Klinefelter syndrome
Klinefelter syndrome is a condition affecting males who carry an extra X chromosome, with a typical karyotype of 47,XXY rather than the usual 46,XY. The presence of a Y chromosome directs male development, while the additional X adds an extra dose of X-linked genes — like a copy machine that runs one page more than intended.
It usually arises from nondisjunction during the formation of the egg or sperm, so an extra X ends up in the fertilized embryo. Because cells inactivate spare X chromosomes, much of the extra X is silenced, which is why effects can be subtle; many individuals are diagnosed only in adulthood, or not at all.
Common features can include reduced fertility, smaller testes and lower testosterone, taller stature, and sometimes learning or language differences, though the spectrum is broad and many people lead unremarkable lives. Hormone therapy and fertility options are part of modern care.
Variants with more than one extra sex chromosome (such as 48,XXXY) tend to produce more pronounced effects, reflecting the greater dosage imbalance, even though X-inactivation still buffers part of it.
A man investigated for infertility is found to have a 47,XXY karyotype, identifying Klinefelter syndrome.
An extra X alongside a Y is the cytogenetic signature of Klinefelter syndrome.
It is named after Harry Klinefelter, who described the syndrome in 1942; the 47,XXY cause was found in 1959. This entry is educational, not medical advice.