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Pentasomy X, also known as 49,XXXXX, is a chromosomal disorder in which a female has five, rather than two, copies of the X chromosome. Pentasomy X is associated with short stature, intellectual disability, characteristic facial features, heart defects, skeletal anomalies, and pubertal and reproductive abnormalities. The condition is exceptionally rare, with an estimated prevalence between 1 in 85,000 and 1 in 250,000.
The condition has a large variety of symptoms, and it is difficult to paint a conclusive portrait of its phenotypes. Though significant disability is characteristic, there are so few diagnosed cases that confident conclusions about the presentation and prognosis remain impossible. Pentasomy X may be mistaken for more common chromosomal disorders, such as Down syndrome or Turner syndrome, before a conclusive diagnosis is reached.
Pentasomy X is not inherited, but rather occurs via nondisjunction, a random event in gamete development. In rare cases, it may be related to a parent's chromosomal mosaicism. The karyotype observed in pentasomy X is formally known as 49,XXXXX, which represents the 49 chromosomes observed in the disorder as compared to the 46 in normal human development.

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