Everyone's genetic material, or DNA, is contained in tiny structures called chromosomes that are found inside their cells. A person's gender is determined by their sex chromosomes. Females have two X chromosomes, or XX. Most males have one X chromosome and one Y chromosome, or XY. Males with Klinefelter syndrome are born with cells that have an extra X chromosome, or XXY.

Klinefelter syndrome is a genetic condition, meaning people are born with it, but it's not passed down through families like some genetic diseases. Instead, Klinefelter syndrome occurs randomly due to an error in cell division when a parent's reproductive cells are being formed. If one of these defective cells contributes to a successful pregnancy, the baby will have the XXY condition in some or all of his cells. Some boys will even have more than two X chromosomes, which increases the risk of severe symptoms and other health concerns.

When a boy is born with the XXY condition in only some of his cells, it's called mosaic Klinefelter syndrome. Often, boys and men with this condition can have milder signs and symptoms than others with the XXY condition.