XXYY Syndrome

XXYY or 48,XXYY syndrome or 48,XXYY is a rare intersex trait occurring in approximately 1 in 18,000 to 1 in 40,000 people assigned male at birth. It is a chromosomal condition that causes them to have an extra X and an extra Y chromosome (XXYY). It may be considered a variant of Klinefelter syndrome. Those with the the condition typically have small testes that do not produce enough testosterone, which can lead to reduced facial and body hair, poor muscle development, breast growth, and infertility. Other common symptoms can include increased height, dental problems, problems with blood vessels in the legs, difficulty with language development, delayed development of motor skills, tremors that typically start in adolescence and increase with age, and an increased likelihood of several other physical and neurological conditions.

History
The first published report of a boy with a 48,XXYY karyotype was by Sylfest Muldal and Charles H. Ockey in Manchester, England in 1960. It was described in a 15-year-old mentally challenged boy who showed signs of Klinefelter syndrome; however, chromosome testing revealed 48,XXYY instead of the 47, XXY arrangement known to cause Klinefelter syndrome. Because of this, 48,XXYY syndrome was originally considered a variation of Klinefelter syndrome.

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