Turner syndrome is a genetic condition caused by a missing or incomplete sex chromosome.
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 the other comes 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). The Y chromosome determines "maleness", so if it is missing – as in Turner syndrome – the sex of the child will invariably be female.
A female with Turner syndrome is missing part or all of one sex chromosome. This means she has just one complete X chromosome.
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 one of the X chromosomes is completely missing
- mosaic Turner syndrome – in most cells one X chromosome is complete and the other is partially missing or abnormal in some way, but in some cells there may be just one X chromosome or, rarely, two complete X chromosomes