Tiny DNA Edits Can Flip Gender
A small change in DNA can switch genders—female to male and male to female.
Posted April 15, 2026 | Reviewed by Lybi Ma
Gender is one of the most defining features of our identity . Most people think it is fixed. If you have two X chromosomes, you are female; if you have an X and a Y chromosome, you are male. This idea seems as certain as any fact in biology.
In a time when what defines gender is being questioned and discussed, a new study reveals that single changes in DNA can make dramatic differences. Add one letter of the genetic code in the right place, and an otherwise female XX embryo develops a male form. Remove a small stretch of code in that same spot, and a normally male XY embryo develops a female form. These shifts reveal how delicate the controls on gender development truly are and how a change can overturn the usual rules of chromosomes.
How Chromosomes Normally Determine Gender
Every embryo starts with the same early organs that can turn into either testes or ovaries. Gender begins with a genetic switch called the sex ‑determining region Y ( Sry ) found on the Y chromosome. Think of it as the “starter gene ” for building a male form.
In an embryo with XY chromosomes, the starter gene acts as a switch. It turns on a second gene called SRY-box transcription factor 9 ( Sox9 ), which is a “builder gene.” It is located on an entirely different chromosome. The builder gene then causes these early organs to develop into testes, which produce hormones that shape the rest of a male's body.
In embryos with two X chromosomes, the starter gene isn’t there because the Y chromosome is absent. Without its signal, the builder gene stays quiet, and the organs naturally develop into ovaries. Gender development, at its core, depends on whether the builder gene is activated. That builder gene doesn’t flip on by itself. It relies on a stretch of DNA known as an enhancer . It acts as a key on-off switch for the builder gene. When the enhancer is activated, the builder gene is turned on. When not activated, the builder gene remains off.
In male embryos with XY chromosomes, the starter signal reaches the enhancer. It activates the builder. This, in turn, spurs development toward the male form.
In female XX embryos, molecular “blockers” attach to the enhancer. These hold it in the off position. This allows the female pathway to continue.
One Letter Can Change Everything
The introduction of a single extra letter, one base pair, into the DNA sequence using precision gene editing can make a big difference. The one-letter insertion makes it easier for the starter proteins to bind. This small rearrangement allows the enhancer to remain switched on, even without the Y chromosome's starter signal.
The result is that otherwise female XX embryos develop male forms. These were complete with testes and male-pattern anatomy. Male XY embryos with the same single-letter addition developed normally, unaffected by the change. The opposite can also occur. If mutations weaken the enhancer’s ability to respond to starter signals, the builder gene doesn’t reach full activity. The organs then follow the female path even in the presence of an XY male chromosome set.
Gender development can flip in either direction with only a slight change to how these molecular switches interact. Adding just one extra DNA letter changes the balance, turning the switch on and causing the embryo to develop as male, regardless of the chromosomes.
What makes this story more than a curiosity is that humans appear to use the same hidden wiring. Humans have a nearly identical enhancer near the gene that controls testis formation. The human equivalent is called sex reversal enhancer-A or eSR-A. It sits in the same position relative to the human SOX9 gene.
Every so often, that switch behaves in a way that defies the textbook rules. In rare cases, people born with two X chromosomes develop male forms. The condition is known as 46,XX male development . It occurs in roughly one in 20,000 births. About 80 percent of these cases trace to a misplaced copy of the starter gene that ends up on an X chromosome. The remaining 20 percent lack the starter altogether, yet their bodies develop along a male pathway. In some of these cases, extra copies of the DNA region containing eSR‑A appear to push the builder gene past the critical threshold needed for testis development. This mirrors what the mouse experiment did with a single edited letter.
When Biology Breaks the Binary
At a time when laws, sports policies, and public debates often appeal to chromosomes as the ultimate arbiter of who counts as male or female, these findings show that biology itself does not follow such rigid rules. For anyone whose body or identity does not fit neatly into “XX equals female, XY equals male,” this research does not settle social arguments. Instead, it offers something powerful: evidence that variation and complexity are not exceptions to biology.
Gender development is not a simple yes-no determined by XX or XY. It is the outcome of a finely tuned network of switches, thresholds, and feedback loops scattered across the genome. The line between male form and female form can rest on the smallest possible change in the genome, one letter among billions, in a region that produces no protein and long seemed to serve no purpose at all.
Abberbock, E., Ridnik, M., Stévant, I., Weiss, R., Bamberger, C., Ziv Lhermann, S., ... & Gonen, N. (2026). A single-nucleotide enhancer mutation overrides chromosomal sex to drive XX male development. Nature Communications , 17 (1), 3186.
Share this post Facebook Bluesky Linkedin Email
There was a problem adding your email address. Please try again.
By submitting your information you agree to the Psychology Today Terms & Conditions and Privacy Policy
William A. Haseltine, Ph.D., is known for his pioneering work on cancer, HIV/AIDS, and genomics. He is Chair and President of the global health think tank Access Health International. His recent books include My Lifelong Fight Against Disease.
Get the help you need from a therapist near you–a FREE service from Psychology Today.
This article is part of the Bringwise Psychology Journal — daily insights on human behavior, mental health, and personal growth.