What is XYY syndrome (Jacobs syndrome)?

XYY syndrome is caused by an extra “Y” chromosome in each of a male's cells. While males with this condition may be taller than average, this chromosomal change typically causes no unusual physical features. Most males with 47,XYY syndrome have normal sexual development and are fertile.

XYY syndrome can be associated with an increased risk of learning disabilities and delayed speech and language skills. Delayed development of motor skills, weak muscle tone, and behavioral and emotional difficulties are also possible. A small percentage of males with 47,XYY syndrome are diagnosed with autistic spectrum disorders, which are developmental conditions that affect communication and social interaction.

Each person with XYY syndrome is unique and the severity of symptoms varies greatly among individuals.

How common is XYY syndrome?

This condition occurs in about 1 in 1,000 newborn boys. The condition is not related to race, nationality, religion or socioeconomic status. There is usually no family history of the condition.

How is XYY syndrome treated?

Treatment depends on the needs of the child. Boys with XYY syndrome may need to be seen by physical, developmental, occupational, or speech therapists if they have developmental or speech problems. Additionally, a pediatric psychologist or group therapy may be helpful if they have social difficulties.

Because boys with XYY syndrome are healthy and have a normal appearance, they may remain undiagnosed all their lives, or the diagnosis may be discovered while investigating other medical issues.

Other names for
XYY syndrome

  • Jacobs syndrome


  • Am J Med Genet. 2002 Jun 1:110(1):11-18.
  • Am J Med Genet. 2002 Jun 1;110(1):3-10.
  • Gardner, R. J. M., & Sutherland, G. R. (2011). Chromosome abnormalities and genetic counseling (4th ed.). New York: Oxford University Press, Inc.
  • Genet Couns. 2003;14(3):267-79.
  • Jones, K. L. (Ed.). (2013). Smith’s recognizable patterns of human malformation (7th ed.). Philadelphia: Elsevier Inc.