Sex determination in mammals comes down to a single gene on the Y chromosome called SRY, short for “sex-determining region Y.” When a developing embryo inherits a Y chromosome from its father, SRY activates during the early weeks of fetal life and redirects what would otherwise become an ovary into becoming a testis. That triggering event sets off a hormonal cascade that shapes the rest of male anatomical development. But the story has considerably more texture than “Y equals male,” from the active genetic program required for female development to species that have lost their Y chromosome entirely and still produce males.
How SRY Triggers Testis Formation
Every human embryo starts with a pair of undifferentiated gonads, structures called genital ridges that have the potential to become either ovaries or testes. Around the sixth week of development, SRY switches on in XY embryos and acts as a transcription factor, a protein that turns other genes on or off. Its immediate job is twofold: it activates genes involved in forming testes while simultaneously repressing genes that would steer the gonad toward an ovarian fate.
1PubMed Central. Switching on sex: transcriptional regulation of the testis-determining gene SrySRY’s window of activity is surprisingly brief. It does its work and then hands off to another gene called SOX9, which takes over the job of maintaining testis development long-term. SRY binds to genes involved in ovarian differentiation and suppresses them, while simultaneously promoting the formation of Sertoli cells, the structural cells of the testis that organize into cord-like structures. SOX9 then sustains and expands that program, activating its own set of downstream targets beyond what SRY initiated.
2PubMed. The sex-determining factors SRY and SOX9 regulate similar target genes and promote testis cord formation during testicular differentiationOnce the testes form, they produce two hormones that drive the rest of male anatomical development. Testosterone and its more potent derivative promote the growth of male external anatomy, while a second hormone called anti-Müllerian hormone causes the ducts that would otherwise develop into a uterus and fallopian tubes to regress. The entire process depends on a tightly choreographed sequence: SRY activates, SOX9 takes over, testes form, hormones flow. A disruption at any step can send development down a different path.
3PubMed. Sex determination and gonadal development in mammalsFemale Development Is Not a Default
For decades, the standard framing was that female development happened passively whenever SRY was absent. The ovary was treated as the “default” outcome. That view has been thoroughly overturned. Ovarian development requires its own active genetic program, centered on a signaling pathway involving genes like WNT4 and RSPO1. WNT4 actively suppresses male sexual differentiation, promotes the development of the Müllerian ducts (which become the uterus and fallopian tubes), and helps maintain the health of oocytes, the cells that become eggs.
4PubMed. WNT4, RSPO1, and FOXL2 in sex developmentRSPO1 sits upstream of WNT4 in this cascade and activates a signaling pathway called β-catenin signaling. When researchers knocked out RSPO1 in mice with XX chromosomes, those mice developed masculinized gonads, complete with male-type blood vessel patterns and steroid production, because the female-specific activation of WNT4 failed to occur. In other words, without the active push toward ovarian development, even XX gonads can drift toward a testis-like state.
5Human Molecular Genetics. Activation of β-catenin signaling by Rspo1 controls differentiation of the mammalian ovarySex determination is better understood as two competing genetic programs, male and female, that actively suppress each other. In XY embryos, SRY tips the balance toward the testis program. In XX embryos, RSPO1 and WNT4 tip the balance toward the ovary program. Neither outcome is a default; both require active gene regulation to proceed.
The Y Chromosome’s Unusual Architecture
The human Y chromosome is small, carrying far fewer genes than the X. But its structure is more complex than its size suggests. At each tip, the X and Y chromosomes share short stretches of DNA called pseudoautosomal regions, known as PAR1 and PAR2. These regions are essentially identical on both chromosomes and pair up during the cell division that produces sperm and eggs. Genes within these regions are not inherited in a sex-linked pattern because they can freely swap between X and Y.
6PubMed Central. The Human Pseudoautosomal Region (PAR): Origin, Function and FutureRecombination in these pseudoautosomal regions behaves differently in males and females. In males, crossover activity throughout PAR1 is intense, with recombination rates far above the genome-wide average across most of the region.
7PubMed Central. Recombination in the Human Pseudoautosomal Region PAR1 In females, recombination in both PAR1 and PAR2 is much lower, nearly negligible in PAR2.
8PLOS Genetics. Evolutionary dynamics of the human pseudoautosomal regions This obligatory recombination in males is critical: it allows the X and Y chromosomes to physically pair and separate properly during sperm production. When that pairing fails, it can lead to chromosomal errors passed on to offspring.
Between the pseudoautosomal regions lies the male-specific region, which makes up the vast majority of the Y chromosome and does not recombine with the X. This isolation has consequences for how the Y evolves and maintains itself over time.
How the Y Chromosome Avoids Falling Apart
The Y chromosome evolved from a regular chromosome and has lost most of its original genes over hundreds of millions of years. This process of massive gene decay happened because the male-specific region cannot swap DNA with a partner chromosome during cell division, meaning harmful mutations that accumulate there have no easy way to be repaired.
