A chiasma (plural: chiasmata), in genetics, is thought to be the point where two homologous non-sister chromatids exchange genetic material during chromosomal crossover during meiosis (sister chromatids also form chiasmata between each other (also known as a chi structure), but because their genetic material is identical, it does not cause any change in the resulting daughter cells). The chiasmata become visible during the diplotene stage of prophase I of meiosis, but the actual "crossing-over" of genetic material is thought to occur during the previous pachytene stage. When each tetrad, which is composed of two pairs of sister chromatids, begins to split, the only points of contact are at the chiasmata.
- chiasma frequency = 2 x recombination frequency
where recombination frequency is:
- recombination frequency = (no. of recombinants x 100) / (total no. of progeny)
The phenomenon of genetic chiasmata (chiasmatypie) was discovered and described in 1909 by Frans Alfons Janssens, a Jesuit professor at the University of Leuven in Belgium.  "Bivalent" refers to the two homologous chromosomes (4 chromatids); "chiasma" refers to the actual break of the phosphodiester bond during crossing over. The larger the number of map units between the genes, the more crossing over occurs.
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- Whitby, M. (2009), "Recombination and the formation of chiasmata in meiosis", in Millar, J. (ed.), The Cell Division Cycle: Controlling when and where cells divide and differentiate, The Biomedical & Life Sciences Collection, Henry Stewart Talks Ltd, London (online at http://hstalks.com/?t=BL0422198)