XYY

XYY, also known as Jacob’s syndrome, is a chromosomal condition in which a male has an extra Y chromosome, resulting in a 47,XYY karyotype. It is a sex chromosome aneuploidy and occurs in males who typically have 47 chromosomes instead of the usual 46. The condition is usually identified through genetic testing conducted for various reasons, and many individuals with XYY lead typical lives with few or no symptoms.

Prevalence and clinical features vary widely. XYY occurs in about 1 in 1,000 live male births, but many cases remain undiagnosed. Common presentations can include being taller than average in adolescence and adulthood. Some individuals have mild learning difficulties, speech and language delays, or motor coordination differences. Behavioral and neurodevelopmental concerns, such as attention-deficit/hyperactivity disorder or autism spectrum features, may appear in some cases, but there is no uniform phenotype. Fertility is generally normal, and many individuals have normal sex development and reproductive capability.

Causes and diagnosis are genetic in origin. XYY results from nondisjunction during paternal meiosis II or, less commonly, during early mitotic divisions after fertilization. Mosaic cases, with a mix of 46,XY and 47,XYY cells, can occur but are less common. Diagnosis is made by karyotyping or other chromosome analysis, and can be detected before birth via prenatal testing or after birth if developmental concerns arise.

Management focuses on supportive care. There is no cure for XYY. Interventions may include speech and language therapy, occupational therapy, and educational accommodations tailored to the individual’s needs. Regular medical follow-up is advised to monitor growth, development, and any associated health issues. Most individuals with XYY have a normal life expectancy and can achieve typical adult functioning with appropriate supports. Misconceptions about links to crime have been debunked by research, and the condition is best understood as a spectrum of outcomes rather than a single clinical trajectory.