XYY, or "XYY syndrome," is a trisomy of the sex chromosomes in which a human male receives an extra Y chromosome in each cell, hence having a karyotype of 47,XYY.
XYY syndrome typically causes no unusual physical features or medical problems. Persons with this syndrome may be slightly taller than average, or have more severe acne than normal. Skeletal malformations may also accompany XYY syndrome at a higher rate than in the general population. Several other physical characteristics, including large hands and feet, have been associated (although not definitively) with XYY syndrome. Any physical characteristics, however, are usually so slight that they are insufficient evidence to suggest a diagnosis.
Most males with XYY syndrome have normal sexual development and are able to conceive children.
Since there are no distinct physical characteristics, the condition usually is only detected during genetic analysis for other reasons.
There are reports that men with XYY syndrome may be more likely to have behavior problems, possibly due to a higher testosterone level, but these are usually avoided if the syndrome is detected early. There are also reports of learning disabilities and delayed speech and language skills, but they may be caused by the behavior problems.
An earlier study suggested that XYY males were more likely to be incarcerated and be criminal, but the study is now widely considered as procedurally flawed. The misleading data resulted from the study being carried out in prisons (a classic self-selection error).
Researchers are not yet certain why an extra copy of the Y chromosome should be associated with tall stature and learning problems.
Cause and prevalence
XYY syndrome is not inherited, but usually occurs as a random event during the formation of sperm cells. An error in cell division called nondisjunction can result in sperm cells with an extra copy of the Y chromosome. If one of these atypical reproductive cells contributes to the genetic makeup of a child, the child will have an extra Y chromosome in each of the body's cells. In some cases, the addition of an extra Y chromosome occurs as an accident during cell division in early fetal development.
The incidence of this condition is about one in 850 males.
- public domain text from the NLM Genetics Home Reference at  (http://ghr.nlm.nih.gov/condition=47xyysyndrome).