Causes of Turner Syndrome

What Causes Turner Syndrome?


It is estimated that 1 girl out of every 2,000- 4,000 baby girls delivered has TS, so those featured throughout our website were born with TS despite the odds.  Turner syndrome occurs when all or part of one of the X chromosome is lost before or soon after the time of conception. To read a more detailed description of how this happens, please refer to the Guide for Families, pages 3-6.


There is nothing that either parent did before or at the start of the pregnancy that caused or increased the risk of TS for their daughter. The age of a parent, ethnicity, diet, or other factors are unrelated to the conception of a child with Turner syndrome.

Almost 50% of those with TS are missing the X chromosome in all of the cells that are tested (usually blood cells). This is written 45X and the medical term is monosomy X.

About 30% of those with TS have what is called mosaicism. This means that some cells have the complete number of chromosomes, written 46,XX, and the other cells are missing an X. It is believed that mosaicism is a result of the loss of an X chromosome after fertilization.

A Typical Human Karyotype
A Turner Syndrome Karyotype (45X)

Mosaicism in Turner Syndrome

There are different types of mosaicism. A few of these are:

45,X/46,XX - This can be harder to diagnose because they can, but not always, have fewer features related to TS, because fewer cells are missing an X.

45,X/46,XY - This occurs in a small percentage of girls with TS. There are small fragments of Y chromosome in their cells, which can increase the risk of developing tumors, called gonadoblastomas, in their gonads (internal sex organs). These tumors are benign, but can lead to cancerous tumors, so it is usually recommended that the ovaries be surgically removed.

It is important to remember that having Y chromosome material will not make a girl with TS have male features. 

45,X/47XXX - This is a rare type of mosaicism in which there are 45,X cells and another cell type that has an extra X. These girls are typically taller.

About 20% of those with TS will have two X chromosomes, but one will be incomplete or have a different structure.

Ring chromosome - There are two X chromosomes, but one is shaped like a circle with the ends joined.

Isochromosome- The X chromosome has two long arms instead of a long and short arm.

Deletion- This occurs when the X chromosome is missing some of its genetic material.

NOTE: Males may sometimes have the 45,X/46,XY mosaic karyotype, but this is not Turner syndrome. A TS diagnosis only applies to phenotypical females (those born with female sexual organs internally and externally). In males, this is simply referred to as 45,X/46,XY,male.   TSSUS recognizes physicians diagnose males with TS, at this time our professional advisers disagree with the practice of diagnosing males with TS.  The professional advisers encourage physicians to contact TSSUS and request to consult with a TS genetic specialist for questions related to TS karyotypes.


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