Tetrasomy X

Tetrasomy X, also called 48,XXXX or poly-X Klinefelter, is a chromosomal disorder in which a female carries four copies of the X chromosome instead of the usual two. It arises from nondisjunction, the failure of chromosomes to separate properly during the formation of egg or sperm cells, and is not inherited from a parent with the same condition. The extra chromosomes are associated with intellectual disability, distinctive facial features, heart defects, skeletal anomalies, and unusually tall stature, though severity varies widely between individuals. The condition was first recorded in 1961 in two women with intellectual disability who were living in an institution, making it one of the later-described sex chromosome aneuploidies. It remains rare, estimated to affect roughly one in fifty thousand females, and is managed through the specialty of medical genetics.