An XY female is a person who carries XY chromosomes, the combination typically associated with male development, but who develops female anatomy and usually grows up identifying as a girl or woman. This happens because the genetic signals that would ordinarily steer a developing embryo toward male anatomy are disrupted at some point along the way. The disruption can occur at different stages, from the initial gene that triggers testis formation to the hormones those testes would later produce, and the specific point of disruption determines what the person’s body looks like and what medical care they may eventually need. Though often described as extremely rare, XY females are more common than most people assume, and several distinct biological paths can lead to the same outcome.
How Sex Determination Normally Works
In most mammals, the presence of a Y chromosome sets off a chain reaction that pushes an initially undecided embryonic gonad to become a testis rather than an ovary. The key player is a gene called SRY, which sits on the short arm of the Y chromosome and acts as the master switch for male development. SRY encodes a protein that binds to DNA and bends it, triggering the activity of another gene called SOX9, which in turn launches a cascade of signals promoting testis formation, the production of testosterone, and the development of male internal and external anatomy.1PubMed. The role of SRY in mammalian sex determination When SRY is absent or not working, a parallel set of genetic pathways that promote ovarian development takes over instead.2PubMed. Sry and SoxE genes: How they participate in mammalian sex determination and gonadal development?
This is where things get interesting: the embryo does not passively “default” to female. Active genetic pathways involving genes like WNT4, RSPO1, and FOXL2 work to promote ovarian development and actively suppress testis formation.3PubMed. WNT4, RSPO1, and FOXL2 in sex development 4PubMed Central. Complementary pathways in mammalian female sex determination Imbalances anywhere in these competing networks can redirect development, which is why an XY individual can end up with female anatomy through several biologically distinct routes.
Swyer Syndrome
Swyer syndrome, also called pure 46,XY gonadal dysgenesis, is one of the most straightforward paths to being an XY female. In this condition, the SRY gene is mutated or otherwise non-functional, so the embryonic gonads never develop into testes. Instead, they become what clinicians call “streak gonads,” thin ribbons of tissue that produce neither testosterone nor estrogen in meaningful amounts. Without testosterone to masculinize the body, and without the anti-Müllerian hormone that testes normally secrete, the person develops female external genitalia along with a uterus and fallopian tubes, though these structures tend to be underdeveloped.5PubMed Central. Swyer syndrome in a woman with pure 46, XY gonadal dysgenesis and a hypoplastic uterus
Most mutations found in XY females with Swyer syndrome cluster in the HMG domain of the SRY gene, the part of the protein responsible for binding and bending DNA. Research on these mutant proteins has shown that both binding and bending are essential for SRY to do its job.6Development. Sry: the master switch in mammalian sex determination That said, SRY mutations only explain a fraction of Swyer syndrome cases. In many individuals, SRY itself is perfectly normal, and the disruption lies in one of the downstream genes in the sex-determination cascade.
People with Swyer syndrome typically come to medical attention during their teenage years when puberty does not begin on its own. Because their streak gonads produce almost no sex hormones, breast development, menstruation, and the typical growth spurt of adolescence do not occur without hormone replacement therapy.
Complete Androgen Insensitivity Syndrome
Complete androgen insensitivity syndrome, or CAIS, takes a very different route to the same general outcome. Here, the SRY gene works fine, testes form during fetal development, and those testes produce testosterone at typical male levels. The problem is that the body’s cells cannot respond to testosterone because the androgen receptor is non-functional due to mutations in the AR gene.7PubMed. Androgen insensitivity and the evolving genetic heterogeneity Without androgen signaling, the external genitalia develop along female lines, and the person grows up appearing unambiguously female.
The internal anatomy in CAIS differs from Swyer syndrome in a revealing way. Because the testes do form and do produce anti-Müllerian hormone, the uterus, fallopian tubes, and upper part of the vagina do not develop. A person with CAIS typically has a shorter vagina with a closed end, undescended testes located somewhere along the inguinal canal or in the abdomen, and absent or sparse pubic and underarm hair.8Archives of Medical Science. The challenges of androgen insensitivity syndrome Breast development does occur at puberty because the testes produce estrogen as well as testosterone, and without androgen opposition, the estrogen drives feminization. Many individuals with CAIS are not diagnosed until their teens, when they are evaluated for not menstruating despite otherwise normal female puberty.
