Stature

• The most common physical effect of XYY syndrome is a taller stature. However, it is not a telltale sign of XYY syndrome. According to Dennis O'Neil of the Palomar College Anthropology Department, males with XYY syndrome are generally above 6 feet tall. Still, this it is not an obvious difference because they only average 3 inches taller than their XY counterparts. The vast majority of men standing 3 inches above the average male height do not have the syndrome. Additionally, men with XYY syndrome tend to be lankier with poor coordination, and they may have difficulty gaining weight.
  
  

Acne

• While the cause is not yet understood, a higher incidence of severe acne has been found in the population of XYY men. It is possible that these men have higher amounts of testosterone, which is linked to acne. However, there is conflicting information as to whether males with an extra Y chromosome produce greater amounts of testosterone.
  
  

Sexual Development

• Aside from a more rapid growth spurt and severe cases of acne during pubescent years, the sexual development of males with XYY is normal in comparison to XY males. Unlike several other chromosomal disorders, the syndrome is not linked to problems with fertility. Additionally, Genetics Education states that men with XYY chromosomes do not have a greater likelihood of fathering children with the disorder. The occurrence of an extra Y chromosome appears to be random.
  
  

Other Possible Effects

• While the physical effects of XYY syndrome are generally harmless, the syndrome has been linked to cognitive and behavioral disorders. The Genetics Home Reference website states that XYY syndrome is associated with learning disabilities and slower development of language skills. Delayed motor skill development, such as the ability to walk, is another potential problem. Men with XYY syndrome may also suffer from involuntary motor movements, known as tics. Although it is a controversial notion, behavioral problems may include antisocial personality disorder and aggression. The disorder has been used as a defense in criminal cases.