VARIATION IN THE RATIO OF PHYSICAL TO GENETIC-DISTANCE IN INTERVALS ADJACENT TO THE MLA LOCUS ON BARLEY CHROMOSOME 1H

Variants of the pulsed-field gel electrophoresis technique were used i n conjunction with two-dimensional DNA gel electrophoresis (2-DDGE) to determine the ratio of physical to genetic distance in two geneticall y defined intervals on barley chromosome 1H. 2-DDCE analysis demonstra ted that two loci that define a 0.3 cM interval, as determined by hybr idization with BCD249, reside on a single 450-kb MluI fragment. This r esult indicates a maximum ratio of physical to genetic distance in thi s interval of 1500 kb/cM as compared to 3.7-4.2 Mb/cM for the barley g enome as a whole. High molecular weight (HMW) DNA restricted with NotI and probed sequentially with MWG068 and BCD249 yield diffuse bands at approximately 2.5 Mb and 3.0 Mb in the C.I. 16151 and C.I. 16155 pare ntal lines, respectively. These results suggest the maximum ratio of p hysical to genetic distance in the interval defined by these probes is 7.5 Mb/cM. Unique HMW DNA restriction fragment length polymorphisms ( RFLP) were attributed to the presence of recombination breakpoints. Da ta from the recombination breakpoint analysis were used to estimate a ratio of physical to genetic distance of 2.5 Mb/cM in the Xbcd249.2-Xm wg068 interval and 0.465 Mb/cM in the Xbcd-349.1-Xbcd249.2 interval. B oth physical linkage and recombination breakpoint analysis indicate th e Xbcd249.1-Xbcd249.2 interval is approximately five-fold smaller, phy sically, than the Xbcd249.2-Xmwg068 interval.