Turner Syndrome

First described by Dr. Henry Turner in 1938, Turner syndrome was later found to be due to a loss of genetic material in one of the two X-chromosomes. There are several ways in which Turner syndrome arises. Females have two X-chromosomes, while males have an X-chromosome and a Y-chromosome. While a Y-chromosome contains genes required for development of testicles in males, both X-chromosomes are required for normal ovarian development in females. The majority of Turner syndrome cases are caused by a sporadic event during a specific stage of cellular division, where an X-chromosome is lost. Normally, when sex cells divide, an equal amount of genetic material is divided into each cell. During fertilization of the egg, if the sex cell void of the X-chromosome is fertilized by a sex cell that is missing the other X-chromosome, the result is only one sex chromosome. If the fetus survives, the baby will be born with Turner syndrome. A fertilized egg with only one Y-chromosome is incompatible with life.

Turner syndrome can also result from the loss of a single X-chromosome after fertilization, sometime during embryonic development. This particular condition is referred to as mosaicism, with clinical manifestations being proportional to the percentage of cells missing the X-chromosome. Finally, Turner syndrome can result due to a defective X-chromosome such as large deletion. The clinical consequences vary depending on the nature of the structural abnormality of the X-chromosome. In the case X-chromosome deletions, where affected individuals are fertile, there is potential for a recurrence risk in pregnancies from an affected female. Otherwise, there is typically little recurrence risk (if any) in subsequent pregnancies or in individuals with Turner syndrome since the majority of these individuals are infertile.