Crossing-over occurs during prophase I, and it is the exchange of genetic material between non-sister chromatids of homologous chromosomes. During prophase I, homologous chromosomes line up in pairs, gene-for-gene down their entire length, forming a configuration with four chromatids, known as a tetrad. The process of pairing the homologous chromosomes is called synapsis. During synapsis, non-sister chromatids may cross-over at points called chiasmata. Within a chiasma, the genetic material from two non-sister chromatids actually intertwine around each other, and some material from non-sister chromatids switch chromosomes, that is, the material breaks off and reattaches at the same position on the homologous chromosome (Figure below). This exchange of genetic material can happen many times within the same pair of homologous chromosomes, creating unique combinations of alleles. This process is also known as homologous recombination.
