<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> <P> The process of bacterial transformation also shares many similarities with chromosomal cross over, particularly in the formation of overhangs on the sides of the broken DNA strand, allowing for the annealing of a new strand . Bacterial transformation itself has been linked to DNA repair many times . The second theory comes from the idea that meiosis evolved from bacterial transformation, with the function of propagating genetic diversity. . Thus, this evidence suggests that it is a question of whether cross over is linked to DNA repair or bacterial transformation, as the two do not appear to be mutually exclusive . It is likely that crossing over may have evolved from bacterial transformation, which in turn developed from DNA repair, thus explaining the links between all three processes . </P>

Point of attachment between each pair of chromatids