<P> Their experiment revealed linkage between the P and L alleles and the p and l alleles . The frequency of P occurring together with L and p occurring together with l is greater than that of the recombinant Pl and pL . The recombination frequency is more difficult to compute in an F2 cross than a backcross, but the lack of fit between observed and expected numbers of progeny in the above table indicate it is less than 50% . </P> <P> The understanding of linkage was expanded by the work of Thomas Hunt Morgan . Morgan's observation that the amount of crossing over between linked genes differs led to the idea that crossover frequency might indicate the distance separating genes on the chromosome . The centimorgan, which expresses the frequency of crossing over, is named in his honour . </P> <P> A linkage map (also known as a genetic map) is a table for a species or experimental population that shows the position of its known genes or genetic markers relative to each other in terms of recombination frequency, rather than a specific physical distance along each chromosome . Linkage maps were first developed by Alfred Sturtevant, a student of Thomas Hunt Morgan . </P> <P> A genetic map is a map based on the frequencies of recombination between markers during crossover of homologous chromosomes . The greater the frequency of recombination (segregation) between two genetic markers, the further apart they are assumed to be . Conversely, the lower the frequency of recombination between the markers, the smaller the physical distance between them . Historically, the markers originally used were detectable phenotypes (enzyme production, eye colour) derived from coding DNA sequences; eventually, confirmed or assumed noncoding DNA sequences such as microsatellites or those generating restriction fragment length polymorphisms (RFLPs) have been used . </P>

Who proposed the coupling and repulsion theory of linkage