The incidence at birth is about 1 in 1000 males. There is no spermatogenesis. These individuals are generally severely retarded. 47, XYY Incidence 1 in 1000 male births. May be without any symptoms.
Males are tall but normally proportioned.
Strangely, although they are fertile they do not seem to transmit the either this condition or Kleinfelter's syndrome. XXX females About one woman in 1000 has an extra X chromosome. It seems to do little harm, individuals are fertile and do not transmit the extra chromosome. They do have a reduction in IQ comparable to that of Kleinfelter's males. This may lead to chromosome breakage.