Turner syndrome (genotype 45, XO) is a chromosomal anomaly arising from the failure of chromosomes to separate properly during meiosis (cell division in sex cells in which the chromosomal number is halved). In 60 percent of the cases an egg lacking an X chromosome (or a sperm lacking an X or Y chromosome) unites with a normal sex cell to produce a zygote, or fertilized egg, bearing a single X chromosome. In the remaining cases, an X chromosome is lost from some cells during early embryonic development, resulting in mosaics that have both normal cells and X-deficient cells (genotype XO/XX). Mosaics include individuals with two or more distinct cell populations due to a genetic change or error soon after conception.
Turner syndrome occurs in 1 out of 2,500 live female births, although miscarriage is the result for 99 percent of the fetuses. Clinical features, which vary widely, include short stature, webbed neck, low-set ears, drooping eyelids, skeletal deformities, hearing problems, reduced secondary sexual development, and sterility. Behavioral features include poor directional sense and poor mathematical ability. Verbal intelligence has been considered normal, although studies conducted during the last decade of the twentieth century suggest an increased risk for speech and language problems. Families may be advised to seek hormonal treatment and educational assistance for these children.
See also: BIRTH DEFECTS; GENOTYPE
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Nancy L. Segal