds, many reptiles, and some fish use a ZW system, where females are the heterogametic sex (carrying one Z and one W chromosome) and males carry two Z chromosomes.² Other species determine sex through environmental cues like temperature, and some can change sex during their lifetime. The XY system in mammals evolved from an ordinary pair of chromosomes, and the SRY gene that makes the human Y chromosome so important likely arose as a mutation on one member of that ancestral pair. The diversity of sex-determination systems across species underscores that chromosomal sex determination is one evolutionary solution among many, not a universal law of biology. Mammals are simply locked into the version that happens to involve a Y chromosome and its SRY gene.