A genetically inherited sex chromosome abnormality only affecting males. Men with XYY syndrome inherit an extra Y chromosome--their genotype is XYY. They are usually quite tall (i.e., above 6 feet) and generally appear and act normal. During adolescence, they frequently have severe facial acne. They are usually fertile and lead ordinary lives as adults. Many, if not most, are unaware that they have a chromosomal abnormality. The frequency of XYY syndrome is not certain due to statistical differences between different studies of this condition. It may be as common as 1 in 900 male births to as rare as 1 in 1,500 or even 1 in 2,000.