A girl with Turner syndrome only has one normal X sex chromosome, rather than the usual two (XX).
Everyone is born with 23 pairs of chromosomes. One pair of chromosomes, the sex chromosomes, determines the baby's gender.
One sex chromosome comes from the father and one 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 two X chromosomes (XX), and boys have an X and a Y chromosome (XY). A female with Turner syndrome is missing part or all of one sex chromosome. This means she has just one 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: