XYY Syndrome

Written by Kimberly Holland
Medically Reviewed by George Krucik, MD, MBA on May 29, 2013

What Is XYY Syndrome?

XYY Syndrome is a genetic condition that occurs when a male’s genes have an extra Y chromosome. Females typically have two X chromosomes (XX). Males typically have one X chromosome and one Y chromosome (XY). Males with this genetic condition have an extra copy of the Y chromosome in each of their cells (XYY).

Most people have 46 chromosomes in each cell. Males with XYY syndrome have 47 because of the extra copy of the Y chromosome. However, sometimes this mutation is only present in some cells. According to the National Institutes of Health, XYY syndrome occurs in 1 out of every 1,000 boys (Genetics Home Reference).

XYY Syndrome is also sometimes called Jacob’s Syndrome, XYY Karyotype, or YY Syndrome.

Males with XYY Syndrome live typical lives for the most part. Some males with XYY Syndrome may be taller than average and face learning difficulties or speech problems. They may also grow up with minor physical differences, such as weaker muscles and bones and delayed puberty. When they are older, men with the syndrome may face fertility issues and complications related to low testosterone levels. Besides these complications, however, males with XYY syndrome do not usually have any distinguishing physical features or complications, and have normal sexual development.

What Causes XYY Syndrome?

XYY Syndrome is the result of a random mix-up during the creation of a male’s genetic code. Most cases of XYY Syndrome are not inherited, and researchers do not believe that there is any genetic predisposition to it. Men with XYY Syndrome are not more or less likely to pass the syndrome on to their male children. Because the error is random and occurs at different times during cell division, a male with XYY syndrome may have some cells that are not affected—some cells may have XY chromosomes while others have XYY chromosomes.

Who Is at Risk for XYY Syndrome?

XYY Syndrome is not inherited. All males are at the same risk for the condition.

What Are the Symptoms of XYY Syndrome?

The signs and symptoms of XYY Syndrome differ from patient to patient and age to age.

The following signs may be an indication that a baby boy has XYY Syndrome:

  • hypotonia, or weak muscle tone
  • delayed motor skill development, such as walking or crawling
  • delayed or difficult speech
  • not easily excitable (is normally quiet and calm)
  • undescended testicles at birth

The following signs may be an indication that a young boy or teenager has XYY Syndrome:

  • an autism diagnosis
  • an inability to grow facial or body hair
  • attention difficulties
  • delayed motor skill development, such as writing
  • delayed or difficult speech
  • delayed or absent puberty
  • emotional or behavioral issues
  • gynecomastia, or enlarged breast tissue
  • hand trembling or involuntary muscle movements
  • hypogonadism, or low testosterone levels
  • hypotonia, or weak muscle tone
  • learning disabilities
  • low energy levels
  • small penis
  • small, undersized testicles
  • taller than average height
  • weak bones

In addition to the signs above, the following signs may be an indication that an adult male has XYY Syndrome:

  • decreased sex drive
  • decreased sperm counts
  • infertility

How Is XYY Syndrome Diagnosed?

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

Two tests may be used to diagnose XYY Syndrome:

  • Hormone testing. Your doctor may request a Sex Hormone-Binding Globulin (SHBG) test to check your testosterone levels. They may request a total testosterone test, too. Both tests analyze blood or urine samples to determine if a man’s testosterone levels are low, a possible indication that he has a problem. If the testosterone levels are low, your doctor may request additional tests to determine the cause.
  • Chromosome analysis. Genetic disorders, such as XYY Syndrome, can be diagnosed with a chromosome analysis. If your doctor is unable to find an explanation for symptoms that may indicate XYY Syndrome, you may be requested to undergo a chromosome analysis to check for XYY Syndrome.

How Is XYY Syndrome Treated?

The XYY Syndrome cannot be cured, but treatments can help reduce symptoms and the effects caused by it, especially if it is discovered and diagnosed early. Patients can work with health care providers to address any symptoms they may have, such as speech disabilities, hormonal imbalances, and learning problems. As a man gets older, he may want to work with a reproductive specialist to address any infertility concerns.

The following treatment options may be used to address some of the most common effects of XYY Syndrome:


  • Testosterone replacement therapy. Males who do not produce adequate testosterone may need hormone replacement therapy as they grow older. Hormone therapy can help teenage boys undergo normal puberty. It can also help older men improve their sexual drive and energy levels, or increase bone strength and improve muscle mass. This therapy is typically given as an injection, but some patients may be able to use a rub-on gel.
  • Fertility treatment. Men with XYY Syndrome may have low sperm counts and difficulty conceiving a child. Testosterone replacement therapy will not increase fertility, but it can help increase sexual drive. Men wishing to have a child may need to see a fertility specialist to discuss options, such as intracytoplasmic sperm injections (ICSI). During this procedure, sperm is removed from a man’s testicle and then injected directly into a woman’s egg before it is planted back inside of her.


Gynecomastia. Men with excess breast tissue as a result of XYY Syndrome may want plastic surgery to remove the breast tissue.

Alternative Therapies

  1. Speech therapy. Patients with XYY Syndrome may experience speech or motor skill disabilities as a result of the condition. Healthcare professionals can help treat these and provide plans for future improvements.
  2. Physical or occupational therapy. Some younger patients with XYY Syndrome experience delayed motor skill development or difficulty with muscle strength. Physical therapists and occupational therapists can help patients overcome these limitations.
  3. Educational therapy. Some patients with XYY Syndrome experience learning disabilities. Talk with your child’s teacher, principal, and special education coordinators to arrange a schedule that is best suited to your child’s needs. Outside tutors and educational instruction may be necessary.

What Is to Be Expected in the Long Term?

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

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