Crossing over is the process by which a section on one homologous chromosome breaks off and reattaches on the other homologous chromosome. Homologous chromosomes come in pairs, with one inherited from the mother and the other coming from the father.
Crossing over occurs in meiosis, a type of cell division that makes egg and sperm cells. During the stage of meiosis called Prophase I, the homologous chromosomes pair up. The arms on the homologous chromosomes overlap with one another, and pieces on the arms detach, migrating to the other chromosomes and attaching. Crossing over causes variability during the process of meiosis, so that the resulting egg or sperm cells have more genetic variation to pass on.