9PubMed Central. Y-chromosome evolution: emerging insights into processes of Y-chromosome degenerationSo how has the Y chromosome survived at all? A big part of the answer is palindromes, stretches of DNA that read the same forward and backward, like mirror images. About a quarter of the male-specific region consists of eight large palindromes. These structures allow a form of self-repair: the two arms of each palindrome can swap sequences with each other through a process called gene conversion, effectively copying a good version of a gene over a damaged one.
10PubMed Central. New insights into the evolution of human Y chromosome palindromes through mutation and gene conversionThis self-correction mechanism has been a major factor in keeping essential male-fertility genes functional. It helps purge harmful mutations and may even speed up the spread of beneficial ones. Researchers have proposed that the palindromic organization also helps maintain unusual chromatin structures that optimize this repair process.
11PubMed. Y chromosome palindromes and gene conversion Without palindromes, the Y chromosome’s gene loss would likely have been far more severe, and the chromosome itself might not have persisted in its current form.
Balancing the Books on X Chromosome Dosage
Because females have two X chromosomes and males have only one, mammals face a dosage problem. Two active copies of every X-linked gene in female cells would produce twice the protein that male cells make. The solution is X-chromosome inactivation: early in development, each female cell randomly shuts down one of its two X chromosomes. The silenced X gets physically compacted and largely stays quiet for the life of that cell and all its descendants.
The process is orchestrated by a long non-coding RNA molecule called Xist. Produced from the X chromosome destined for silence, Xist physically coats the chromosome and recruits the molecular machinery that locks it down.
12PubMed Central. The Role of Xist in X-Chromosome Dosage Compensation Xist plays roles in both choosing which X to inactivate and in spreading and establishing the silencing.
13PubMed. Xist RNA and the mechanism of X chromosome inactivationInactivation is not total, though. Somewhere between 15 and 23 percent of genes on the silenced X escape inactivation and continue to be expressed. This creates a mosaic: some female cells express certain X-linked genes from both chromosomes, while others do not. That cellular mosaicism has been linked to the striking female predominance in autoimmune diseases, though the exact causal chain is still being worked out.
14PubMed Central. Escape from X Chromosome Inactivation and the Female Predominance in Autoimmune DiseasesWhen the Pathway Does Not Follow the Textbook
The XY system usually produces males and the XX system usually produces females, but exceptions reveal how the underlying machinery actually works. One of the most instructive is 46,XX testicular disorder of sex development, in which a person has two X chromosomes but develops testes and a male body. In most of these cases, the SRY gene has broken off the Y chromosome during sperm production and landed on one of the X chromosomes. That transplanted SRY gene is enough to trigger testis formation, confirming that SRY itself, not the rest of the Y, is the critical factor.
15PubMed Central. Clinical, molecular and cytogenetic analysis of 46, XX testicular disorder of sex development with SRY-positiveThese individuals do face consequences from missing the rest of the Y chromosome, however. The Y carries regions essential for sperm production called AZF (azoospermia factor) regions. Without them, individuals with translocated SRY are typically infertile, producing no sperm at all.
16PubMed Central. Sex-determining Region of Y-gene Translocation and 46,XX Testicular Disorders of Sex Development: Cytogenetic and Molecular Insights into Male InfertilityThe mirror-image condition also exists: people with a Y chromosome and an intact SRY gene who develop a female body. In complete androgen insensitivity syndrome, the androgen receptor gene on the X chromosome carries a mutation that prevents cells from responding to testosterone. The testes form and produce testosterone normally, but the body’s tissues cannot detect it. As a result, the external anatomy develops along female lines. The uterus, cervix, and upper vagina are absent because the testes still produce anti-Müllerian hormone, which causes those structures to regress as they would in typical male development.
17The Lancet. Androgen insensitivity syndromeLosing the Y Chromosome With Age
The Y chromosome is not only small and gene-poor; it can also be lost from cells during a person’s lifetime. A condition called mosaic loss of Y (mLOY) involves a growing fraction of a person’s cells, particularly blood cells, shedding their Y chromosome as they age. Large cohort studies have identified mLOY as strongly age-dependent and a significant risk factor for all-cause mortality and worse outcomes in age-related diseases.
18PubMed Central. A complex systems approach to mosaic loss of the Y chromosomeThe mechanism behind this increased disease risk is still under investigation, but it underscores that the Y chromosome has functions in the body beyond sex determination. Genes on the Y that are expressed in immune cells and other somatic tissues appear to contribute to normal physiological maintenance throughout life. Losing them may impair immune surveillance or other protective functions, though researchers are still teasing apart which specific genes matter most.
Sex Determination in Other Species
The XY system is far from universal, even among vertebrates. Birds use a ZW system in which males carry two Z chromosomes and females carry one Z and one W. Rather than a single master switch gene like SRY, avian sex determination appears to depend on the dosage of a Z-linked gene called DMRT1. Males, with two copies, express more of it during embryonic development, and this higher expression drives testis formation.
