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Abstract
SUMMARY: Mitotic mapping in the cellular slime mould Dictyostelium discoideum was investigated by analysing the gene order and map distances of four genetic markers on linkage group II: whi, acrA, tsgD (previously reported) and a new spore shape marker sprB. The previously suggested gene order has been revised to centromere, whi, acrA, tsgD/sprB, on the basis of the analysis of four different diploids. One class of diploid which was previously thought to arise by mitotic crossing-over between tsgD and acrA probably arose by mutation at the acrA locus or by reversion of the tsgD locus followed by mitotic crossing-over at another interval. These and other problems associated with mitotic mapping are discussed. Evidence is presented that for a particular class of cross-over diploid (white, temperature-resistant, methanol-resistant) an origin from a single cross-over event is likely rather than a multiple cross-over origin. It is suggested that multiple mitotic cross-overs, on the same arm of a chromosome, are rare in D. discoideum.
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