Crossing Over

Crossing over is the biological process by which corresponding segments of DNA are physically exchanged between the two non-sister chromatids of a pair of homologous chromosomes, generating new combinations of alleles that did not exist on either original chromosome. Morgan coined the term in 1912 while studying Drosophila genetics. It occurs almost exclusively during the pachytene stage of prophase I of meiosis in the germ cells that will become gametes, which is why it is also called meiotic or germinal crossing over; very rarely, an equivalent exchange can happen between sister chromatids in ordinary body cells during mitosis, called somatic or mitotic crossing over, though this is far less common and genetically less significant. Crossing over occurs universally, in essentially every sexually reproducing organism, and its importance goes well beyond simply shuffling alleles: it is the physical event that makes incomplete linkage possible, and consequently it is also the basis for buil …