Turner syndrome occurs when part or all of an X chromosome is missing from most or all of the cells in a girl’s body. A girl normally receives one X chromosome from each parent. The error that leads to the missing chromosome appears to happen during the formation of the egg or sperm.
Most commonly, a girl with Turner syndrome has only one X chromosome. Occasionally, she may have a partial second X chromosome. Because she is missing part or all of a chromosome, certain genes are missing. The loss of these genes leads to the symptoms of Turner syndrome.1
Sometimes, girls with Turner syndrome have some cells that are missing one X chromosome (45,X) and some that are normal. This is because not every cell in the body is exactly the same, so some cells might have the chromosome, while others might not. This condition is called mosaicism (pronounced moh-ZEY-uh-siz-uhm). If the second sex chromosome is lost from most of a girl’s cells, then it’s likely that she will have symptoms of Turner syndrome. If the chromosome is missing from only some of her cells, she may have no symptoms or only mild symptoms.