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XYY is a genetic disorder that only affects males and is marked by the presence of an extra Y chromosome — males typically are born with one X and one Y chromosome. The condition is also known as a 47 XYY karyotype; people normally have a karyotype of 46 chromosomes. Males affected by XYY syndrome don't usually suffer from any kind of mental retardation, nor do they usually display any other adverse symptoms. Some males do, however, suffer from behavioral and learning issues. The condition has also been known to result in lanky and tall body types, and it can induce severe acne.
For a period of time in the 1960s and 1970s, XYY syndrome was referred to as the super male syndrome. The term originated from research gathered from studies done on prisoners. The studies led researchers to believe that males with XYY syndrome were more likely to display aggressive behavior and, therefore, more likely to engage in criminal activity. Further studies suggested that the former studies didn't take into account the total number of XYY males; there were found to be, in fact, many men with the syndrome who displayed perfectly normal behavior. As a result, the label of super male syndrome was ultimately shot down as inaccurate and offensive.
Even though the super male label represented a skewed viewpoint, it is true that males with XYY syndrome are considered to be at a greater risk for developing behavioral problems, including some aggressive traits, such as a quick temper. XYY males are also at a higher risk for having learning difficulties, such as a difficulty learning to read. Another drawback of the condition is that many XYY males have an IQ that's slightly lower than their peers, but only very slightly—often not enough to be consequential. It also should be noted that males with the syndrome will not necessarily develop any behavioral or learning problems.
Males who have XYY syndrome may appear slightly different than others, sporting a thinner and taller physique. The physical difference usually isn't that conspicuous; they may not actually be any taller or thinner than many of their peers who do not have XYY syndrome. Some XYY males also struggle with severe acne for which they may need medical treatment.
XYY syndrome is a rare condition, although it may be slightly more common than experts think: a good number of cases may go undiagnosed because of the often asymptomatic nature of the condition. The condition is also not thought to be hereditary. As such, there really isn’t any way of knowing ahead of time who is likely to develop XYY syndrome and who isn’t.