When a baby is conceived, a normal egg cell and normal sperm cell start with 46 chromosomes. The egg and sperm cells then divide in half. The egg and sperm cells then have 23 chromosomes each. When a sperm with 23 chromosomes fertilizes an egg with 23 chromosomes, the baby will then have a complete set of 46 chromosomes, or 23 pairs. Half are from the father and half are from the mother. The 23rd pair is called the sex chromosomes. In females, the 23rd pair is two X chromosomes. In males, the 23rd pair is one X and one Y chromosome.
There are two types of Turner syndrome: monosomy X TS and mosaic TS. About half of all girls with Turner syndrome have a monosomy disorder. Monosomy means that a person is missing one chromosome in the pair. Instead of 46 chromosomes, the person has only 45 chromosomes. This means a girl with TS has only one X chromosome in her 23rd pair. Sometimes an error occurs when an egg or sperm cell is forming. This causes it to have a missing sex chromosome. But it is often an error that happened by chance when the father's sperm cell was forming. The missing sex chromosome error can occur in either the mother's egg cell or the father's sperm cell.
Girls with mosaic TS have chromosome changes in only some cells, but not all cells. A small number of cases have the normal number of 46 chromosomes, but with part of the X chromosome missing. When only part of an X chromosome is missing (deletion), a girl with the syndrome will often have milder signs of TS. The features of TS depend on which part of the X chromosome is missing.