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Chromosomal crossover

Crossing over occurs between prophase I and metaphase I and is the process where two homologous non-sister chromatids pair up with each other and exchange different segments of genetic material to form two recombinant chromosome sister chromatids. It can also happen during mitotic division,[1] which may result in loss of heterozygosity. Crossing over is important for the normal segregation of chromosomes during meiosis.[2] Crossing over also accounts for 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.
Thomas Hunt Morgan's illustration of crossing over (1916)
A double crossing over

Chromosomal crossover, or crossing over, is the exchange of genetic material during sexual reproduction between two homologous chromosomes' non-sister chromatids that results in recombinant chromosomes. It is one of the final phases of genetic recombination, which occurs in the pachytene stage of prophase I of meiosis during a process called synapsis. Synapsis is usually initiated before the synaptonemal complex develops and is not completed until near the end of prophase I. Crossover usually occurs when matching regions on matching chromosomes break and then reconnect to the other chromosome, resulting in chiasma which are the visible evidence of crossing over.

  1. ^ Griffiths AJ, Gelbart WM, Miller JH, et al. (1999). "Mitotic Crossing-Over". Modern Genetic Analysis. New York: W. H. Freeman.
  2. ^ Wang S, Zickler D, Kleckner N, Zhang L (1 February 2015). "Meiotic crossover patterns: obligatory crossover, interference and homeostasis in a single process". Cell Cycle. 14 (3): 305–314. doi:10.4161/15384101.2014.991185. PMC 4353236. PMID 25590558.

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