Pentasomy X

Pentasomy X, also written 49,XXXXX, is a chromosomal disorder in which a female has five copies of the X chromosome instead of the usual two. It is associated with short stature, intellectual disability, characteristic facial features, heart defects, skeletal anomalies and abnormalities of puberty and reproduction. The condition is exceptionally rare, with an estimated prevalence of between 1 in 85,000 and 1 in 250,000, and because so few cases have been diagnosed it is hard to give a firm account of its presentation or outlook. It can be mistaken for more common chromosomal conditions such as Down syndrome or Turner syndrome before a diagnosis is confirmed. It is not inherited but arises from nondisjunction, a random event in the formation of an egg cell, which produces 49 chromosomes rather than the usual 46.