English: Crossing over occurs during meiosis I, and is the process where homologous chromosomes pair up with each other and exchange different segments of their genetic material to form recombinant chromosomes. In some species crossing over is essential for the normal segregation of chromosomes during meiosis. Crossing over also increases genetic variation, because due to the swapping of genetic material during crossing over, the chromatids held together by the centromere are no longer identical. So, when the chromosomes go on to meiosis II and separate, some of the daughter cells receive daughter chromosomes with recombined alleles. Due to this genetic recombination, the offspring have a different set of alleles and genes than their parents do. In the diagram, genes B and b are crossed over with each other, making the resulting recombinants after meiosis Ab, AB, ab, and aB.