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A girl with Turner syndrome only has 1 normal X sex chromosome, rather than the usual 2 (XX).

Everyone is born with 23 pairs of chromosomes. One pair of chromosomes, the sex chromosomes, determines the baby's gender.

1 sex chromosome comes from the father and 1 from the mother. The mother's contribution is always an X chromosome. The father's contribution can either be an X or a Y chromosome.

A baby girl usually has 2 X chromosomes (XX), and boys have an X and a Y chromosome (XY). A female with Turner syndrome is missing part or all of 1 sex chromosome. This means she has just 1 complete X chromosome.

The Y chromosome determines "maleness", so if it's missing, as in Turner syndrome, the sex of the child will invariably be female.

This chromosome variation happens randomly when the baby is conceived in the womb. It is not linked to the mother's age.

The syndrome can either be described as:

• classic Turner syndrome – where 1 of the X chromosomes is completely missing
• mosaic Turner syndrome – in most cells, 1 X chromosome is complete and the other is partially missing or incomplete in some way, but in some cells there may be just 1 X chromosome or, rarely, 2 complete X chromosomes