Most females carry two X chromosomes, but a small number of people who are phenotypically female, meaning they look and develop as female on the outside, carry one X and one Y chromosome instead. This happens through several distinct biological pathways, from single-gene mutations to whole-chromosome mosaicism to rare chimeric fusions. The simplest version of the textbook rule (“XX = female, XY = male”) is a useful shorthand, but biology treats it more like a default setting that can be overridden under specific circumstances.
Why XY Usually Means Male Development
Sex determination in mammals hinges on a single gene called SRY, which sits on the Y chromosome. When SRY is present and functional, it triggers a cascade that turns the early, undifferentiated gonad into a testis. The testis then produces hormones that drive the rest of male development: external genitalia, internal reproductive structures, and the masculinizing changes of puberty. Without a working SRY signal, the default developmental path leads to female anatomy, including ovaries, a uterus, and female external genitalia.1PubMed Central. Generation and mutational analysis of a transgenic mouse model of human SRY
Researchers have confirmed this role through gene-editing experiments. When the SRY gene was knocked out in pigs using CRISPR, genetically male (XY) animals developed with complete female external and internal genitalia, though the organs were smaller than in control females.2PubMed Central. Knockout of the HMG domain of the porcine SRY gene causes sex reversal in gene-edited pigs The takeaway: the Y chromosome does not force male development on its own. What matters is whether SRY works and whether the body responds to the hormones that follow. When either of those steps breaks down, a person with XY chromosomes develops as female.
Swyer Syndrome
Swyer syndrome, sometimes called 46,XY complete gonadal dysgenesis, is one of the clearest examples of an XY female. A person with Swyer syndrome has a 46,XY karyotype but carries a mutation in the SRY gene (or, less commonly, in other genes in the sex-determination pathway) that prevents the gonads from developing into testes. Instead, the gonads remain as undeveloped “streak” tissue that produces neither testosterone nor significant amounts of estrogen.3Europe PMC. The Mysteries of Primary Amenorrhea: Swyer Syndrome
Because there is no testosterone signal during fetal life, the external genitalia develop along the female pathway. A uterus and fallopian tubes form normally. The person is typically raised as a girl and has no reason to suspect anything unusual until puberty fails to arrive on its own. The most common reason a diagnosis is eventually made is primary amenorrhea: a teenage girl who never gets her first period.4PubMed Central. 46, XY Complete Gonadal Dysgenesis (Swyer Syndrome) Presenting as Primary Amenorrhea in a Normomorphic Adult Female From Kakamega, Kenya A karyotype test then reveals the XY chromosomes, and further genetic testing often identifies the specific SRY mutation.5PubMed Central. Primary amenorrhea in a 46,XY adolescent girl with partial gonadal dysgenesis: identification of a new SRY gene mutation
With hormone replacement therapy, people with Swyer syndrome go through puberty, develop breasts, and have menstrual cycles. Because they have a functional uterus, pregnancy through donor eggs and IVF is possible, though rare. Fewer than 13 live births have been reported in the medical literature, and at least one successful twin pregnancy has been documented.6Elsevier. Rare successful pregnancy in a patient with Swyer Syndrome7Europe PMC. A Successful New Case of Twin Pregnancy in a Patient with Swyer Syndrome The XY genotype does not prevent the uterus from functioning normally during pregnancy, which reinforces how little the Y chromosome itself contributes to female reproductive anatomy once the SRY signal has failed.
Complete Androgen Insensitivity Syndrome
If Swyer syndrome is a case of the male signal never being sent, complete androgen insensitivity syndrome (CAIS) is a case of the signal being sent but never received. In CAIS, the SRY gene works fine and the gonads do develop into testes, which produce testosterone. But the body’s androgen receptors are non-functional due to a mutation in the androgen receptor (AR) gene, so cells cannot respond to testosterone or its derivatives.8Frontiers in Pediatrics / PubMed Central. Exon 1 deletion of the androgen receptor gene causing complete androgen insensitivity syndrome in a newborn: a case report
The result is a person with XY chromosomes, internal testes (usually undescended), and a completely female external appearance. Because a separate hormone pathway causes the uterus and fallopian tubes to regress in the presence of functioning testes, people with CAIS typically lack those structures. Externally, though, they look unambiguously female. Breast development happens at puberty because the testes produce some estrogen and because testosterone that cannot bind to receptors gets partially converted to estrogen. A case report described a 16-year-old girl with CAIS who presented with primary amenorrhea and had a normal female external appearance; her XY karyotype was discovered only during the diagnostic workup.9Semantic Scholar. Complete androgen insensitivity syndrome in a Saudi adolescent Girl: A case report
CAIS differs from Swyer syndrome in several practical ways. Because the testes are present and somewhat functional, gonad management is handled differently. And because there is no uterus, pregnancy is not an option, unlike in Swyer syndrome. But both conditions produce people who are, by every outward measure, female despite carrying XY chromosomes.
