Explain Crossing Over in genetics?

Crossing over is when the arms of homologous chromosomes exchange segments (and therefore genes) with each other. Recall that one of the homologues originally came from the father's sperm and the other came from the mother's egg.

During Prophase I of Meiosis the homologous chromosomes pair up to form a tetrad. The arms of the chromatids overlap, and can sometimes swap portions. This exchange results in a genetic recombination of the parental genes, providing for genetic variation, the raw material for evolution.