Klinefelter syndrome – NHS

Klinefelter syndrome is not directly inherited – the additional X chromosome occurs as a result of either the mother’s egg or the father’s sperm having the extra X chromosome (an equal chance of this happening in either), so after conception the chromosome pattern is XXY rather than XY. This change in the egg or sperm seems to happen randomly.

https://www.nhs.uk/conditions/klinefelters-syndrome/