Turner syndrome

Turner syndrome is caused by the complete or partial loss of one of the two X chromosomes in girls. When all of a chromosome is missing, this can be called a monosomy. When part of a chromosome is missing, this can be called a deletion. In rare instances, Turner syndrome is caused because genetic material on the X chromosome is duplicated or the chromosome forms a ring or also has genetic material from the Y chromosome. The altered or missing X chromosome may occur in all the cells of a girl's body, or may occur only in a percentage of cells with the remaining cells having two unaltered X chromosomes. This is called mosaicism. Generally, more cells that carry the abnormality, the more severe a disorder is. Turner syndrome is usually not inherited; the complete or partial loss (deletion) of the X chromosome usually occurs after the egg has been fertilized and appears to be a random event. There is nothing the parents did to cause the disorder.

Chromosomes are found in the nucleus of every cell. Chromosomes carry the genetic material of a person in the form of genes. They have a long arm called 'q' and a short arm called 'p'. Genes on the X chromosome influence many different systems in the body. It is the loss of these genes that causes the signs and symptoms of Turner syndrome. Most people have 46 chromosomes; 23 inherited from their mother and 23 from their father. Twenty-two chromosomes are the same in girls and boys, but girls have two X chromosomes and boys have one X and one Y chromosome. These are called the sex chromosomes because they determine the sex of a person. Because boys only have one X chromosome, Turner syndrome is not compatible with life in a boy and leads to miscarriage or stillbirth.

About half of the girls with Turner syndrome, have monosomy X, which means that one entire X chromosome is missing in some (mosaicism) or all of their cells. Some girls have a partial loss (deletion) of one X chromosome in some or all of their cells. There are other abnormalities of the X chromosome that can cause Turner syndrome. Ring chromosome X is when both ends of the one of the X chromosomes break off and then the remaining ends combine to form a ring. This can cause severe problems including intellectual disability. Isochromosome Xq is when there is an extra (duplicated) copy of the long arm (q) of the X chromosome. Sometimes, in addition to a partial or complete loss of one X chromosome, there is genetic material from the Y chromosome in some cells of the body.

The genetic changes causing Turner syndrome are complex. Parents and individuals should talk with a medical geneticist or a genetic counselor. To find a medical professional nearby who can discuss information about Turner syndrome, a listing of medical geneticists can be found at the American College of Medical Geneticists website and genetic counselors can be found on the National Society of Genetic Counselors website.