Klinefelter syndrome is a genetic condition affecting males that occurs when a person carries an extra copy of the X chromosome. Instead of the typical male chromosome pattern, written as 46,XY, a person with Klinefelter syndrome usually has the pattern 47,XXY. This is not a change within a single gene the way many other genetic conditions are; it is a difference in the total number of chromosomes, called an aneuploidy, and it is the most common of all human chromosome differences, affecting an estimated 1 in 500 to 1 in 1,000 males.

The extra X chromosome arises from an error called nondisjunction, in which chromosome pairs fail to separate properly as an egg or sperm cell is forming, or, less often, during one of the earliest cell divisions after fertilization. This error can happen on either the mother’s or the father’s side, and current evidence suggests the two are roughly equally likely to be the source, at close to half and half. When the error occurs in the egg, it can happen at either of two stages of cell division; when it occurs in the sperm, it can only happen at the first stage, since an error at the second stage would produce a different chromosome combination entirely. A mother’s age at conception does appear to matter: pregnancies in mothers older than 40 show about four times the likelihood of Klinefelter syndrome compared with pregnancies in mothers younger than 24, though the reasons for this are not fully worked out and are thought to relate to how the egg’s internal machinery manages chromosome separation as it ages.

Not everyone with Klinefelter syndrome has the same chromosome pattern in every cell. Between 10 and 20 percent of affected individuals are mosaic, meaning some of their cells carry the extra X chromosome and others carry the typical pattern; this generally happens because the error occurred after fertilization, in only some of the descendant cells. Mosaic Klinefelter syndrome tends to produce a milder pattern of features. Rare variants exist too, with two or three extra X chromosomes instead of one, which generally cause more pronounced effects on development and physical features than the standard 47,XXY pattern.