Turner syndrome is caused by a missing or structurally altered X chromosome. Instead of having two complete sex chromosomes, individuals with Turner syndrome have one intact X chromosome and a second that is either entirely absent, partially deleted, or rearranged. This chromosomal difference occurs randomly and is not inherited from either parent. It affects roughly 1 in every 3,200 female live births.
The Missing X Chromosome
Every cell in your body typically carries 46 chromosomes, including two sex chromosomes. Females usually have two X chromosomes (one from each parent), while males have an X and a Y. In Turner syndrome, the second sex chromosome is either completely gone or damaged in some way. The result is a set of physical features and health considerations that vary widely from person to person, depending on which cells are affected and how much genetic material is missing.
About 45% of people diagnosed with Turner syndrome after birth have what’s called complete monosomy X: every cell in the body contains only a single X chromosome instead of two. The remaining cases involve either mosaicism (a mix of cell types) or structural changes to the second X chromosome.
Where the Error Happens
The chromosomal change behind Turner syndrome occurs during the formation of a parent’s egg or sperm, or very early in embryonic development. When egg or sperm cells divide, chromosomes are supposed to separate evenly so each resulting cell gets one copy. Sometimes this separation fails, producing a reproductive cell with a missing sex chromosome. If that cell goes on to form an embryo, every cell in the baby’s body will carry the error.
In about 75 to 80% of cases, the single X chromosome a person with Turner syndrome retains comes from the mother. That means the paternal sex chromosome, whether it was an X or a Y, is the one most often lost. Research points to instability in the Y chromosome during sperm cell division as a major driver. Importantly, Turner syndrome is not linked to advanced maternal age, which distinguishes it from other chromosomal conditions like Down syndrome. It also doesn’t run in families. Parents who have one child with Turner syndrome are not at increased risk of having another.
Mosaicism: A Mixed Picture
Not everyone with Turner syndrome is missing an X chromosome in every cell. In mosaic Turner syndrome, the error occurs after fertilization, during the early rounds of cell division as the embryo grows. One cell line ends up with the typical two X chromosomes, while another line loses one. The body becomes a patchwork of cells with different chromosome counts.
The ratio of affected to unaffected cells, and which tissues they end up in, plays a large role in how the condition presents. Someone with a high proportion of cells carrying two X chromosomes may have very mild features or go undiagnosed until puberty or even adulthood. Someone with mostly single-X cells will typically show more of the classic signs, including shorter stature and ovarian differences.
Structural X Chromosome Changes
In some cases, a second X chromosome is present but incomplete or rearranged. Several types of structural changes can cause Turner syndrome:
- Isochromosome: The most common structural abnormality. Instead of having one short arm and one long arm, the chromosome forms with two copies of the same arm. This effectively deletes all the genes on the missing arm.
- Ring chromosome: Both tips of the X chromosome break off and the remaining piece fuses into a ring shape, losing genetic material from both ends. This occurs in roughly 6% of Turner syndrome cases, usually alongside a mosaic pattern.
- Partial deletions: A segment of the short or long arm of the X chromosome is simply missing, removing specific genes that contribute to normal development.
The specific genes lost in these rearrangements help explain why symptoms vary so much. A small deletion on the short arm of the X chromosome may primarily affect height, while a larger deletion or complete absence of the chromosome affects a broader range of systems.
Why a Missing X Affects the Body
You might wonder why losing one X chromosome matters, given that in typical females, one X chromosome in each cell is largely silenced anyway (a process called X-inactivation). The answer lies in the 20 to 30% of genes on the X chromosome that escape this silencing and remain active on both copies. When one X is missing or damaged, those genes lose their second working copy, and the resulting drop in gene activity drives the features of Turner syndrome.
One of the best-understood examples involves a gene critical for bone growth. This gene is normally active on both X chromosomes and plays a key role in limb and skeletal development, particularly around the elbows, knees, wrists, and forearms. With only one working copy, growth plate cells in bones don’t develop normally. Short stature, the single most consistent feature of Turner syndrome (present in at least 95% of cases), is a direct consequence. On average, adult height is reduced by about 20 centimeters, or roughly 8 inches, compared to the general female population.
Other genes that escape X-inactivation are involved in lymphatic development, heart formation, and ovarian function. Their reduced activity helps explain why Turner syndrome can involve puffy hands and feet at birth (from lymphatic fluid buildup), certain heart defects, and ovaries that don’t develop or function typically. The ovaries often form initially but lose their egg cells much faster than usual, leading to early loss of ovarian function and, in most cases, the need for hormone support to go through puberty.
What Doesn’t Cause It
Because Turner syndrome results from a random chromosomal event, there is nothing either parent did or didn’t do to cause it. It’s not related to diet, lifestyle, environmental exposures, medications during pregnancy, or anything within a parent’s control. It’s also not a condition that’s passed down through generations. The vast majority of cases are completely sporadic, meaning they appear without any family history of chromosomal conditions.
The randomness of the event also means the risk doesn’t increase with the number of pregnancies or with age. This sets Turner syndrome apart from several other chromosomal conditions where parental age is a known factor.