What is XYY syndrome?

Most people have 46 chromosomes in each cell. In males, this typically includes one X chromosome and one Y chromosome (XY). XYY syndrome is a genetic condition that occurs when a male has an extra copy of the Y chromosome in each of their cells (XYY). Sometimes, this mutation is only present in some cells. Males with XYY syndrome have 47 chromosomes because of the extra Y chromosome.

This condition is also sometimes called Jacob’s syndrome, XYY karyotype, or YY syndrome. According to the National Institutes of Health, XYY syndrome occurs in 1 out of every 1,000 boys.

For the most part, people with XYY syndrome live typical lives. Some may be taller than average and face learning difficulties or speech problems. They may also grow up with minor physical differences, such as weaker muscle tone. Besides these complications, though, males with XYY syndrome don’t usually have any distinguishing physical features, and they have normal sexual development.

XYY syndrome is the result of a random mix-up, or mutation, during the creation of a male’s genetic code. Most cases of XYY syndrome are not inherited. Researchers don’t believe that there’s any genetic predisposition to it. That is, men with XYY syndrome are not more or less likely than other men to have children with XYY syndrome. The random error can occur during the formation of sperm or at different times during the formation of an embryo. In the latter case, a male may have some cells that are not affected. This means that some cells may have XY genotype while others have XYY genotype.

The signs and symptoms of XYY syndrome differ from person to person and age to age.

Symptoms in a baby who has XYY syndrome can include:

  • hypotonia (weak muscle tone)
  • delayed motor skill development, such as with walking or crawling
  • delayed or difficult speech

Symptoms in a young child or teenager with XYY syndrome can include:

  • an autism diagnosis
  • attention difficulties
  • delayed motor skill development, such as with writing
  • delayed or difficult speech
  • emotional or behavioral issues
  • hand trembling or involuntary muscle movements
  • hypotonia (weak muscle tone)
  • learning disabilities
  • taller-than-average height

In adults, infertility is a possible symptom of XYY syndrome.

XYY syndrome may remain undiscovered and undiagnosed until adulthood. That’s when fertility problems such decreased sperm counts alert doctors to a possible condition.

Genetic disorders can be diagnosed with a chromosome analysis. If your doctor can’t find another explanation for symptoms that may indicate XYY syndrome, they may ask you to undergo a chromosome analysis to check for XYY syndrome.

XYY syndrome can’t be cured. But treatments can help reduce its symptoms and effects, especially if it’s diagnosed early. People with XYY syndrome can work with healthcare providers to address any symptoms they may have, such as speech and learning problems. As they get older, they may want to work with a reproductive specialist to address any infertility concerns.

The following treatment options may help address some of the most common effects of XYY syndrome.

Speech therapy: People with XYY syndrome may have speech or motor skill disabilities. Healthcare professionals can help treat these issues. They can also provide plans for future improvements.

Physical or occupational therapy: Some younger people with XYY syndrome have delayed motor skill development. They may also have difficulty with muscle strength. Physical therapists and occupational therapists can help people overcome these issues.

Educational therapy: Some people with XYY syndrome have learning disabilities. If your child has this syndrome, talk with their teacher, principal, and special education coordinators. Arrange a schedule that’s best suited to your child’s needs. Outside tutors and educational instruction may be necessary.

People with XYY syndrome can — and very often do — live completely normal lives with the condition. In fact, XYY syndrome can remain undiagnosed throughout a person’s lifetime. If it is diagnosed, however, individuals with XYY syndrome can find the help they might need.