Crossing over

Crossing over is the swapping of corresponding stretches of DNA between nonsister chromatids of a pair of homologous chromosomes, one inherited from each parent. It happens in prophase I of meiosis, when the two homologs line up tightly along their whole length, held by a protein scaffold called the synaptonemal complex. The DNA of the two chromatids is cut at matching points and rejoined crosswise, and the sites of exchange later show under the microscope as X-shaped links called chiasmata.

Crossing over has two main jobs. It creates recombinant chromosomes carrying a new mix of maternal and paternal alleles, which, together with the random orientation of homolog pairs at metaphase I, makes nearly every gamete genetically unique. It also keeps each pair of homologs physically connected until anaphase I, and at least one crossover per pair is normally needed for the chromosomes to separate correctly.

Genes far apart on a chromosome are split up by crossing over more often than genes close together. Alfred Sturtevant used this in 1913 to draw the first genetic map, and distances are still given in map units, or centimorgans, where 1 percent recombination equals one unit. Recombination between two genes never exceeds 50 percent.