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As its name indicates, a person with the syndrome has one Y chromosome and four X chromosomes on the 23rd pair, thus having 49 chromosomes rather than the normal 46. As is common with aneuploidy disorders, 49 XXXXY syndrome is often accompanied by mental retardation. It can be considered a form of Klinefelter syndrome,  or a variant of it.
It is genetic, but not hereditary. This means that while the genes of the parents cause the syndrome, there is a small chance of more than one child having the syndrome. The probability of inheriting the disease is about 1%.
The individuals with this syndrome are males, but a female version also exists with similar characteristics as the male version.
Aneuploidy is often fatal, but in this case there is "X-inactivation" where the effect of the additional gene dosage due to the presence of extra X chromosomes is greatly reduced.
The mental effects of 49 XXXXY Syndrome vary, much like Down syndrome. Males with the syndrome tend to have impaired speech and behavioral problems. Those with 49 XXXXY syndrome tend to exhibit infantile secondary sex characteristics with sterility in adulthood and have some skeletal anomalies. Skeletal anomalies include:
- genu valgus
- pes cavus
- fifth finger clinodactyly
The effects on the males also include:
- cleft palate
- club feet
- respiratory conditions
- short or/and broad neck
- low birth weight
- hyperextensible joints
- short stature
- narrow shoulders
- coarse features in older age
- epicanthal folds
- gynecomastia (rare)
- muscular hypotonia
- hypoplastic genitalia
- congenital heart defects
- a very round face in infancy
- What is XXXXY syndrome? Retrieved March 26, 2008.
- Visootsak J, Graham JM (2006). Klinefelter syndrome and other sex chromosomal aneuploidies. Orphanet J Rare Dis 1: 42.
- Cotran, Ramzi S.; Kumar, Vinay; Fausto, Nelson; Nelso Fausto; Robbins, Stanley L.; Abbas, Abul K. (2005). Robbins and Cotran pathologic basis of disease, St. Louis, Mo: Elsevier Saunders.
- Webspawner.com article on 49 XXXXY syndrome. Retrieved 26 March 2008.
- Visootsak J, Rosner B, Dykens E, Tartaglia N, Graham JM (June 2007). Behavioral phenotype of sex chromosome aneuploidies: 48,XXYY, 48,XXXY, and 49,XXXXY. Am. J. Med. Genet. A 143A (11): 1198–203.