Other Conditions That Produce XY Females
Swyer syndrome and CAIS are the most well-known routes to an XY female phenotype, but they are not the only ones. Several other conditions land in the same territory, each through a different mechanism.
- 5-alpha reductase deficiency: This enzyme converts testosterone into a more potent form called dihydrotestosterone, which drives the development of male external genitalia. When the enzyme is absent or impaired, a person with XY chromosomes may be born with genitalia that appear female or ambiguous and may be raised as a girl.10PubMed Central. Gender identity in patients with 5-alpha reductase deficiency raised as females At puberty, rising testosterone levels can cause virilization, sometimes prompting a reassessment of the diagnosis.
- Mixed gonadal dysgenesis: This involves a mosaic karyotype, most commonly 45,X/46,XY, meaning some cells have only one X chromosome (like Turner syndrome) and others have both X and Y. The physical presentation varies widely, from fully female to ambiguous to fully male.11Cureus. Catamenial Hematuria in Mixed Gonadal Dysgenesis (45,X/46,XY): A Case Report of a Rare Presentation
The thread connecting all these conditions is that having a Y chromosome is necessary but not sufficient for male development. Every step downstream of the Y chromosome, from SRY activation to hormone production to hormone reception to enzyme conversion, is a point where the process can be interrupted, resulting in female or partially female development.
Chimerism and Naturally Occurring XX/XY Individuals
All the conditions above involve a single genetic identity where something in the sex-determination pathway went differently than expected. Chimerism is something else entirely: a person who is literally made of cells from two different genetic individuals, each with its own DNA. When two early embryos fuse into one (or exchange cells through a shared placenta), the resulting person can carry both XX and XY cell lines in different tissues.
A review of documented cases found at least 50 people with confirmed 46,XX/46,XY chimerism. Of those, only 28 had ambiguous or mixed gonadal tissue. The remaining 22 had typical male or female development, with 11 showing entirely normal sexual anatomy: seven appeared male and four appeared female.12Elsevier / ScienceDirect (or PubMed Central). Natural human chimeras: A review Most chimeras are never detected unless they happen to undergo genetic testing for an unrelated reason, like a blood group mismatch or fertility workup.
One striking case involved a 30-year-old woman with normal ovarian function and a prior pregnancy who turned out to have a 46,XY karyotype in her blood cells. When researchers tested cells from her ovaries, muscles, and skin, those were 46,XX. She was a chimera, and her history revealed the likely explanation: she had a twin brother who died shortly after birth. Fetal cell exchange during their shared time in the womb had given her a population of XY cells in her blood, while her reproductive tissues were entirely XX.13Oxford Academic. Chimerism in a fertile woman with 46,XY karyotype and female phenotype: Case report Her fertility was completely normal.
Microchimerism and Y Chromosome Traces in Women
There is an even subtler way that XY cells end up in a female body. During pregnancy, small numbers of fetal cells cross the placenta and take up residence in the mother’s tissues, a phenomenon called microchimerism. When a woman carries a male fetus, some of those transferred cells are XY.
Studies have found that male-origin microchimerism is common in the female liver and is correlated with having previously carried a male pregnancy.14Hepatology. Male cell microchimerism in normal and diseased female livers from fetal life to adulthood A more recent analysis found that women who had given birth to a male fetus had about 60% higher odds of testing positive for male-origin microchimerism compared with women who had never carried a male pregnancy.15Elsevier. Factors associated with the acquisition and retention of male-origin microchimerism in women
These are tiny populations of cells, not enough to change a person’s sex or produce detectable hormonal effects. But they do mean that many women technically harbor some cells with a Y chromosome. What these cells do, if anything, remains an open question. Researchers have found them in organs ranging from the liver to the brain, and some studies have explored whether they play a role in tissue repair or, conversely, in autoimmune disease. The evidence so far is inconclusive in both directions.
Gonadal Health Risks in XY Females
For people with conditions like Swyer syndrome, the streak gonads that replace functional ovaries or testes pose a real medical risk. Dysgenic gonadal tissue that contains Y chromosome material has an elevated risk of developing certain tumors, particularly gonadoblastoma and germinoma. A study of pediatric and adolescent phenotypic females with XY chromosomes found that those with complete gonadal dysgenesis had a significantly increased tumor risk, and prophylactic removal of the streak gonads is standard practice once the diagnosis is confirmed.16Frontiers in Pediatrics. Gonadal tumor risk in pediatric and adolescent phenotypic females with disorders of sex development and Y chromosomal constitution with different genetic etiologies17Europe PMC. A Risk of Gonadoblastoma in Familial Swyer Syndrome
The risk is not uniform across all XY conditions. People with CAIS, for instance, have functioning testes rather than streak gonads, and their tumor risk is lower. The timing of gonadectomy in CAIS is more debated: some clinicians recommend leaving the testes in place through puberty to allow natural estrogen production, then removing them afterward with hormone replacement to follow. For people with mixed gonadal dysgenesis and Turner syndrome mosaicism involving Y material, the risk falls somewhere in between. The management strategy depends on the specific diagnosis, the type of gonadal tissue present, and individual patient factors.
