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47,XYY syndrome is the presence of 2 Y chromosomes and 1 X chromosome in a phenotypic male.
(See also Overview of Chromosomal Abnormalities and see Overview of Sex Chromosome Abnormalities.)
The 47,XYY syndrome occurs in approximately 1/1000 live male births.
Affected boys tend to be taller than average and have a 10- to 15-point IQ reduction compared with family members. There are few physical problems. Minor behavior disorders, hyperactivity, attention-deficit disorder, and learning disorders are more common.
XYY syndrome was once thought to cause aggressive or violent criminal behavior, but this theory has been disproved.
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