What Is XYY Syndrome?

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XYY syndrome (Jacobs syndrome or 47,XYY) is a genetic condition that occurs when a person assigned male at birth is born with an extra copy of the Y chromosome in some or all of their cells. Males are usually born with one X and one Y chromosome.

About 1 in 1,000 males are born with XYY syndrome and may have physical characteristics such as increased height, larger heads, and low muscle tone, though some have no noticeable signs or symptoms. 

Most people born with XYY syndrome have average intelligence but may experience developmental delays or learning disabilities. XYY syndrome is associated with an increased risk of behavioral, social, and emotional challenges and conditions such as attention deficit hyperactivity disorder (ADHD) and autism spectrum disorder

This article discusses the symptoms, causes, diagnosis, and treatment of XYY syndrome.

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What Are the Symptoms of XYY Syndrome? 

Many boys and men with XYY syndrome have no significant or noticeable symptoms. When signs and symptoms of XYY syndrome occur, they can include a combination of physical and behavioral differences that vary from person to person. 

Physical Characteristics 

Boys born with XYY syndrome may have some subtle physical characteristics associated with the condition, including the following:

  • Taller than average height 
  • Hypotonia (low muscle tone) 
  • Clinodactyly (curved pinky finger) 
  • Hypertelorism (widely spaced eyes) 
  • Cystic acne (during adolescence) 
  • Delayed motor skills development, such as sitting and walking
  • Dental problems, such as baby teeth not falling out, overcrowded teeth, missing teeth, and poor tooth enamel 
  • A forward-jutting jaw or an underbite

Learning and Behavioral Symptoms 

Some boys with XYY syndrome may have behavioral, emotional, learning, and social difficulties, such as:

  • Speech and language delays
  • Expressive language disorder 
  • Learning disability (e.g., dyslexia)
  • Depression
  • Anxiety
  • Social skills impairment
  • Impulsivity 
  • Low-self esteem

Boys with XYY syndrome are more likely to have autism spectrum disorder and ADHD than those who do not have XYY syndrome.

What Causes XYY Syndrome?

In most cases, XYY syndrome occurs due to a random chromosomal error before conception, when sperm cells form in the father's body. Usually, a single sperm cell carries either an X or Y chromosome.

If a cell division error occurs during spermatogenesis (production of sperm), a single sperm cell can contain two Y chromosomes rather than one. When this sperm cell fertilizes an egg and results in a pregnancy, a male is born with XYY syndrome, meaning every cell in his body has XYY chromosomes rather than XY chromosomes. 

In rare cases, cell division errors can occur after conception and cause XYY syndrome. When this happens, a boy born with XYY syndrome may have an extra Y chromosome in some of his cells but not all. This is known as 46,XY or 47,XYY mosaicism.

The exact cause of cell division errors is not fully understood. XYY syndrome is genetic and not inherited, which means that XYY cannot be passed down through generations and does not tend to run in families. 

How Is XYY Syndrome Diagnosed?

A healthcare provider can diagnose XYY syndrome before birth (prenatally) or after birth (postnatally). However, up to 90% of people with XYY syndrome are never diagnosed.

Prenatal Testing

Some prenatal screening tests for genetic and chromosomal disorders can detect XYY syndrome during pregnancy. The most common test that detects chromosomal disorders is noninvasive prenatal testing (NIPT).

For a NIPT test, a blood sample taken from the vein of the gestational parent (parent giving birth) is sent to the lab, where small fragments of fetal DNA are isolated and analyzed to screen for chromosomal disorders. If the NIPT test result is positive for XYY syndrome, additional testing such as amniocentesis or chorionic villus sampling (CVS) tests can confirm the diagnosis.

Postnatal Testing 

After birth, a healthcare provider can diagnose XYY syndrome through a karyotype genetic test. A karyotype test analyzes the size, shape, and number of chromosomes in some of the body's cells. Karyotype testing may involve: 

  • Blood test: Taking a small sample of blood from a vein 
  • Buccal (cheek) swab: A swab is inserted into the mouth and rubbed across the inside of the cheek to gather cells

The blood or cheek swab is sent to the lab for analysis. In males with XYY syndrome, cells will contain XYY chromosomes rather than the usual XY.