A comparison of these two conditions highlights a useful principle: which internal and external structures develop depends on which hormones are produced and which ones the body can actually use. In Swyer syndrome, the absence of both testosterone and anti-Müllerian hormone allows Müllerian structures like the uterus to form. In CAIS, anti-Müllerian hormone is present and blocks those structures, but the absence of functional androgen signaling still results in female external anatomy.9PubMed Central. Pure 46, XY gonadal dysgenesis and 46, XY complete androgen insensitivity syndrome: A case report
Enzyme Deficiencies That Blur the Lines
Not every XY female has a clear-cut female appearance from birth. Some enzyme deficiencies produce external genitalia that look female at birth but masculinize at puberty, creating a more complex situation. Two of the better-studied examples involve 5-alpha reductase deficiency and 17β-hydroxysteroid dehydrogenase type 3 deficiency.
In 5-alpha reductase deficiency, the enzyme that converts testosterone into its more potent form, dihydrotestosterone, is missing or reduced. Dihydrotestosterone is responsible for masculinizing the external genitalia during fetal development, so without it, the newborn may appear female or have ambiguous anatomy. At puberty, however, the surge of testosterone itself can cause significant virilization, including voice deepening and muscle growth. In 17β-HSD3 deficiency, the enzyme responsible for the final step of testosterone production is impaired, resulting in low testosterone and high levels of its precursor. Again, the newborn often appears female, and the diagnosis may not be made until puberty triggers unexpected changes.10PubMed. 46,XY disorder of sex development (DSD) due to 17β-hydroxysteroid dehydrogenase type 3 deficiency
These enzyme deficiencies sit in a gray zone. The individuals may be raised as girls but experience masculinization later, which introduces difficult questions about identity, medical management, and whether early intervention is appropriate. They are sometimes categorized alongside Swyer syndrome and CAIS under the broader umbrella of 46,XY differences of sex development, but their clinical trajectory is quite different.
Chromosomal Mosaicism
Some XY females do not carry a straightforward 46,XY karyotype in every cell. In mixed gonadal dysgenesis, a person has a mosaic pattern, commonly 45,X/46,XY, meaning some cells have one X chromosome and no Y while others have a normal XY pair. The physical outcome depends on which cell lines dominate in which tissues, and the range of possibilities is enormous. About ninety percent of babies born with this mosaic pattern have male-appearing genitalia, roughly five percent have female-appearing genitalia, and the remaining five percent have genitalia that are neither fully male nor fully female.11PubMed Central. 45,X/46,XY mixed gonadal dysgenesis: a case report from Saudi Arabia The phenotypic presentation ranges widely and can include features associated with Turner syndrome, such as short stature, alongside the consequences of partial gonadal development.12PubMed Central. Mixed Gonadal Dysgenesis: A Comprehensive Review of Clinical Spectrum, Diagnostic Strategies, and Management Approaches
The significant phenotypic variability in mixed gonadal dysgenesis has contributed to ongoing confusion about the condition among specialists themselves.13PubMed. Mixed Gonadal Dysgenesis: A Narrative Literature Review and Clinical Primer for the Urologist Because the mixture of cell lines differs from person to person and from tissue to tissue, no two cases look exactly the same, making management decisions highly individualized.
Gonadal Tumor Risk
One of the most urgent clinical concerns for XY females is the risk of gonadal tumors. Dysgenetic gonads, particularly streak gonads that contain Y-chromosome material, can give rise to gonadoblastoma and related germ cell tumors. The risk is not trivial. A study of nearly 300 phenotypic females with Y-chromosome-containing differences of sex development found an overall germ cell tumor risk of about fifteen percent, with Swyer syndrome carrying the highest risk at roughly twenty-three percent.14PubMed. Gonadal tumour risk in 292 phenotypic female patients with disorders of sex development containing Y chromosome or Y-derived sequence The risk in CAIS was the next highest, while the risk in mixed gonadal dysgenesis and certain enzyme deficiencies was lower, under ten percent.
In familial cases of Swyer syndrome, the tumor risk appears to climb even higher, with one literature review suggesting rates around two-thirds in familial versus sporadic cases.15PubMed Central. A Risk of Gonadoblastoma in Familial Swyer Syndrome-A Case Report and Literature Review Because of this elevated risk, preventive removal of streak gonads is generally recommended in Swyer syndrome, often shortly after diagnosis. The decision is more nuanced in CAIS, where the testes contribute to pubertal feminization and some individuals and their families prefer to delay gonadectomy until after puberty.