19PubMed. DMRT1 is upregulated in the gonads during female-to-male sex reversal in ZW chicken embryosSome reptiles bypass genetic sex determination entirely. In species with temperature-dependent sex determination, the temperature at which eggs incubate during a critical developmental window determines whether the offspring become male or female. In the Australian central bearded dragon, chromosomal sex determination exists but can be overridden by high temperatures, producing sex-reversed females from chromosomally male embryos. Researchers have linked this reversal to changes in how certain chromatin-modifying genes process their RNA, which alters the molecular landscape enough to flip the sex-determination outcome.
20PubMed Central. Differential intron retention in Jumonji chromatin modifier genes is implicated in reptile temperature-dependent sex determinationEven within mammals, the XY system is not sacrosanct. The Amami spiny rat, a rodent found only on a single Japanese island, has completely lost its Y chromosome and SRY gene. Both males and females carry a single X chromosome with no Y present. Researchers discovered that males carry a duplicated enhancer near the SOX9 gene, the same gene that SRY normally activates in other mammals. This duplication boosts SOX9 expression specifically in males and drives testis formation without any need for SRY or the Y chromosome at all.
21PubMed Central. Turnover of mammal sex chromosomes in the Sry-deficient Amami spiny rat is due to male-specific upregulation of Sox9The spiny rat provides a living example of something evolutionary biologists have long theorized: that a new sex-determining gene can arise and replace SRY, leading to a complete turnover of the sex chromosome system. It also reinforces the idea that SOX9, not SRY, is the fundamental driver of testis development. SRY is essentially a switch that flips SOX9 on, but evolution can find other ways to flip that same switch.
The First Complete Y Chromosome Sequence
For decades, the Y chromosome was the most poorly assembled chromosome in the human genome reference. Its repetitive sequences and palindromic structures made it extremely difficult for standard sequencing technologies to read accurately. In 2023, the Telomere-to-Telomere (T2T) consortium published the first truly complete sequence of a human Y chromosome: 62,460,029 base pairs from one end to the other. The effort corrected multiple errors in the previous reference assembly and added over 30 million base pairs of previously unresolved sequence. It also revealed 41 additional protein-coding genes, mostly from the TSPY gene family, and resolved the full structure of the heterochromatic region at the end of the long arm, a stretch of highly repetitive DNA that had resisted all prior sequencing efforts.
22PubMed Central. The complete sequence of a human Y chromosomeHaving a complete reference changes what researchers can study. Genes buried in repetitive regions that were previously invisible to analysis can now be examined for their roles in fertility, development, and disease. The Y chromosome’s full architecture, including the precise layout of its palindromes and gene family clusters, is now available for comparative studies across human populations and across species, opening questions that were literally unresolvable before.
Sex Chromosome Aneuploidies
Errors during cell division can produce individuals with extra or missing sex chromosomes. One of the more common results is Klinefelter syndrome, in which a person carries 47 chromosomes with an XXY configuration. These individuals typically develop male anatomy because SRY on their Y chromosome still triggers testis formation, but the extra X chromosome often leads to reduced testosterone production, taller stature, and infertility.
How the extra chromosome arises varies. When the error occurs in the mother, it usually happens during the first round of cell division that produces eggs and frequently involves X chromosomes that failed to swap DNA properly or that swapped DNA in the wrong locations. Advancing maternal age worsens the risk because the protein structures holding chromosomes together deteriorate over time. When the error occurs in the father, it typically results from a failure of the obligatory recombination in the pseudoautosomal region that is needed for the X and Y to pair and separate correctly during sperm production.
23PubMed. Meiotic Origins of Non-Mosaic Klinefelter Syndrome (47, XXY): Mechanisms, Dimorphism, and Emerging Genetic SusceptibilityOther sex chromosome aneuploidies include 47,XXX (triple X syndrome) and 45,X (Turner syndrome, in which only one X is present and no Y). Turner syndrome is the only viable human monosomy, reflecting how essential at least one X chromosome is for survival. Research into the origins of these conditions has identified multiple distinct types of cell-division errors, some involving a complete absence of recombination, others involving recombination in abnormal locations, and still others where recombination appeared normal but the chromosomes failed to separate anyway.
24Human Molecular Genetics. The origin of 47, XXY and 47, XXX aneuploidy: heterogeneous mechanisms and role of aberrant recombinationThe variety of mechanisms behind these errors complicates any simple story about why sex chromosome aneuploidies happen. It is not one process going wrong in one way; it is several processes, each with its own failure mode, all converging on a similar outcome of an extra or missing chromosome. That diversity is part of why these conditions occur at relatively high rates compared to other chromosomal abnormalities, and why preventing them has proven so difficult.