Turner syndrome (TS), commonly known as 45,X, or 45,X0, is a chromosomal disorder in which cells of females have only one X chromosome instead of two, or are partially missing an X chromosome (sex chromosome monosomy) leading to the complete or partial deletion of the pseudoautosomal regions (PAR1, PAR2) in the affected X chromosome. Humans typically have two sex chromosomes, XX for females or XY for males. The chromosomal abnormality is often present in just some cells, in which case it is known as Turner syndrome with mosaicism. 45,X0 with mosaicism can occur in males or females, but Turner syndrome without mosaicism only occurs in females. Signs and symptoms vary among those affected but often include additional skin folds on the neck, arched palate, low-set ears, low hairline at the nape of the neck, short stature, and lymphedema of the hands and feet. Those affected do not normally develop menstrual periods or mammary glands without hormone treatment and are unable to reproduce without assistive reproductive technology. Small chin (micrognathia), loose folds of skin on the neck, slanted eyelids and prominent ears are found in Turner syndrome, though not all will show it. Heart defects, Type II diabetes, and hypothyroidism occur in the disorder more frequently than average. Most people with Turner syndrome have normal intelligence; however, some have problems with spatial visualization that can hinder learning mathematics. Ptosis (droopy eyelids) and conductive hearing loss also occur more often than average.
Turner syndrome is caused by one X chromosome (45,X), a ring X chromosome, 45,X/46,XX mosaicism, or a small piece of the Y chromosome in what should be an X chromosome. They may have a total of 45 chromosomes or will not develop menstrual periods due to loss of ovarian function genes. Their karyotype often lacks Barr bodies due to lack of a second X or may have Xp deletions. It occurs during formation of the reproductive cells in a parent or in early cell division during development. No environmental risks are known, and the mother's age does play a role. While most people have 46 chromosomes, people with Turner syndrome usually have 45 in some or all cells. In cases of mosaicism, the symptoms are usually fewer, and possibly none occur at all. Diagnosis is based on physical signs and genetic testing.
