<P> Crossing over was described, in theory, by Thomas Hunt Morgan . He relied on the discovery of Frans Alfons Janssens who described the phenomenon in 1909 and had called it "chiasmatypie". The term chiasma is linked, if not identical, to chromosomal crossover . Morgan immediately saw the great importance of Janssens' cytological interpretation of chiasmata to the experimental results of his research on the heredity of Drosophila . The physical basis of crossing over was first demonstrated by Harriet Creighton and Barbara McClintock in 1931 . </P> <P> The linked frequency of crossing over between two gene loci (markers) is the crossing - over value . For fixed set of genetic and environmental conditions, recombination in a particular region of a linkage structure (chromosome) tends to be constant and the same is then true for the crossing - over value which is used in the production of genetic maps . </P> <P> There are two popular and overlapping theories that explain the origins of crossing - over, coming from the different theories on the origin of meiosis . The first theory rests upon the idea that meiosis evolved as another method of DNA repair, and thus crossing - over is a novel way to replace possibly damaged sections of DNA . The second theory comes from the idea that meiosis evolved from bacterial transformation, with the function of propagating diversity . </P> <P> Crossing over and DNA repair are very similar processes, which utilize many of the same protein complexes . Recombinases and primases lay a foundation of nucleotides along the DNA sequence . One such particular protein complex that is conserved between processes is RAD51, a well conserved recombinase protein that has been shown to be crucial in DNA repair as well as cross over . Several other genes in D. melanogaster have been linked as well to both processes, by showing that mutants at these specific loci cannot undergo DNA repair or crossing over . Such genes include mei - 41, mei - 9, hdm, spnA, and brca2 . This large group of conserved genes between processes supports the theory of a close evolutionary relationship . Furthermore, DNA repair and crossover have been found to favor similar regions on chromosomes . In an experiment using radiation hybrid mapping on wheat's (Triticum aestivum L .) 3B chromosome, crossing over and DNA repair were found to occur predominantly in the same regions . Furthermore, crossing over has been correlated to occur in response to stressful, and likely DNA damaging, conditions </P>

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