Normally, a girl inherits one X chromosome from her mother and one X chromosome from her father. But girls who
have Turner syndrome are missing one of their X chromosomes.
Turner syndrome is typically caused by what is called nondisjunction. Nondisjunction happens when a pair of sex chromosomes fails to separate during the formation of a sperm (or egg). When sperm with no X chromosome unites with a normal egg to form an embryo, that embryo will have just one X chromosome (X rather than XX). As the embryo grows and the cells divide, the X chromosome will be missing from every cell of the baby's body.
The abnormality is not inherited from an affected parent (not passed down from parent to child), because women with
Turner syndrome are usually sterile and cannot have children.
In about 20 percent of Turner syndrome cases, both X chromosomes are present, but one is abnormal. It may be shaped like a ring or missing some genetic material.
About 30 percent of girls with the disorder are missing the X chromosome in just some of their cells. This mixed
chromosome pattern is known as mosaicism. Girls with this pattern may have fewer symptoms because they
still have some normal (XX) cells.
One of the missing genes on the X chromosome is the SHOX gene, which is responsible for long bone growth. The missing
SHOX gene is the reason girls who have the disorder are unusually short. Other missing genes regulate ovarian development, which influences sexual characteristics.
Nondisjunction happens when chromosomes are distributed incorrectly during egg and sperm formation. The gametes above either are missing or have an extra X chromosome.