Can You Treat XYY Syndrome?

There is no cure for XYY syndrome, so treatment focuses on addressing specific symptoms. Early intervention and therapies support boys with XYY-associated learning disabilities, behavioral issues, or communication struggles. Treatment options vary, depending on the age at diagnosis and symptom severity. 

XYY syndrome treatment options include:

  • Early intervention support: Children with XYY syndrome may benefit from early intervention services, such as speech and language therapy and occupational therapy, physical therapy, and behavioral therapy, to help them reach developmental milestones. These therapies can help improve gross and fine motor, communication, language, and social skills. 
  • Educational support: Classroom support can help boys with XYY syndrome succeed in school. Boys who attend mainstream schools may need additional support, such as an individualized education plan (IEP) or a one-to-one or small instruction setting for certain subjects, particularly reading and writing. 
  • Counseling: Families affected by XYY syndrome may benefit from counseling to better understand living with the syndrome and how best to support their family members. Those with emotional or behavioral issues, such as depression, anxiety, or social issues, may benefit from psychotherapy to help improve self-esteem, emotional well-being, and communication. 
  • Medication: Healthcare providers may prescribe certain medications for conditions common in children with XYY syndrome, such as ADHD and acne. 
  • Regular medical checkups: Regular visits with a healthcare provider are important for identifying any health problems or delays early so boys can receive the appropriate treatment as soon as possible. 

XYY Syndrome Prognosis 

Research suggests that the life span of people with XYY syndrome is approximately 12 years shorter than males without the condition. The increased risk of pulmonary (lung) and neurological conditions, such as asthma and seizure disorders, may affect the shorter life span. 

Most boys and men with XYY syndrome live full lives. They go through puberty normally and can have children and careers, as well as a positive impact on their communities. 

Coping With XYY Syndrome

Getting a diagnosis of XYY syndrome, whether your own or your child's, can be difficult to process. Living with XYY syndrome can present unique challenges, and it's normal to have concerns about what the future may hold. 

It can be helpful to learn as much as possible about XYY syndrome and the various therapies and early interventions that can positively impact the lives of those with the condition. Work closely with a healthcare provider to identify and treat any health, behavioral, or developmental concerns, and be in contact family members and friends for emotional support.

You may find value in reaching out to an organization that provides support, education, and resources to families affected by chromosome variations, such as AXYS, the Association for X and Y Chromosome Variations.


XYY syndrome occurs when a male is born with two Y chromosomes rather than one, as usual. Some people born with the condition have physical characteristics and behavioral symptoms, while others do not. Men and boys with XYY syndrome are often taller than average, and may have a curved pinky finger, low muscle tone, or flat feet.

Developmental delays, learning disabilities, and social and emotional challenges are common in boys born with an extra Y chromosome. XYY syndrome is associated with an increased risk of autism spectrum disorder and ADHD. 

9 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. MedlinePlus. 47,XYY syndrome.

  2. Unique: Understanding Rare Chromosome and Gene Disorders. XYY.

  3. National Organization for Rare Disorders. XYY syndrome.

  4. Association for X and Y Chromosome Variations. About 47, XYY.

  5. Nemours KidsHealth. XYY syndrome.

  6. National Organization for Rare Disorders. XYY syndrome.

  7. MedlinePlus. Karyotype genetic test.

  8. Gravholt CF. Sex-chromosome abnormalitiesEmery and Rimoin’s Principles and Practice of Medical Genetics. 2013:1-32. doi.org/10.1016/B978-0-12-383834-6.00050-1

  9. Jodarski C, Duncan R, Torres E. et al. Understanding the phenotypic spectrum and family experiences of XYY syndrome: Important considerations for genetic counseling. J Community Genet 14, 17–25 (2023). doi:10.1007/s12687-022-00630-y