THE DISTRIBUTION OF RFLP MARKERS ON CHROMOSOME 2(2H) OF BARLEY IN RELATION TO THE PHYSICAL AND GENETIC LOCATION OF 5S RDNA

The 5S rDNA locus on the long arm of barley chromosome 2(2H) was genet ically mapped in two crosses in relation to 30 other RFLP loci. Compar ison of the genetic maps with the previously published physical positi on of the 5S rDNA, determined by in-situ hybridization, showed that th ere was a marked discrepancy between physical and genetic distance in both crosses, with recombination being less frequent in the proximal p art of the arm. Pooled information from the present study and other pu blished genetic maps showed that at least 26 of the 44 (59 %) RFLPs th at have been mapped on 2(2H)L lie distal to the 5S rDNA locus even tho ugh this region is only 27 % of the physical length of the arm. The di stribution of RFLP markers is significantly different from expected (P < 0.01), implying that the low-copy sequences used for RFLP analysis occur more frequently in distal regions of the arm and, or, that seque nces in distal regions are more polymorphic.