Chiasmata are X-shaped structures that form at the point of contact between two non-sister chromatids belonging to homologous chromosomes during meiosis). They hold the homologous chromosomes together as a bivalent until anaphase I. At the point of chiasmata, an exchange of chromosomal segments can occur between both chromatids, which is called a chromosomal crossover). This exchange of genetic material is most easily shown if homologous chromosomes are differentially color-coded. The phenomenon of genetic chiasmata (chiasmatypie) was discovered and described in 1909 by Frans Alfons Janssens, a Professor at the University of Leuven in Belgium). The absence of a chiasma generally results in improper chromosomal segregation and aneuploidy). Sister chromatids also form chiasmata between each other, but because their genetic material is identical, it does not cause any noticeable change in the resulting daughter cells).