XYY – Stereotype of the karyotype
XX female fetuses can acquire an extra X chromosome, leading to XXX females (triple X syndrome), and XY male fetuses can acquire an extra Y chromosome, leading to XYY syndrome (which affects one male in 1000).
Triple X females are usually fertile, but sometimes go through the menopause early. XYY men tend to be taller and more wiry than other men, and tend to suffer from severe acne. They also have higher average blood testosterone than XY men.
XYY men in particular have been negatively stereotyped by society. When it was first discovered, popular science writers speculated that the extra Y would make males more aggressive – and more prone to criminality – than their XY peers. (This view persists: the film 'Aliens 3' was set in a penal colony for XYY prisoners.)
The myth was backed up by some rather misleading statistics. A paper published in the 'Lancet' in 1968 claimed that the prevalence of XYY men in prison was 25-60 times as high as the prevalence in the general population. This prompted some to suggest that screening for XYY would identify potentially violent and criminal males.
But the quality of this research was soon questioned (it was later pointed out that only two XYY male prisoners were identified in the study) and the consensus today is there is no strong link between XYY and criminality. XYY men may tend to share certain features, but these are well within the 'normal' range of human variation.