Klinefelter syndrome is a genetic condition in which boys are born with an extra sex chromosome in their cells.
Klinefelter syndrome (also known as XXY syndrome) is a genetic syndrome that is found only in males. Boys with Klinefelter Syndrome are born with an extra sex chromosome in their cells.
Typically, females have two X chromosomes, while males have one X and one Y chromosome. In males with Klinefelter syndrome, there are two X chromosomes and one Y chromosome (XXY). Rarely there are additional X chromosomes that are present in the cells (XXXY).
As the boys develop into adolescents (12-17 years of age*), their condition becomes more apparent, as they can be taller than other boys their age, with disproportionally long arms and legs.
Males with Klinefelter Syndrome typically demonstrate symptoms during the onset of puberty, including:
In addition to physical symptoms, boys with Klinefelter Syndrome can also have language, emotional or learning delays. As adult males, they can also be affected by infertility due to low sperm count.
*Age of adolescents as defined by the Centers for Disease Control (CDC).
While Klinefelter Syndrome is congenital (present at birth), is not hereditary (passed down in families). It occurs during fetal development when the chromosomes divide.
American Board of Pediatrics/Endocrinology