Turner syndrome is caused by a missing or structurally altered X chromosome. Instead of the typical two sex chromosomes (XX), a person with Turner syndrome has only one fully functioning X chromosome. This affects roughly 3.2 out of every 10,000 female live births and is the most common sex chromosome abnormality in females.
The chromosome loss happens randomly during cell division, either before or shortly after conception. It is not inherited, not caused by anything a parent did, and not linked to maternal age.
How the Missing Chromosome Happens
Every egg and sperm cell is supposed to carry exactly one sex chromosome. During the process of dividing a parent cell’s chromosomes in half to make eggs or sperm, the two sex chromosomes occasionally fail to separate properly. This error, called nondisjunction, produces a reproductive cell with either two sex chromosomes or none at all. If a sperm or egg carrying no sex chromosome joins with a normal partner at conception, the resulting embryo ends up with just a single X chromosome (written as 45,X) instead of the usual pair.
About 75 to 80% of people with Turner syndrome retain their mother’s X chromosome, meaning the father’s sex chromosome was the one lost. This strongly suggests that paternal cell division errors, possibly involving an unstable Y chromosome that gets discarded during sperm formation, account for the majority of cases. This is one reason Turner syndrome doesn’t follow the pattern of other chromosomal conditions that become more common as a mother’s age increases. Parental age plays no known role.
Three Genetic Patterns Behind Turner Syndrome
Complete Monosomy X
The most common form. Every cell in the body has just one X chromosome and no second sex chromosome at all. This results from the nondisjunction error described above, happening before conception so that every cell descended from the original embryo carries the same 45,X pattern. People with complete monosomy X tend to have the most noticeable physical features and the highest rates of ovarian insufficiency, with about 88% never experiencing a first menstrual period without medical intervention.
Mosaic Turner Syndrome
In mosaic cases, the chromosome error happens after conception, during one of the early rounds of cell division as the embryo grows. This means some cells end up with the normal two sex chromosomes while others have only one X. The body becomes a patchwork of genetically different cell lines. The ratio of affected to unaffected cells varies widely from person to person, and so do symptoms. Someone whose body is mostly made up of normal cells may have very mild features, while someone with a higher proportion of 45,X cells may look more like the classic presentation.
Structural X Abnormalities
Sometimes both X chromosomes are present, but one is physically damaged. Several types of structural problems can cause Turner syndrome features:
- Isochromosome: One arm of the X chromosome gets duplicated while the other is lost, creating a lopsided chromosome. When the short arm is missing, short stature is the dominant feature. When the long arm is missing, ovarian problems are more likely.
- Ring chromosome: Both tips of the X chromosome break off and the remaining piece curls into a ring. The lost genetic material at the tips produces Turner syndrome features.
- Partial deletion: A segment of one X chromosome is simply missing. Which genes are lost determines which symptoms appear, and the severity can range from very mild to indistinguishable from complete monosomy.
Why a Missing X Chromosome Matters
You might wonder why losing one X chromosome causes problems when males function with only one X (plus a Y). The answer is that certain genes on the X chromosome are meant to be active on both copies, even in typical female cells where most of one X is turned off. When the second copy is missing, those genes operate at half their intended dose. The effects ripple through development in specific ways.
Short Stature and Bone Growth
The most universal feature of Turner syndrome is short stature, and it traces back to a single gene on the X chromosome called SHOX. This gene controls the growth and organization of cartilage cells in growth plates, the zones near the ends of bones where lengthening occurs. With only one working copy of SHOX instead of two, cartilage cells in the growth plate become disorganized. Instead of stacking in neat columns that drive bone lengthening, they arrange side by side. Growth plate fusion can also happen prematurely in certain bones. The result is shorter limbs and an average adult height around 4 feet 8 inches without treatment.
Ovarian Function and Fertility
Ovarian development begins normally in a fetus with Turner syndrome, but egg cells die off at a dramatically accelerated rate. By 20 weeks of gestation, roughly 70% of egg cells in a 45,X fetus are already undergoing programmed cell death, compared to just 3 to 5% in a typical female fetus of the same age. Three mechanisms drive this loss: chromosomal pairing fails during the early stages of egg cell development because there is no second X to pair with; tight junctions between egg cells and their surrounding support cells break down, cutting off critical communication; and the reduced dose of X chromosome genes directly weakens egg cell survival.
By the time most girls with Turner syndrome reach puberty, the ovaries have been largely replaced by streaks of fibrous tissue with few or no remaining eggs. Only about 10 to 15% of those with Turner syndrome experience their first period spontaneously. The rest need hormone therapy to initiate and maintain puberty. Some people with mosaic Turner syndrome or certain partial deletions retain enough ovarian function to menstruate and, in rare cases, conceive naturally.
Effects on the Heart and Kidneys
The missing genetic material doesn’t just affect height and fertility. It also disrupts the development of certain organs during fetal life, particularly the heart and kidneys.
About 19% of people with Turner syndrome have a bicuspid aortic valve, where the main valve leading out of the heart has two flaps instead of the usual three. Another 19% have a narrowing of the aorta called coarctation. Mitral valve problems affecting the valve between the heart’s left chambers occur in roughly 21% of cases. Many of these are mild and discovered only through screening, but some require monitoring or treatment over time.
Kidney differences are also common. Around 25% have a duplicated collecting system (extra drainage tubes inside the kidney), 25% have hydronephrosis (swelling from backed-up urine), and about 21% have a horseshoe kidney, where the two kidneys are fused at the bottom. Most kidney variations cause no symptoms but are important to identify early so any complications can be caught.
Why It’s Not Inherited or Preventable
Turner syndrome is a sporadic event. It is not passed down through families, and having one child with Turner syndrome does not meaningfully increase the chance of having another. There is no environmental exposure, medication, or lifestyle factor that causes it. The cell division error that leads to a missing or damaged X chromosome appears to be a matter of chance during the formation of sperm or eggs, or during the earliest divisions of the embryo itself.
Because the error most often originates in the father’s sperm rather than the mother’s egg, it sidesteps the well-known connection between maternal age and chromosomal conditions like Down syndrome. A 20-year-old mother and a 40-year-old mother face essentially the same odds.