<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 . In 1931, Barbara McClintock discovered a triploid maize plant . She made key findings regarding corn's karyotype, including the size and shape of the chromosomes . McClintock used the prophase and metaphase stages of mitosis to describe the morphology of corn's chromosomes, and later showed the first ever cytological demonstration of crossing over in meiosis . Working with student Harriet Creighton, McClintock also made significant contributions to the early understanding of codependency of linked genes . </P> <P> Crossing over and DNA repair are very similar processes, which utilize many of the same protein complexes . In her report, "The Significance of Responses of the Genome to Challenge", McClintock studied corn to show how corn's genome would change itself to overcome threats to its survival . She used 450 self - pollinated plants that received from each parent a chromosome with a ruptured end . She used modified patterns of gene expression on different sectors of leaves of her corn plants show that transposable elements ("controlling elements") hide in the genome, and their mobility allows them to alter the action of genes at different loci . These elements can also restructure the genome, anywhere from a few nucleotides to whole segments of chromosome . 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>

The exchange of genetic material during crossing over causes which of the following