Hormone Therapy and Bone Health
Whether the gonads are removed surgically or were never functional to begin with, XY females generally need hormone replacement therapy. In Swyer syndrome, estrogen and progesterone are prescribed to initiate and maintain the development of secondary sex characteristics like breast growth, to protect bone density, and to support uterine health.16PubMed. Swyer syndrome: presentation and outcomes In CAIS after gonadectomy, estrogen replacement becomes necessary for the same bone-protective reasons, since the testes that had been providing it are now gone.
Without adequate hormone replacement, XY females are at substantial risk of osteoporosis. This is not a distant or theoretical concern; it begins early in life when the skeleton would normally be building its peak bone mass. For individuals who identify as female, standard estrogen-based regimens are used. For those whose gender identity differs, therapy is adjusted accordingly. A case report described a non-binary individual with Swyer syndrome receiving gender-affirming hormone therapy tailored to their identity while still addressing bone health.17PubMed Central. Gender Affirming Hormone Therapy in a Non-Binary Individual with Swyer Syndrome
Can XY Females Have Children?
This is one of the questions people find most surprising, and the answer depends heavily on the specific condition. In CAIS, pregnancy is not possible because there is no uterus. In Swyer syndrome, however, the uterus is present, and although it is typically small and underdeveloped, estrogen therapy can improve its size and shape. With donor eggs and in vitro fertilization, pregnancies have been achieved and carried to term.
Multiple case reports document successful pregnancies in women with Swyer syndrome, including twin pregnancies delivered by cesarean section at or near full term.18PubMed Central. Rare successful pregnancy in a patient with Swyer Syndrome 19PubMed Central. A Successful New Case of Twin Pregnancy in a Patient with Swyer Syndrome-An Up-to-Date Review on the Incidence and Outcome of Twin/Multiple Gestations in the Pure 46,XY Gonadal Dysgenesis These pregnancies require careful monitoring and a tailored fertility program, but they demonstrate that having XY chromosomes does not categorically prevent a person from carrying and delivering a baby. The key requirement is a functional uterus, which Swyer syndrome provides but CAIS does not.
Gender Identity and Psychological Well-Being
One of the most persistent misconceptions about XY females is that their chromosomes must create some kind of internal conflict about gender. The evidence tells a different story. A study comparing 22 women with CAIS to matched controls found no statistically significant differences in quality of life, self-esteem, gender identity, sexual orientation, or gender role behavior. The women with CAIS were psychologically indistinguishable from other women on every measure the researchers assessed.20PubMed. Psychological outcomes and gender-related development in complete androgen insensitivity syndrome The researchers noted that these findings argue against the idea that two X chromosomes or functional ovaries are needed for typically feminine psychological development.
That said, gender identity is not rigidly determined by any single factor. Rare cases of male gender identity have been reported in individuals with CAIS, including one case that led to full sex reassignment with androgen treatment and phalloplasty. The researchers who documented this case emphasized that it did not change the general recommendation for female sex assignment in CAIS, but it did challenge assumptions about the role of androgen receptors in shaping gender identity.21PubMed. Male gender identity in complete androgen insensitivity syndrome Within a single family carrying the same CAIS mutation, one member developed male gender identity while others did not, illustrating that even identical genetic conditions can produce different identity outcomes.22PubMed. Complete androgen insensitivity syndrome associated with male gender identity or female precocious puberty in the same family
How the Diagnosis Is Made
Many XY females are not diagnosed until adolescence, when the absence of menstruation prompts investigation. The initial workup typically involves hormone levels, pelvic imaging, and a karyotype to check the chromosomes. When the karyotype comes back 46,XY in a person who appears female, genetic testing becomes critical for pinpointing the exact cause.
Whole-exome sequencing has emerged as the most effective genetic tool for reaching a molecular diagnosis, outperforming more targeted gene panels.23PubMed Central. Worldwide cohort study of 46, XY differences/disorders of sex development genetic diagnoses: geographic and ethnic differences in variants Even with advanced sequencing, however, reaching a definitive genetic diagnosis is far from guaranteed. One study of over 200 individuals with 46,XY differences of sex development found a genetic cause in about thirty percent of those tested by modern sequencing methods.24The Journal of Clinical Endocrinology & Metabolism. Contribution of Clinical and Genetic Approaches for Diagnosing 209 Index Cases With 46,XY Differences of Sex Development That means the majority of XY females still do not have a fully explained molecular cause for their condition, which gives a sense of how much remains to be understood about the genetics of sex determination.
