XYY syndrome, also known as 47,XYY syndrome or Jacobs syndrome, is a rare genetic condition where a male is born with an extra Y chromosome in each of his cells. While many individuals with XYY syndrome have no symptoms or only mild physical differences, the condition can be associated with increased height, learning challenges, and potential delays in speech or motor skill development. What causes XYY syndrome? XYY syndrome is caused by a random error in cell division during the formation of sperm cells or shortly after fertilization. Instead of the typical 46 chromosomes, individuals with XYY syndrome have 47 chromosomes. Importantly, this is not an inherited condition; it is a sporadic genetic event that occurs by chance, meaning parents do not "pass it on" through their own genetic makeup. How common is XYY syndrome? Research indicates that XYY syndrome occurs in approximately 1 in every 1,000 newborn males. Because many people with the condition never experience significant health issues, many remain undiagnosed throughout their lives. Within the DiseaseMaps.org community, 82 people with XYY syndrome have already joined to share their lived experiences and connect with others navigating similar paths. What are the primary characteristics of XYY syndrome? The clinical presentation of XYY syndrome varies significantly from person to person. While there are no distinct "subtypes," clinicians monitor the following common features: Physical growth: A tendency toward above-average height during childhood and adolescence. Developmental milestones: Potential for mild delays in speech, language, and motor skills. Learning and behavior: An increased likelihood of learning disabilities, such as dyslexia, or attention-deficit hyperactivity disorder (ADHD). Physical health: Generally, individuals with XYY syndrome have normal sexual development and fertility, similar to the general population. How does XYY syndrome differ from other conditions? Unlike other chromosomal aneuploidies that often result in severe physical or intellectual disabilities, XYY syndrome is characterized by a much milder phenotype. Most individuals lead healthy, productive lives and often do not require medical intervention beyond routine developmental monitoring. Next steps Consult a clinical geneticist or a pediatrician if you have concerns about developmental milestones. Connect with the 82 members of the DiseaseMaps.org community to share support and insights. Schedule a consultation with a speech or educational therapist if learning challenges are identified. Medical disclaimer: This information is for educational purposes only and does not replace professional medical advice, diagnosis, or treatment. References NIH Genetic and Rare Diseases Information Center (GARD): 47,XYY Syndrome Overview. Orphanet: 47,XYY syndrome (ORPHA:96144). OMIM (Online Mendelian Inheritance in Man): 47,XYY Syndrome entry #400046.

Short answer · Medically reviewed summary · Last updated: 2026-05-08

XYY syndrome, also known as 47,XYY syndrome or Jacobs syndrome, is a rare genetic condition where a male is born with an extra Y chromosome in each of his cells. While many individuals with XYY syndrome have no symptoms or only mild physical differences, the condition can be associated with increased height, learning challenges, and potential delays in speech or motor skill development. What causes XYY syndrome? XYY syndrome is caused by a random error in cell division during the formation of sperm cells or shortly after fertilization.

1 people with XYY Syndrome have shared their first-person experience on this question at DiseaseMaps.

What is XYY Syndrome

Name: What is XYY Syndrome
Creator: DiseaseMaps Editorial Team
Published: 2016-08-19UTC15:00:31.0000000

What is XYY Syndrome? Plain-language, medically reviewed definition plus the lived reality told by patients.

What causes XYY syndrome?

XYY syndrome is caused by a random error in cell division during the formation of sperm cells or shortly after fertilization. Instead of the typical 46 chromosomes, individuals with XYY syndrome have 47 chromosomes. Importantly, this is not an inherited condition; it is a sporadic genetic event that occurs by chance, meaning parents do not "pass it on" through their own genetic makeup.

How common is XYY syndrome?

Research indicates that XYY syndrome occurs in approximately 1 in every 1,000 newborn males. Because many people with the condition never experience significant health issues, many remain undiagnosed throughout their lives. Within the DiseaseMaps.org community, 82 people with XYY syndrome have already joined to share their lived experiences and connect with others navigating similar paths.

What are the primary characteristics of XYY syndrome?

The clinical presentation of XYY syndrome varies significantly from person to person. While there are no distinct "subtypes," clinicians monitor the following common features:

Physical growth: A tendency toward above-average height during childhood and adolescence.

Developmental milestones: Potential for mild delays in speech, language, and motor skills.

Learning and behavior: An increased likelihood of learning disabilities, such as dyslexia, or attention-deficit hyperactivity disorder (ADHD).

Physical health: Generally, individuals with XYY syndrome have normal sexual development and fertility, similar to the general population.

How does XYY syndrome differ from other conditions?

Unlike other chromosomal aneuploidies that often result in severe physical or intellectual disabilities, XYY syndrome is characterized by a much milder phenotype. Most individuals lead healthy, productive lives and often do not require medical intervention beyond routine developmental monitoring.