XxY

XXY refers to a sex chromosome aneuploidy in humans in which males have one extra X chromosome, most commonly described as 47,XXY. The condition is often called Klinefelter syndrome, a name derived from the physician who first described it in the 1940s. In many cases, individuals with XXY have no distinguishing external signs at birth and diagnoses are made later in childhood, adolescence, or adulthood.

The cause is typically nondisjunction during meiosis, leading to an extra X chromosome in the fertilized egg. Most cases arise sporadically; there is no simple inheritance pattern. Affected individuals may be mosaic, having a mix of 46,XY and 47,XXY cell lines, which can influence the severity of features.

Clinical features vary but commonly include signs of testicular dysfunction and hypogonadism, such as smaller testes, reduced testosterone levels, delayed or incomplete puberty, and gynecomastia. Most individuals have normal or near-normal intelligence, but some experience learning difficulties, language delays, or social and functional challenges. Infertility is common due to impaired spermatogenesis, though rare cases of sperm production can occur in mosaic cases. Tall stature with long limbs is frequent, and there may be mild metabolic or bone health concerns.

Diagnosis is made through chromosomal analysis, usually via blood tests, karyotyping, or more modern genetic testing. Management focuses on addressing hormone levels with testosterone replacement when appropriate, educational and psychosocial support, and fertility counseling. Some individuals may pursue assisted reproduction in select cases. The prognosis for quality of life is generally good with appropriate medical and psychosocial care.