Terminology and Why It Matters
The language used to describe XY females has shifted considerably. Older terms like “intersex,” “hermaphrodite,” and “pseudohermaphrodite” have largely been replaced in clinical settings by “differences of sex development” (DSD), a change recommended by a 2006 consensus statement that recognized the older terms as unhelpful and often stigmatizing.25PubMed. Disorders of sex development: a new definition and classification Under this system, a person with Swyer syndrome would be described as having “46,XY DSD” rather than “male pseudohermaphroditism.”
Not everyone is comfortable with the DSD terminology either. Some advocacy groups prefer “variations in sex characteristics” or simply “intersex” as an identity term. The tension between clinical precision and personal identity has not been resolved, and different communities use different language. What has been broadly accepted is that terms implying deception or abnormality serve no one well.
XY Females in Elite Sports
Few topics involving XY females have generated as much public controversy as eligibility rules in women’s athletics. World Athletics and other governing bodies have introduced regulations that restrict female athletes with certain differences of sex development from competing in the women’s category, primarily based on testosterone levels. The assumption underlying these rules is that higher testosterone confers an unfair advantage.
This assumption has been challenged on scientific grounds. One analysis pointed out that in many cases of 46,XY DSD found among elite female athletes, androgens are either non-functional (as in CAIS) or absent entirely, meaning testosterone cannot be responsible for their athletic success. The researchers argued that genes for stature and possibly other traits on the Y chromosome, rather than circulating testosterone, may explain the overrepresentation of 46,XY DSD among elite female athletes.26PubMed. Natural selection for genetic variants in sport: the role of Y chromosome genes in elite female athletes with 46,XY DSD A separate paper argued that even if an advantage does exist, the regulations requiring testosterone suppression are not justified, partly because they rely on contradictory definitions of sex and partly because the required medical interventions carry their own harms.27PubMed Central. World Athletics regulations unfairly affect female athletes with differences in sex development
The debate remains unresolved and deeply contentious. For XY females caught in it, the consequences are intensely personal: public scrutiny of their bodies, pressure to undergo medical treatments they would not otherwise choose, and the existential question of whether they are being told they are not “really” women. The science does not support a simple answer, which makes it an uncomfortable fit for the binary categories that sports governance demands.
XY Females in Other Mammal Species
Humans are not the only mammals that produce fertile XY females. Several rodent species have evolved sex-determination systems that deviate from the standard XX/XY pattern. The African pygmy mouse, for example, naturally produces three types of females: standard XX females, females carrying a modified X chromosome paired with a Y, and others. The XY females in this species have typical female anatomy and are fertile, making them a valuable research model for understanding how sex reversal can occur without pathology.28PubMed Central. Unusual Mammalian Sex Determination Systems: A Cabinet of Curiosities
Research on the African pygmy mouse has revealed that XY females show some behavioral traits that are shifted in a masculine direction compared to XX females, despite having the same female reproductive anatomy. This suggests that certain genes on the sex chromosomes influence behavior independently of which gonads develop.29Scientific Reports. Masculinised Behaviour of XY Females in a Mammal with Naturally Occurring Sex Reversal Some mammal species have gone even further, losing the Y chromosome entirely while maintaining functional sex determination through other genetic mechanisms. These natural experiments illustrate that the XY system, for all its importance in human biology, is not the only way mammals can solve the problem of producing two sexes.
Epigenetic Regulation and the Timing of SRY
Beyond mutations in SRY itself, the timing and level of SRY expression matter enormously. If SRY is expressed too late or at too low a level during embryonic development, male sex determination can fail even when the gene sequence is perfectly normal. Epigenetic mechanisms, which regulate when and how strongly genes are turned on without changing the DNA sequence itself, play a role in ensuring SRY fires at the right moment. Processes like DNA methylation, modifications to the proteins that DNA wraps around, and non-coding RNA regulation all contribute to the precise control of SRY’s activation. Disruptions to any of these epigenetic layers are a potential cause of sex development differences that genetic sequencing alone would miss, which may help explain why so many cases of 46,XY DSD remain without a molecular diagnosis even after whole-exome sequencing.