Identity and Psychosocial Dimensions
Finding out that your chromosomes do not match your lifelong gender identity is, unsurprisingly, psychologically complex. Many people with 46,XY DSD learn their diagnosis during adolescence or young adulthood, often after seeking medical help for absent periods. Research into gender development in these individuals has historically focused on hormones and genetics, but more recent work emphasizes the role of socialization, family dynamics, and the quality of medical care in shaping outcomes.18Europe PMC. Gender Development in 46,XY DSD: Influences of Chromosomes, Hormones, and Interactions with Parents and Healthcare Professionals
The vast majority of people with Swyer syndrome or CAIS who were raised as girls identify as women. Their lived experience, social identity, and self-concept are female. The chromosomal finding is medically relevant but does not, for most, override a lifetime of female identity. Studies on quality of life in related conditions, like 45,X/46,XY mosaicism, have found that clinical scores do not always align with self-reported well-being, highlighting that individual temperament and social support matter as much as the medical details.19CrossRef. Sex attribution, gender identity and quality of life in disorders of sex development due to 45,X/46,XY mosaicism
Historically, the medical approach to children born with ambiguous or atypical sex anatomy leaned heavily on early surgical intervention, sometimes before the child could participate in the decision. This approach developed in an era when physician authority was less questioned than it is now, and the concept of informed consent had not yet been fully extended to these cases.20Oxford Academic (Medical Law Review). Did Bioethics Matter? A History of Autonomy, Consent, and Intersex Genital Surgery The current trend, especially in multidisciplinary DSD clinics, favors delaying irreversible procedures when medically safe to do so and involving the patient in decisions as they mature.
XY Females in Sports
The existence of XY females has been thrust into public debate through eligibility controversies in women’s athletics. Some female athletes with 46,XY DSD have been flagged by sex verification testing, leading to questions about whether their physiology gives them an unfair advantage. The assumption behind most eligibility regulations has been that elevated testosterone is the key factor.
That assumption has been challenged. A detailed analysis of elite female athletes with 46,XY DSD argued that Y chromosome genes affecting stature, rather than testosterone, were responsible for the increased frequency of these athletes at the elite level. In many of the identified cases, androgens were either non-functional (as in CAIS) or entirely absent, making testosterone an implausible explanation for their athletic success.21SpringerLink. Natural selection for genetic variants in sport: the role of Y chromosome genes in elite female athletes with 46,XY DSD The researchers argued that eligibility rules based on testosterone levels are not scientifically supported and risk subjecting athletes to invasive and unnecessary medical procedures. This remains one of the more contentious intersections of biology, policy, and individual rights.
XY Females in Other Species
Humans are not the only mammals where XY individuals develop as females, and in some species it happens routinely rather than as an anomaly. The wood lemming and the collared lemming both have naturally occurring XY females alongside XX females and XY males. In these species, a variant X chromosome carries a genetic element that suppresses the male-determining effect of SRY, producing fertile females with XY chromosomes as a normal part of the population.22Nature. Fertile XX- and XY-type females in the wood lemming Myopus schisticolor23PubMed Central. Aberrant chromosomal sex-determining mechanisms in mammals, with special reference to species with XY females
Beyond mammals, the entire framework shifts. Birds use a ZW/ZZ system in which the female is the one with two different sex chromosomes (ZW) and the male has two of the same (ZZ), essentially the mirror image of the mammalian pattern.24Karger Publishers. Avian sex determination: what, when and where? Some reptiles skip chromosomal sex determination altogether and let egg incubation temperature decide. The X/Y system humans use is just one solution among many that evolution has come up with.
The Slow Erosion of the Y Chromosome
The X and Y chromosomes started out as a matching pair hundreds of millions of years ago. Over time, the Y has lost the vast majority of its original genes, a process researchers describe as progressive degeneration. The X chromosome still carries over a thousand genes. The Y has been whittled down to a few dozen that serve male-specific functions, primarily sex determination and sperm production.25PubMed Central. Mammalian Y chromosome evolution and the male-specific functions of Y chromosome-borne genes26Europe PMC. Y-chromosome evolution: emerging insights into processes of Y-chromosome degeneration
This decay happens because the Y does not recombine with a partner chromosome the way other chromosome pairs do during reproduction. Without recombination, harmful mutations accumulate and beneficial ones cannot easily spread, a kind of evolutionary trap. Some researchers have speculated that the Y chromosome is headed for extinction entirely, though others argue it has stabilized around its current minimal gene set. Either way, the Y’s ongoing erosion underscores that it is already a stripped-down specialist rather than a full partner to the X.27PubMed. Sex chromosome specialization and degeneration in mammals If the Y does eventually disappear in some mammalian lineage, as it already has in certain rodent species, sex determination will have to be taken over by a different gene on a different chromosome. The lemmings that already have XY females offer a preview of how flexible the system is.