Turner syndrome

Turner syndrome is a condition affecting females who have only one fully functional X chromosome instead of two. It is the human example of a survivable monosomy: the karyotype is usually written 45,X, meaning 45 total chromosomes with a single X and no second sex chromosome. Think of it as missing the spare copy that the body normally keeps in reserve.

Because cells normally silence one X anyway through X-inactivation, having only one X is partly tolerable. But a few genes on the X escape silencing and need two working copies for typical development, so a single X is not fully equivalent to the usual pair. This explains why Turner syndrome has real effects despite the underlying logic of dosage.

Characteristic features can include shorter stature, ovaries that do not develop typically (often causing infertility), and certain heart and kidney differences, though severity varies widely. Many people with Turner syndrome have a normal lifespan and intelligence, and modern care addresses growth, hormones and associated conditions.

Not every case is a clean 45,X. Some individuals are mosaic, with some cells 45,X and others 46,XX or carrying a structurally altered X, which often produces a milder picture.